Basic Meteorologi
Department of Atmospheric Sciences (DAS)
at
the University of Illinois at Urbana-Champaign.
the University of Illinois at Urbana-Champaign.
The Online Meteorology Guide is a
collection of web-based instructional modules that use multimedia technology
and the dynamic capabilities of the web. These resources incorporate text,
colorful diagrams, animations, computer simulations, audio and video to
introduce fundamental concepts in the atmospheric sciences.
The target audience for the Online
Meteorology Guide is high school and undergraduate level students. Contents of
the Online Meteorology Guide were developed by graduate students and faculty
through our efforts in the Collaborative
Visualization Project (CoVis), which was funded by the National Science
Foundation. These resources have been reviewed by faculty and
scientists at the University of
Illinois and the Illinois State
Water Survey. Many of these resources were tested in a classroom
environment and have been modified based upon teacher and student feedback.
Atmospheric Pressure
force exerted by the weight of the air
Atmospheric pressure
is defined as the force per unit area exerted against a surface by the weight
of the air above that surface. In the diagram below, the pressure at point
"X" increases as the weight of the air above it increases. The same
can be said about decreasing pressure, where the pressure at point
"X" decreases if the weight of the air above it also decreases.
Thinking in terms of
air molecules, if the number of air molecules above a surface increases, there
are more molecules to exert a force on that surface and consequently, the
pressure increases. The opposite is also true, where a reduction in the number
of air molecules above a surface will result in a decrease in pressure.
Atmospheric pressure is measured with an instrument called a
"barometer", which is why atmospheric pressure is also referred to as
barometric pressure.
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In aviation and
television weather reports, pressure is given in inches of mercury
("Hg), while meteorologists use millibars (mb), the unit of pressure
found on weather maps.
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As an example,
consider a "unit area" of 1 square inch. At sea level, the weight of
the air above this unit area would (on average) weigh 14.7 pounds! That means
pressure applied by this air on the unit area would be 14.7 pounds per square
inch. Meteorologists use a metric unit for pressure called a millibar and the
average pressure at sea level is 1013.25 millibars.
Pressure with Height
pressure decreases with increasing altitude
The number of air
molecules above a surface changes as the height of the surface above the ground
changes. For example, there are fewer air molecules above the 50 kilometer (km)
surface than are found above the 12 km surface. Since the number of air
molecules above a surface decreases with height, pressure likewise decreases
with height.
Most of the atmosphere's
molecules are held close to the earth's surface by gravity. Because of this,
air pressure decreases rapidly at first, then more slowly at higher levels.
Since more than half
of the atmosphere's molecules are located below an altitude of 5.5 km, atmospheric
pressure decreases roughly 50% (to around 500 mb) within the lowest
5.5 km. Above 5.5 km, the pressure continues to decrease, but at an
increasingly slower rate (to about 1 mb at 50 km).
Isobars
lines of constant pressure
A line drawn on a
weather map connecting points of equal pressure
is called an "isobar". Isobars are generated from mean sea-level pressure reports
and are given in millibars.
The diagram below
depicts a pair of sample isobars. At every point along the top isobar, the
pressure is 996 mb while at every point along the bottom isobar, the pressure
is 1000 mb. Points above the 1000 mb isobar have a lower pressure and points
below that isobar have a higher pressure.
Any point lying in
between these two isobars must have a pressure somewhere between 996 mb and
1000 mb. Point A, for example, has a pressure of 998 mb and is therefore
located between the 996 mb isobar and the 1000 mb isobar.
Sea-level pressure
reports are available every hour, which means that isobar maps are
likewise available every hour. The solid blue contours (in the map below)
represent isobars and the numbers along selected contours indicate the pressure
value of that particular isobar.
Such maps are
useful for locating areas of high and low pressure, which correspond to the
positions of surface cyclones
and anticyclones.
A map of isobars is also useful for locating strong pressure gradients,
which are identifiable by a tight packing of the isobars. Stronger winds are
associated with stronger pressure gradients.
Constant Pressure Surfaces
a surface of equal pressure, also called an isobaric
surface
A constant pressure
(or isobaric) surface is a surface in the atmosphere where the pressure
is equal everywhere along that surface. For example, the 100 millibar (mb)
surface is the surface in the atmosphere where the pressure at every point
along that surface is 100 mb. Since pressure decreases
with height, the altitude of the 100 mb surface is higher than the
500 mb surface, which is likewise higher than 1000 mb. Meteorologists use
pressure as a vertical coordinate to simplify thermodynamic computations which
are performed on a routine basis.
Measurements of the
upper atmosphere (temperature, pressure, winds, etc.) are taken by instruments
on weather balloons as they rise upward from the earth. When referring to the
500 mb surface, we mean a location in the atmosphere where the pressure has
been measured to be 500 mb. The approximate heights and temperatures for
several constant pressure surfaces have been listed below:
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Pressure
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Approximate Height
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Approximate Temperature
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Sea Level
1000mb 850 mb 700 mb 500 mb 300 mb 200 mb 100 mb |
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Chart from: WXP Purdue
The atmospheric
variables typically plotted on isobaric maps include: height of the pressure
surface, temperature, moisture content and wind speed and direction.
Pressure and Temperature
the relationship between pressure surfaces and temperature
The height of a given pressure surface
above the ground varies with temperature. As an example, consider two identical
columns of air (A and B). Since they are identical, the 500 mb surface is found
at the same height in each column.
Cooling column A and
heating column B changes the height of the 500 mb surface in each column. Since
colder air contracts, the height of the 500 mb surface in column A decreases,
while in column B, the warm air expands, raising the height of the 500 mb
surface.
Therefore, where the
temperatures are colder, a given pressure surface will have a lower height than
if the same pressure surface was located in warmer air.
High Pressure Centers
also known as anticyclones
A high pressure center
is where the pressure
has been measured to be the highest relative to its surroundings. That means,
moving in any direction away from the "High" will result in a
decrease in pressure. A high pressure center also represents the center of an
anticyclone and is indicated on a weather map by a blue "H".
Winds flow clockwise
around a high pressure center in the northern hemisphere, while in the southern
hemisphere, winds flow counterclockwise around a high.
Sinking air in the vicinity of a high pressure center suppresses the upward motions needed to support the development of clouds and precipitation. This is why fair weather is commonly associated with an area of high pressure.
Low Pressure Centers
also known as cyclones
A low pressure center
is where the pressure
has been measured to be the lowest relative to its surroundings. That means,
moving in any horizontal direction away from the "Low" will result in
an increase in pressure. Low pressure centers also represent the centers of cyclones.
A low pressure center
is indicated on a weather map by a red "L" and winds flow
counterclockwise around a low in the northern hemisphere. The opposite is true
in the southern hemisphere, where winds flow clockwise around an area of low
pressure.
Rising motion
in the vicinity of a low pressure center favors the development of clouds and
precipitation, which is why cloudy weather (and likely precipitation) are
commonly associated with an area of low pressure.
Pressure Gradient Force
directed from high to low pressure
The change in pressure
measured across a given distance is called a "pressure gradient".
The pressure gradient results in a net force
that is directed from high to
low
pressure and this force is called the "pressure gradient force".
Coriolis Force
an artifact of the earth's rotation
Once air has been set in motion by the pressure gradient
force, it undergoes an apparent deflection from its path, as seen by
an observer on the earth. This apparent deflection is called the "Coriolis
force" and is a result of the earth's rotation.
As air moves from high to
low
pressure in the northern hemisphere, it is deflected to the right by the
Coriolis force. In the southern hemisphere, air moving from high to low
pressure is deflected to the left by the Coriolis force.
The amount of
deflection the air makes is directly related to both the speed at which the air
is moving and its latitude. Therefore, slowly blowing winds will be deflected
only a small amount, while stronger winds will be deflected more. Likewise,
winds blowing closer to the poles will be deflected more than winds at the same
speed closer to the equator. The Coriolis force is zero right at the equator.
Geostrophic Wind
winds balanced by the Coriolis and Pressure Gradient forces
An air parcel
initially at rest will move from high pressure to low pressure because of the pressure gradient
force (PGF). However, as that air parcel begins to move, it is
deflected by the Coriolis force
to the right in the northern hemisphere (to the left on the southern hemisphere).
As the wind gains speed, the deflection increases until the Coriolis force
equals the pressure gradient force. At this point, the wind will be blowing
parallel to the isobars.
When this happens, the wind is referred to as geostrophic.
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The movie below illustrates the process mentioned above,
while the diagram at right shows the two forces balancing to produce the
geostrophic wind. Winds in nature are rarely exactly geostrophic, but to a
good approximation, the winds in the upper troposphere can be close. This is
because winds are only considered truly geostrophic when the isobars are
straight and there are no other forces acting on it -- and these conditions
just aren't found too often in nature.
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Gradient Wind
non-geostrophic winds which blow parallel to isobars
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Geostrophic
winds exist in locations where there are no frictional forces and
the isobars are striaght. However, such locations are quite rare. Isobars
are almost always curved and are very rarely evenly spaced. This changes the
geostrophic winds so that they are no longer geostrophic but are instead in gradient
wind balance. They still blow parallel to the isobars, but are no longer
balanced by only the pressure gradient and Coriolis forces, and do not have
the same velocity as geostrophic winds.
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Winds near the surface
Winds affected by friction
Geostrophic wind
blows parallel to the isobars
because the Coriolis force
and pressure gradient
force are in balance. However it should be realized that the actual
wind is not always geostrophic -- especially near the surface.
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The surface of the Earth exerts a frictional drag on the
air blowing just above it. This friction can act to change the wind's
direction and slow it down -- keeping it from blowing as fast as the wind
aloft. Actually, the difference in terrain conditions directly affects how
much friction is exerted. For example, a calm ocean surface is pretty smooth,
so the wind blowing over it does not move up, down, and around any features.
By contrast, hills and forests force the wind to slow down and/or change
direction much more.
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As we move higher,
surface features affect the wind less until the wind is indeed geostrophic.
This level is considered the top of the boundary (or friction) layer. The
height of the boundary layer can vary depending on the type of terrain, wind,
and vertical temperature profile. The time of day and season of the year also
affect the height of the boundary layer. However, usually the boundary layer
exists from the surface to about 1-2 km above it.
In the friction layer,
the turbulent friction that the Earth exerts on the air slows the wind down.
This slowing causes the wind to be not geostrophic. As we look at the diagram
above, this slowing down reduces the Coriolis
force, and the pressure gradient
force becomes more dominant. As a result, the total wind deflects
slightly towards lower pressure. The amount of deflection the surface wind has
with respect to the geostrophic wind
above depends on the roughness of the terrain. Meteorologists call the
difference between the total and geostrophic winds ageostrophic winds.
Sea Breezes
a result of uneven surface heating
When spending a day at
the beach, a noticeable drop in temperature may occur during the early
afternoon as a cool breeze begins to blow off of the water. This wind is known
as the "sea breeze", which occurs in response to differences in
temperature between a body of water and neighboring land.
Sea-breeze circulations
most often occur on warm sunny days during the spring and summer when the
temperature of the land is normally higher than the temperature of the water.
During the early morning hours, the land and the water start out at roughly the
same temperature. On a calm morning, a given pressure surface
will be at the same height above both the land and water.
A few hours later, the
sun's energy begins to warm the land more rapidly than the water. By later in
the day, the temperature of the land increases while the temperature of the
water remains relatively constant. This occurs because water, especially large
bodies of water like a lake or ocean, are able to absorb more energy than land
without warming.
It is important to
remember that the air is not heated directly from above by the sun. In fact,
most of the incoming solar energy actually passes right through the atmosphere.
However, as the land absorbs this energy, heat is radiated back into the
atmosphere (from the earth), warming the overlying air. Some of this heat is
transported to higher levels in the atmosphere through convection.
On the other hand,
since the temperature of the water remains relatively constant throughout the
day, the air over the water is not heated from below (as over land), resulting
in lower air temperatures over the water.
Land Breezes
begin with the cooling of low-level air
On clear, calm
evenings, temperature differences between a body of water and neighboring land
produce a cool wind that blows offshore. This wind is called a "land
breeze". Land breezes are strongest along the immediate coastline but weaken
considerably further inland.
Land-breeze
circulations can occur at any time of year, but are most common during the fall
and winter seasons when water temperatures are still fairly warm and nights are
cool.
On clear and calm
evenings, the earth's surface cools by radiating (giving off) heat back into
space, and this results in a cooling of the immediately overlying air.
Since the air over
land cools more rapidly than the air over water, a temperature difference is
established, with cooler air present over land and relatively warmer air
located over water.
Air Masses
uniform bodies of air
An air mass is a large
body of air that has similar temperature and moisture properties throughout. The
best source regions for air masses are large flat areas where air can be
stagnant long enough to take on the characteristics of the surface below. Maritime tropical
air masses (mT), for example, develop over the subtropical oceans
and transport heat and moisture northward into the U.S.. In contrast, continental polar
air masses (cP), which originate over the northern plains of Canada,
transport colder and drier air southward.
Once an air mass moves
out of its source region, it is modified as it encounters surface conditions different
than those found in the source region. For example, as a polar air mass moves
southward, it encounters warmer land masses and consequently, is heated by the
ground below. Air masses typically clash in the middle latitudes, producing
some very interesting weather.
Continental Polar Air Masses
cold temperatures and little moisture
Those who live in
northern portions of the United
States expect cold weather during the winter
months. These conditions usually result from the invasion of cold arctic air
masses that originate from the snow covered regions of northern Canada. Because
of the long winter nights and strong radiational cooling found in these
regions, the overlying air becomes very cold and very stable. The longer this
process continues, the colder the developing air mass becomes, until changing
weather patterns transport the arctic air mass southward.
Arctic air masses move
about as a shallow area of high pressure,
commonly known as an "Arctic High". Northerly winds associated with a
cyclone
and trailing anticyclone, (the center of the arctic air mass), transport the
colder air southward. Since the terrain is generally flat and free of any
significant topographical features, arctic air masses entering the United States and can easily slide all the way
to Texas and Florida.
Below is a map of surface
observations and the leading edge of a large arctic air mass
blanketing much of the United
States has been highlighted by the blue
line. The center of this air mass is a high pressure
center located in northern Montana
(indicated by the blue "H").
From these reports,
we see that most stations in the arctic air mass generally exhibit relatively
colder temperatures,
with lower dew point
temperatures, and winds
generally out of the north. Notice that on the other side of the blue boundary,
outside of this air mass, surface conditions are much different, which
indicates the presence of an entirely different air mass.
Maritime Tropical Air Masses
warm temperatures and rich in moisture
Maritime tropical air
masses originate over the warm waters of the tropics and Gulf
of Mexico, where heat and moisture are transferred to the
overlying air from the waters below. The northward movement of tropical air
masses transports warm moist air into the United States, increasing the
potential for precipitation.
Tropical air masses
are generally restricted to the southern states during much of the winter.
However, southerly winds
ahead of migrating cyclones occasionally transport a tropical air
mass northward during the winter season.
Below is a map of surface
observations and the leading edge of a tropical air mass surging
northward into the Ohio
Valley has been
highlighted in red. Southerly winds behind the boundary signify the continued
northward transport of warm moist air.
From these reports,
we see that most stations in the tropical air mass generally exhibit relatively
warmer temperatures,
with higher dew point
temperatures, and winds
generally out of the south. Notice that on the other side of the red boundary,
outside of this air mass, surface conditions are much different, which
indicates the presence of an entirely different air mass.
Fronts
the boundaries between air masses
A front is defined as
the transition zone between two air masses
of different density. Fronts extend not only in the horizontal direction, but
in the vertical as well. Therefore, when referring to the frontal surface (or
frontal zone), we referring to both the horizontal and vertical components of
the front.
Stationary Front
a front that is not moving
When a warm or cold
front stops moving, it becomes a stationary front. Once this boundary resumes
its forward motion, it once again becomes a warm front
or cold front.
A stationary front is represented by alternating blue and red lines with blue
triangles pointing towards the warmer air and red semicircles pointing towards
the colder air.
A noticeable
temperature change and/or shift in wind direction is commonly observed when
crossing from one side of a stationary front to the other.
In the map above, temperatures
south of the stationary front were in the 50's and 60's with winds
generally from the southeast. However, north of the stationary front,
temperatures were in the 40's while the winds had shifted around to the
northeast. Cyclones
migrating along a stationary front can dump heavy amounts of precipitation, resulting
in significant flooding along the front.
Cold Front
transition zone from warm air to cold air
A cold front is
defined as the transition zone where a cold air mass is replacing a warmer air
mass. Cold fronts generally move from northwest to southeast. The air behind a
cold front is noticeably colder and drier than the air ahead of it. When a cold
front passes through, temperatures can drop more than 15 degrees within the
first hour.
Symbolically, a cold
front is represented by a solid line with triangles along the front pointing
towards the warmer air and in the direction of movement. On colored weather
maps, a cold front is drawn with a solid blue line.
There is typically a
noticeable temperature change from one side of a cold front to the other. In
the map of surface temperatures below, the station east of the front reported a
temperature of 55 degrees Fahrenheit while a short distance behind the front,
the temperature decreased to 38 degrees. An abrupt temperature change over a
short distance is a good indicator that a front is located somewhere in
between.
If colder air is
replacing warmer air, then the front should be analyzed as a cold front. On the
other hand, if warmer air is replacing cold air, then the front should be
analyzed as a warm front.
Common characteristics associated with cold fronts have been listed in the
table below.
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Before Passing
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While Passing
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After Passing
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Winds
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south-southwest
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gusty; shifting
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west-northwest
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Temperature
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warm
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sudden drop
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steadily dropping
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Pressure
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falling steadily
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minimum, then sharp rise
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rising steadily
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Clouds
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Precipitation
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short period of showers
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heavy rains, sometimes with hail, thunder and lightning
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showers then clearing
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Visibility
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fair to poor in haze
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poor, followed by improving
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good, except in showers
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Dew Point
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high; remains steady
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sharp drop
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lowering
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Table adapted from: Ahrens, (1994)
Warm Front
transition zone from cold air to warm air
A warm front is
defined as the transition zone where a warm air mass is replacing a cold air
mass. Warm fronts generally move from southwest to northeast and the air behind
a warm front is warmer and more moist than the air ahead of it. When a warm
front passes through, the air becomes noticeably warmer and more humid than
it was before.
Symbolically, a warm
front is represented by a solid line with semicircles pointing towards the
colder air and in the direction of movement. On colored weather maps, a warm
front is drawn with a solid red line.
There is typically a
noticeable temperature change from one side of the warm front to the other. In
the map of surface temperatures below, the station north of the front reported
a temperature of 53 degrees Fahrenheit while a short distance behind the front,
the temperature increased to 71 degrees. An abrupt temperature change over a
short distance is a good indication that a front is located somewhere in
between.
If warmer air is
replacing colder air, then the front should be analyzed as a warm front. If
colder air is replacing warmer air, then the front should be analyzed as a cold front.
Common characteristics associated with warm fronts have been listed in the
table below.
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Before Passing
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While Passing
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After Passing
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Winds
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south-southeast
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variable
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south-southwest
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Temperature
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cool-cold, slow warming
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steady rise
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warmer, then steady
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Pressure
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usually falling
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leveling off
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slight rise, followed by fall
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Clouds
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stratus-type
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Precipitation
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light-to-moderate rain, snow, sleet, or drizzle
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drizzle or none
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usually none, sometimes light rain or showers
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Visibility
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poor
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poor, but improving
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fair in haze
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Dew Point
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steady rise
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steady
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rise, then steady
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Table adapted from: Ahrens, (1994)
The States of Water
solid, liquid, gas
Water is known to exist in three different
states; as a solid, liquid or gas.
Clouds, snow, and rain
are all made of up of some form of water. A cloud is comprised of tiny water
droplets and/or ice crystals, a snowflake is an aggregate of many ice crystals,
and rain is just liquid water.
Water existing as a
gas is called water vapor. When referring to the amount of moisture in the air,
we are actually referring to the amount of water vapor. If the air is described
as "moist", that means the air contains large amounts of water vapor.
Common sources of moisture for the United States
are the warm moist air
masses that flow northward from the Gulf of Mexico and western Atlantic Ocean as well as the moist Pacific air masses
brought onshore by the westerlies.
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As cyclones
move eastward from the Rocky Mountains, southerly winds
ahead of these storm systems transport the
warm moist air northward. Moisture is a necessary ingredient for
the production of clouds and precipitation.
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Relative Humidity
indicates how moist the air is
Relative humidity may
be defined as the ratio of the water vapor density (mass per unit volume) to
the saturation water vapor density, usually expressed in percent:
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Relative Humidity
(RH) =
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X 100%
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Relative humidity is also approximately the
ratio of the actual to the saturation vapor pressure.
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RH =
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X 100%
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Actual vapor pressure
is a measurement of the amount of water vapor in a volume of air and increases
as the amount of water vapor increases. Air that attains its saturation vapor
pressure has established an equilibrium with a flat surface of water. That
means, an equal number of water molecules are evaporating
from the surface of the water into the air as are condensing
from the air back into the water.
Saturation vapor
pressure is a unique function of temperature as given in the table below. Each
temperature in the table may be interpreted as a dew point
temperature, because as the ground cools, dew will begin to form at
the temperature corresponding to the vapor pressure in table below.
For example, if the
water vapor pressure in the air is 10.2 millibars (mb), dew will form when the
ground reaches 45 degrees Fahrenheit (F). The relative humidity for air
containing 10.2 mb of water vapor is simply 100% times 10.2 mb divided by the
saturation vapor pressure at the actual temperature. For example, at 70 F the
saturation vapor pressure is 25 mb, so the relative humidity would be
RH = 100% X (10.21 / 25.0) = 41%
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(C)
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Temp
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(F)
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Sat Vapor Prs (mb)
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(C)
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Temp
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(F)
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Sat Vapor Prs (mb)
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-18
-15 -12 -09 -07 -04 -01 02 04 07 10 13 16 |
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00
05 10 15 20 25 30 35 40 45 50 55 60 |
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1.5
1.9 2.4 3.0 3.7 4.6 5.6 6.9 8.4 10.2 12.3 14.8 17.7 |
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18
21 24 27 29 32 35 38 41 43 46 49 52 |
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65
70 75 80 85 90 95 100 105 110 115 120 125 |
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21.0
25.0 29.6 35.0 41.0 48.1 56.2 65.6 76.2 87.8 101.4 116.8 134.2 |
Chart adapted from: Ahrens
Rising Air
a key process in the production of clouds and precipitation
Imagine a block of
air, or air parcel, rising upward through the atmosphere. The air parcel
expands as it rises and this expansion, or work, causes the temperature of the
air parcel to decrease.
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As the parcel rises, its humidity
increases until it reaches 100%. When this occurs, cloud droplets begin
forming as the excess water vapor condenses on the largest aerosol particles.
Above this point the cloud droplets grow by condensation
in the rising air.
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If the cloud is sufficiently deep or long lived,
precipitation
will develop.
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The upward motions that generate clouds and lead to
precipitation can be produced by convection
in unstable air, convergence
of air near cloud base, lifting of air by fronts
and lifting over elevated topography
such as mountains.
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Cloud Types
common cloud classifications
Clouds are classified
into a system that uses Latin words to describe the appearance of clouds as
seen by an observer on the ground. The table below summarizes the four
principal components of this classification system (Ahrens,
1994).
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Latin Root
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Translation
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Example
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cumulus
stratus cirrus nimbus |
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heap
layer curl of hair rain |
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fair weather cumulus
altostratus cirrus cumulonimbus |
Further classification
identifies clouds by height of cloud base. For example, cloud names containing
the prefix "cirr-", as in cirrus clouds, are located at high levels
while cloud names with the prefix "alto-", as in altostratus, are
found at middle levels. This module introduces several cloud groups. The first
three groups are identified based upon their height above the ground. The
fourth group consists of vertically developed clouds, while the final group
consists of a collection of miscellaneous cloud types.
High-level clouds form above 20,000 feet (6,000 meters) and
since the temperatures are so cold at such high elevations, these clouds are
primarily composed of ice crystals. High-level clouds are typically thin and
white in appearance, but can appear in a magnificent array of colors when the
sun is low on the horizon.
Cirrus Clouds
thin and wispy
The most common form of high-level clouds are
thin and often wispy cirrus clouds. Typically found at heights greater than
20,000 feet (6,000 meters), cirrus clouds are composed of ice crystals that
originate from the freezing of supercooled water droplets. Cirrus generally
occur in fair weather and point in the direction of air movement at their
elevation.
Cirrus can form from
almost any cloud that has undergone glaciation
and can be observed in a variety of shapes and sizes. Possibilities range from
the "finger-like" appearance of cirrus fall streaks to the uniform
texture of more extensive cirrus clouds associated with an approaching warm front.
Fall streaks form when
snowflakes and ice crystals fall from cirrus clouds. The change in wind with
height and how quickly these ice crystals fall determine the shapes and sizes
the fall streaks attain. Since ice crystals fall much more slowly than
raindrops, fall streaks tend to be stretched out horizontally as well as
vertically. Cirrus streaks may be nearly straight, shaped like a comma, or
seemingly all tangled together.
Similar to fall
streaks is virga,
which appears as streamers suspended in the air beneath the base of
precipitating clouds. Virga develops when precipitation falls through a layer
of dry air and evaporates before reaching the ground.
Cirrostratus Clouds
sheet-like and nearly
transparent
Cirrostratus are sheet-like, high-level clouds
composed of ice crystals. Though cirrostratus can cover the entire sky and be
up to several thousand feet thick, they are relatively transparent, as the sun
or the moon can easily be seen through them. These high-level clouds typically
form when a broad layer of air is lifted by large-scale convergence.
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Sometimes the only indication of their presence is given
by an observed halo
around the sun or moon. Halos result from the refraction
of light by the cloud's ice crystals. Cirrostratus clouds, however, tend to
thicken as a warm front
approaches, signifying an increased production of ice crystals. As a result,
the halo gradually disappears and the sun (or moon) becomes less visible.
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When the sun is low on the horizon, cirrostratus
clouds can appear in a magnificent array of colors as longer wavelengths of
sunlight (red, yellow, and orange) are reflected
off of the clouds.
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The cirrus streaks
in this photograph are aligned in a southwest to northeast direction,
indicative of warmer air advancing at higher levels. Lower on the horizon,
thickening cirrostratus clouds effectively hide the sun, signifying changing
weather ahead. As the warm front
approaches, these clouds will thicken and be replaced lower and more
dense cloud types.
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The bases of mid-level clouds typically appear between 6,500 to 20,000 feet (2,000 to 6,000 meters). Because of their lower altitudes, they are composed primarily of water droplets, however, they can also be composed of ice crystals when temperatures are cold enough.
Altocumulus Clouds
parallel bands or rounded
masses
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Altocumulus clouds are composed primarily of water
droplets and are located between 6,500 and 20,000 feet (2,000 to 6,000
meters) above the ground.
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Altocumulus may appear as parallel bands (top photograph)
or rounded masses (bottom photograph). Typically a portion of an altocumulus
cloud is shaded, a characteristic which makes them distinguishable from the
high-level cirrocumulus. Altocumulus clouds usually form by convection
in an unstable layer aloft, which may result from the gradual lifting of air
in advance of a cold front.
The presence of altocumulus clouds on a warm and humid summer morning is
commonly followed by thunderstorms later in the day.
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Also found at
mid-levels are altostratus clouds, which are often confused with high-level cirrostratus.
One distinguishing feature is that a halo is
not observed around the sun (or moon) when viewed through altostratus, but is a
common feature associated with cirrostratus clouds. In fact, the sun (or moon)
is only vaguely visible through altostratus clouds and appears as if it were
shining through frosted glass.
Low clouds are of mostly composed of water droplets since their bases generally lie below 6,500 feet (2,000 meters). However, when temperatures are cold enough, these clouds may also contain ice particles and snow.
Nimbostratus Clouds
dark, low-level clouds
with precipitation
Nimbostratus are dark, low-level clouds
accompanied by light to moderately falling precipitation. Low clouds are
primarily composed of water droplets since their bases generally lie below
6,500 feet (2,000 meters). However, when temperatures are cold enough, these
clouds may also contain ice particles and snow.
The sun (or moon) is
not visible through nimbostratus clouds, which distinguishes them from
mid-level altostratus clouds. Because of the fog and falling precipitation
commonly found beneath and around nimbostratus clouds, the cloud base is
typically very diffuse and difficult to accurately determine.
Stratocumulus Clouds
low, lumpy layer of
clouds
Stratocumulus clouds generally appear as a low,
lumpy layer of clouds that is sometimes accompanied by weak intensity
precipitation. Stratocumulus vary in color from dark gray to light gray and may
appear as rounded masses, rolls, etc., with breaks of clear sky in between.
Since the individual
elements of stratocumulus are larger than those of altocumulus,
one can easily decipher between the two cloud types by extending your arm
toward the sky. Altocumulus elements are about the size of a thumb nail while
stratocumulus are about the size of a fist (Ahrens,
1994).
Probably the most familiar of the classified clouds is the cumulus cloud. Generated most commonly through either thermal convection or frontal lifting, these clouds can grow to heights in excess of 39,000 feet (12,000 meters), releasing incredible amounts of energy through the condensation of water vapor within the cloud itself.
Fair Weather Cumulus
Clouds
puffy cotton balls
floating in the sky
Fair weather cumulus have the appearance of
floating cotton and have a lifetime of 5-40 minutes. Known for their flat bases
and distinct outlines, fair weather cumulus exhibit only slight vertical
growth, with the cloud tops designating the limit of the rising air. Given
suitable conditions, however, harmless fair weather cumulus can later develop
into towering cumulonimbus
clouds associated with powerful thunderstorms.
Fair weather cumulus
are fueled by buoyant bubbles of air, or thermals,
that rise upward from the earth's surface. As they rise, the water vapor within
cools and condenses forming cloud droplets. Young fair weather
cumulus have sharply defined edges and bases while the edges of older clouds
appear more ragged, an artifact of cloud erosion. Evaporation along the cloud
edges cools the surrounding air, making it heavier and producing sinking motion
(or subsidence) outside the cloud.
The downward motion
inhibits further convection
and the growth of additional thermals from below, which is why fair weather
cumulus typically have expanses of clear sky between them. Without a continued
supply of rising air, the cloud begins to erode and eventually disappears.
Cumulonimbus Clouds
reaching high into the
atmosphere
Cumulonimbus clouds (Cb) are much larger and
more vertically developed than fair weather
cumulus. They can exist as individual towers
or form a line of towers called a squall line.
Fueled by vigorous convective updrafts (sometimes in excess 50 knots), the tops
of cumulonimbus clouds can easily reach 39,000 feet (12,000 meters) or higher.
Lower levels of
cumulonimbus clouds consist mostly of water droplets while at higher
elevations, where temperatures are well below 0 degrees Celsius, ice crystals
dominate. Under favorable atmospheric conditions, harmless fair weather
cumulus clouds can quickly develop into large cumulonimbus clouds
associated with powerful thunderstorms known as supercells.
Supercells
are large thunderstorms with deep rotating updrafts and can have a lifetime of
several hours. Supercells can produce frequent lightning,
large hail, damaging winds,
and tornadoes.
These storms tend to develop during the
afternoon and early evening when the effects of heating by the sun are
strongest. For more information about supercells
and other types of severe weather phenomena, visit the Severe Storm
Spotters Guide.
Lifting by Convection
upward moving thermals
In meteorology,
convection refers primarily to atmospheric motions in the vertical direction.
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As the earth is heated by the sun, bubbles of hot air
(called thermals) rise upward from the warm surface. A thermal cools as it
rises and becomes diluted as it mixes with the surrounding air, losing some
of its buoyancy (its ability to rise).
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An air parcel will
rise naturally if the air within the parcel is warmer than the surrounding air
(like a hot air balloon). Therefore, if cool air is present aloft with warm air
at lower levels, thermals can rise to great heights before losing their
buoyancy.
Successive thermals
following the same path usually rise higher than previous ones, and if a
thermal is able to rise high enough to cool to its
saturation point, the moisture within condenses
and becomes visible as a cloud.
When a deep stable
layer exists just above the cloud base, continued vertical growth is restricted
and only fair weather
cumulus are able to form. However, if a deep unstable layer (cold
air aloft) is present, continued vertical growth is likely, leading to the
development of a cumulonimbus cloud,
which contains raindrops. Once the supply of thermals is cut off, the cloud
begins to dissipate and eventually disappears. Convective clouds are typically
much more vertically developed than those clouds generated by convergence
lifting.
Lifting by Convergence
broad lifting of an entire layer of air
Convergence is an
atmospheric condition that exists when there is a horizontal net inflow of air
into a region. When air converges along the earth's surface, it is forced to
rise since it cannot go downward.
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Large scale convergence
can lift a layer of air hundreds of kilometers across.
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Vertical motions
associated with convergence are typically much weaker than the small-scale
vertical motions associated with convective
processes. As a result, clouds generated through convergence, for example cirrostratus
clouds, are typically less vertically developed than convective
clouds.
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Lifting Due To Topography
produces orographic clouds
When air is confronted
by a mountain, it is lifted up and over the mountain, cooling as it
rises. If the air cools to its saturation point, the water vapor condenses
and a cloud forms.
These types of clouds
are called "orographic clouds", which develop in response to lifting
forced by the topography the earth.
exact type of cloud that develops depends upon the moisture
content and stability of the air.
Lifting Along Frontal Boundaries
when air masses interact
Lifting also occurs
along frontal boundaries,
which separate air masses
of different density.
In the case of a cold front,
a colder, denser air mass lifts the warm, moist air ahead of it. As the air
rises, it cools and its moisture condenses
to produce clouds and precipitation. Due to the steep slope of a cold front,
vigorous rising motion is often produced, leading to the development of showers
and occasionally severe thunderstorms.
In the case of a warm front,
the warm, less dense air rises up and over the colder air ahead of the front.
Again, the air cools as it rises and its moisture condenses
to produce clouds and precipitation. Warm fronts have a gentler slope and
generally move more slowly than cold fronts, so the rising motion along warm
fronts is much more gradual. Precipitation
that develops in advance of a surface warm front is typically steady and more
widespread than precipitation associated with a cold front.
Rain or Snow?
dependent upon temperature
Precipitation
typically forms high in the atmosphere where the temperature is below freezing.
As ice crystals form aloft and fall toward the surface, they collect each other
to form large snowflakes. If ground temperature is above 32 F, the freezing
level must be located somewhere above the ground. As the falling snow passes
through the freezing level into the warmer air, the flakes melt and collapse
into raindrops. During the summer months, it is not uncommon for the freezing
level to be found at a level above cloud base.
When the air
temperature at the ground is less than 32 F, the snowflakes do not melt on the
way down and therefore reach the ground as snow.
Occasionally, we
observe snow reaching the ground even though the outside temperature is above
freezing. This occurs when a very thin layer of warm air is found near the
surface.
Since the layer of
warm air is so shallow, the precipitation reaches the ground as snow before it
has a chance to melt and become rain. For more about precipitation, visit the precipitation
section of this module.
Rain and Hail
liquid and ice precipitation
Rain develops when growing cloud droplets become too heavy
to remain in the cloud and as a result, fall toward the surface as rain. Rain
can also begin as ice crystals that collect each other to form large
snowflakes. As the falling snow passes through the freezing level into warmer
air, the flakes melt and collapse into rain drops.
Hail is a large frozen
raindrop produced by intense thunderstorms,
where snow and rain can coexist in the central updraft. As the snowflakes fall,
liquid water freezes onto them forming ice pellets that will continue to grow
as more and more droplets are accumulated. Upon reaching the bottom of the
cloud, some of the ice pellets are carried by the updraft back up to the top of
the storm.
As the ice pellets
once again fall through the cloud, another layer of ice is added and the hail
stone grows even larger. Typically the stronger the updraft, the more times a hail
stone repeats this cycle and consequently, the larger it grows. Once the hail
stone becomes too heavy to be supported by the updraft, it falls out of the
cloud toward the surface. The hail stone reaches the ground as ice since it is
not in the warm air below the thunderstorm long enough to melt before reaching
the ground.
Freezing Rain
supercooled droplets freezing on impact
Ice storms can be the
most devastating of winter weather phenomena and are often the cause of
automobile accidents, power outages and personal injury. Ice storms result from
the accumulation of freezing rain, which is rain that becomes supercooled and
freezes upon impact with cold surfaces. Freezing rain is most commonly found in
a narrow band on the cold side of a warm front,
where surface temperatures are at or just below freezing.
The diagram below
shows a typical temperature profile for freezing rain with the red line
indicating the atmosphere's temperature at any given altitude. The vertical
line in the center of the diagram is the freezing line. Temperatures to the
left of this line are below freezing, while temperatures to the right are above
freezing.
Freezing rain develops
as falling snow
encounters a layer of warm air deep enough for the snow to completely melt and
become rain.
As the rain continues to fall, it passes through a thin layer of cold air just
above the surface and cools to a temperature below freezing. However, the drops
themselves do not freeze, a phenomena called supercooling (or forming
"supercooled drops"). When the supercooled drops strike the frozen
ground (power lines, or tree branches), they instantly freeze, forming a thin
film of ice, hence freezing rain.
Freezing rain
is dangerous because it is almost invisible on smooth surfaces and
consequently, people are often unaware of its presence. Sidewalks become
extremely slick when covered with freezing rain, increasing the likelihood of
someone slipping and injuring themselves. Automobile accidents are more likely
to occur during an ice storm because of the icy roads.
Sleet
frozen raindrops that bounce on impact with the ground
Progressing further
ahead of the warm front,
surface temperatures continue to decrease and the freezing rain eventually
changes over to sleet. Areas of sleet are located on the colder side (typically
north) of the freezing rain band.
Sleet is less
prevalent than freezing rain
and is defined as frozen raindrops that bounce on impact with the ground or
other objects. The diagram below shows a typical temperature profile for sleet
with the red line indicating the atmosphere's temperature at any given
altitude. The vertical line in the center of the diagram is the freezing line.
Temperatures to the left of this line are below freezing, while temperatures to
the right are above freezing.
Sleet is more
difficult to forecast than freezing rain
because it develops under more specialized atmospheric conditions. It is very
similar to freezing rain in that it causes surfaces to become very slick, but
is different because its easily visible.
Snow
an aggregate of ice crystals
Progressing even
further away from the warm front,
surface temperatures continue to decrease and the sleet changes over to snow.
Snowflakes are simply
aggregates of ice crystals that collect to each other as they fall toward the
surface. The diagram below shows a typical temperature profile for snow with
the red line indicating the atmosphere's temperature at any given altitude. The
vertical line in the center of the diagram is the freezing line. Temperatures
to the left of this line are below freezing, while temperatures to the right
are above freezing.
Since the snowflakes
do not pass through a layer of air warm enough to cause them to melt, they
remain in tact and reach the ground as snow.
El Niño
a warm current of water
El Niño (Spanish name for the male child), initially
referred to a weak, warm current appearing annually around Christmas time along
the coast of Ecuador and Peru and lasting only a few weeks to a month or more.
Every three to seven years, an El Niño event may last for many months, having
significant economic
and atmospheric
consequences worldwide. During the past forty years, ten of these major El Niño
events have been recorded, the worst of which occurred in 1997-1998.
Previous to this, the El Niño event in 1982-1983 was the strongest. Some of the
El Niño events have persisted more than one year.
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El Niño Years
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1902-1903
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1905-1906
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1911-1912
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1914-1915
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1918-1919
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1923-1924
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1925-1926
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1930-1931
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1932-1933
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1939-1940
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1941-1942
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1951-1952
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1953-1954
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1957-1958
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1965-1966
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1969-1970
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1972-1973
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1976-1977
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1982-1983
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1986-1987
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1991-1992
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1994-1995
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Selected text from: CPC ENSO Main Page
In the tropical
Pacific, trade winds
generally drive the surface waters westward. The surface water becomes
progressively warmer going westward because of its longer exposure to solar
heating. El Niño is observed when the easterly trade winds weaken, allowing
warmer waters of the western Pacific to migrate eastward and eventually reach
the South American Coast
(shown in orange). The cool nutrient-rich sea water normally found along the
coast of Peru
is replaced by warmer water depleted of nutrients, resulting in a dramatic reduction
in marine fish and plant life.
In contrast to El
Niño, La Niña (female child) refers to an anomaly of unusually cold sea surface
temperatures found in the eastern tropical Pacific. La Niña occurs roughly half
as often as El Niño.
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La Niña Years
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1904-1905
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1909-1910
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1910-1911
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1915-1916
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1917-1918
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1924-1925
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1928-1929
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1938-1939
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1950-1951
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1955-1956
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1956-1957
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1964-1965
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1970-1971
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1971-1972
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1973-1974
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1975-1976
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1988-1989
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1995-1996
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1997-1998 El Niño
the most recent event
The most recent El
Niño event began in the spring months of 1997. Instrumentation placed on Buoys
in the Pacific Ocean after the 1982-1983 El Niño began recording abnormally
high temperatures off the coast of Peru. Over the next couple of
months, these strength of these anomalies grew. The anomalies grew so large by
October 1997 that this El Niño had already become the strongest in the 50+
years of accurate data gathering.
The image below
displays the Sea Surface Temperature (SST) Anomalies in degrees Celsius for the
middle of September, 1997. By this time, the classic El Niño pattern has almost
fully ripened, with maxima above +4 degrees Celsius.
Image by: CPC ENSO Main Page
Droughts in the Western Pacific
Islands and Indonesia
as well as in Mexico and Central America were the early (and sometimes constant)
victims of this El Niño. These locations were consistent with early season El
Niños in the past. A global view of the normal climatic effects of El Niño can
be seen below.
Image by: CPC ENSO Main Page
The effects El Niño have on United States' weather is less obvious. Back in 1982-1983, the U.S. Gulf States and California received excessive rainfall. As the winter approached, forecasters expected excessive rainfall to occur again. Indeed, portions of central and southern California suffered record-breaking rainfall amounts. Damage consisted not only of flooding, but mudslides Some mudslides destroyed communities in a flash -- causing many casualties. Other problems could be found in the Gulf states, as severe weather was above average. Even though no one particular storm can be blamed on El Niño, many forecasters do believe the event did increase the chances for such severe weather to occur.
Upwelling
the transport of deeper water to shallow levels
One oceanic process
altered during an El Niño year is upwelling, which is the rising of deeper
colder water to shallower depths. The diagram below shows how upwelling occurs
along the coast of Peru.
Because of the frictional stresses that exist between ocean layers, surface
water is transported at a 90 degree angle to the left of the winds in the
southern hemisphere, 90 degrees to the right of the winds in the northern
hemisphere. This is why winds blowing northward parallel to the coastline of
Peru "drag" surface water westward away from shore.
Nutrient-rich water
rises from deeper levels to replace the surface water that has drifted away and
these nutrients are responsible for supporting the large fish
population commonly found in these areas. The effectiveness of
upwelling and its ability to support abundant sea life is greatly dependent
upon the depth of the thermocline.
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The thermocline is the transition layer between the mixed
layer at the surface and the deep water layer. The definitions of these
layers are based on temperature.
The mixed layer is
near the surface where the temperature is roughly that of surface water. In
the thermocline, the temperature decreases rapidly from the mixed layer
temperature to the much colder deep water temperature.
The mixed layer and
the deep water layer are relatively uniform in temperature, while the
thermocline represents the transition zone between the two.
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A deeper thermocline
(often observed during El Niño years) limits the amount of nutrients brought to
shallower depths by upwelling processes, greatly impacting
the year's fish crop.
Atmospheric Consequences of El Niño
influencing weather patterns worldwide
During an El Niño year,
tropical rains usually centered over Indonesia shift eastward,
influencing atmospheric wind patterns world wide. Possible impacts include: a
shifting of the jet stream,
storm tracks and monsoons, producing unseasonable weather over many regions of
the globe. During the El Niño event of
1982-1983, some of the abnormal weather patterns observed included:.
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Drought in Southern
Africa, Southern India, Sri Lanka, Philippines, Indonesia, Australia,
Southern Peru, Western Bolivia, Mexico, Central America
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Heavy rain and
flooding in Bolivia, Ecuador, Northern Peru, Cuba, U.S.
Gulf States
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Hurricanes in Tahiti, Hawaii
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The 1982-83 El Niño
strengthened the upper-level ridge
that was present off the West coast of the United States. (This
intensification is represented by the increased amplitude of the wave in the
right panel below).
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Normal Winter
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El Niño Winter
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Images by: DAS, University of Washington
The amplification led
to a warming in the near-Pacific regions of North America, extending from Alaska to the northern Plains of the United States
(orange shading).
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Simultaneously, the deepening of the winter upper-level
trough (typically found over the eastern US) produced heavier than
normal rains in the southern states (blue shading).
As a result of the 1982-83 El Niño
event, wide spread flooding occurred across the southern United States.
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Cyclones
an idealized model
A cyclone is an area
of low pressure
around which the winds flow counterclockwise in the Northern Hemisphere and
clockwise in the Southern Hemisphere.
A developing cyclone
is typically accompanied by a warm front
pushing northward and a cold front
pulling southward, marking the leading edges of air masses being wrapped around
a center of low pressure,
or the center of the cyclone.
The counterclockwise
winds associated with northern hemisphere midlatitude cyclones play a
significant role in the movement air masses,
transporting warm moist air northward ahead of a low while dragging colder,
drier air southward behind it.
** Press "Reload" to restart the animation **
Rising air
in the vicinity of a low pressure center favors the development of clouds and
precipitation, which is why cloudy weather (and likely precipitation) are
commonly associated with an area of low pressure. Cyclones are easily
identifiable on certain types of weather maps by remembering some key
signatures. For example, a cyclone can be found on a map of surface
observations by recognizing a counterclockwise
rotation of the wind barbs for a group of stations, while on
satellite images, cyclones are identifiable by the trademark comma shaped
configuration of cloud bands.
Winds Around Cyclones
flowing counterclockwise in the northern hemisphere
Wind barbs
are useful for locating low pressure
centers on surface weather maps.
Since winds flow in a
counterclockwise direction around low pressure
centers, look for a group of stations where the wind barbs
reflect this type of wind pattern. For example, a counterclockwise wind pattern
was observed in the states of Nebraska, Iowa, Minnesota and South Dakota (highlighted
by the red arrow).
The low pressure
center was located near the center (similar to the center of a
whirlpool) with winds flowing counterclockwise around it.
So when trying to find
a low pressure
center on a surface weather map, use the wind barbs
to identify a counterclockwise wind pattern and the low pressure center will be
found near the center of circulation.
The Movement of Air Masses
transporting warm air northward and colder air southward
Counterclockwise
winds associated with cyclones transport heat and moisture from
lower to higher latitudes and play a significant role in the movement of air masses.
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By superimposing fronts
over the low pressure
center and the air masses,
a top view of a midlatitude cyclone and accompanying air masses might resemble
something like the diagram below:
Southerly winds
east of the low transport warm and moist air northward and this moisture often
contributes to the development of precipitation. A warm front
marks the leading edge of this warm, moist air mass. Behind the low, northerly
winds transport colder and drier air southward, with a cold front
marking the leading edge of this colder, drier air mass.
Cyclones on Satellite Images
comma-shaped cloud configuration
On satellite images, a
midlatitude cyclone
is often identifiable by a comma-shaped cloud mass.
A single cyclone can influence the weather over a large area, (in this case from Texas into Minnesota). This particular storm (in the satellite image above) left more than six inches of snow from Nebraska into Minnesota, while heavy rains occurred from Missouri into Texas.
Jet Stream
current of rapidly moving air
The jet stream is a
current of fast moving air found in the upper levels of the atmosphere. This
rapid current is typically thousands of kilometers long, a few hundred
kilometers wide, and only a few kilometers thick. Jet streams are usually found
somewhere between 10-15 km (6-9 miles) above the earth's surface. The position
of this upper-level jet stream denotes the location of the strongest SURFACE
temperature contrast (as in the diagram below).
During the winter
months, Arctic and tropical air masses create
a stronger surface temperature contrast resulting in a strong jet stream.
However, during the summer months, when the surface temperature variation is
less dramatic, the winds of the jet are weaker.
Below is an ETA Model
forecast panel for 300 mb winds and geopotential
heights (white contours). The color filled regions indicate wind
speed in knots and is color coded according to the legend at the bottom of the
image. The shades of blue indicate winds less than 60 knots, while winds
greater than 120 knots are given in shades of red.
The yellow, green and red ribbon on the image above represents the jet stream, and along the East Coast, the region of strongest winds (shaded in red) is a jet streak.
Hurricanes
a tropical cyclone with winds > 64 knots
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Hurricanes are tropical cyclones with winds that exceed
64 knots (74 mi/hr) and circulate counter-clockwise about their centers in
the Northern Hemisphere (clockwise in the Southern Hemisphere).
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Image by: the GOES Project
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Hurricanes are formed
from simple complexes of thunderstorms.
However, these thunderstorms can only grow to hurricane strength with
cooperation from both the ocean and the atmosphere. First of all, the ocean
water itself must be warmer than 26.5 degrees Celsius (81°F). The heat and
moisture from this warm water is ultimately the source of energy for
hurricanes. Hurricanes will weaken rapidly when they travel over land or colder
ocean waters -- locations with insufficient heat and/or moisture.
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This is a sea surface
temperature map for the northern hemisphere summer. The yellow, orange, and
red colors show water temperatures warm enough to sustain hurricanes
(> 26.5°C).
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Image by: OSDPD
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Related to having warm
ocean water, high relative
humidities in the lower and middle troposphere are also required for
hurricane development. These high humidities reduce the amount of evaporation
in clouds
and maximizes the latent heat
released because there is more precipitation.
The concentration of latent heat is critical to driving the system.
The vertical wind
shear in a tropical cyclone's environment is also important. Wind shear
is defined as the amount of change in the wind's direction or speed with
increasing altitude.
When the
wind shear is weak, the storms that are part of the cyclone grow vertically,
and the latent heat
from condensation
is released into the air directly above the storm, aiding in development. When
there is stronger wind shear, this means that the storms become more slanted
and the latent heat release is dispersed over a much larger area.
Initial Development
the storms that become hurricanes
Hurricanes
initiate from an area of thunderstorms.
These thunderstorms are most commonly formed in one of three different ways.
The first is the InterTropical Convergence Zone (ITCZ). The ITCZ is a
near-solid ring of thunderstorms surrounding the globe found in the tropics. In
the diagram below, the easterly trade winds converge near the equator and
create thunderstorms, which can be seen in the satellite image along the
equator.
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Image by: GOES Server
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The second source for
thunderstorms that can create hurricanes are from eastward moving atmospheric
waves, called easterly waves. Easterly waves are similar to waves in the
mid-latitudes, except they are in the easterly trade-flow. Convergence
associated with these waves creates thunderstorms that can ultimately reach hurricane strength.
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The third mechanism is along old frontal boundaries that
drift into the Gulf of Mexico or coastal Florida. The lift associated with these fronts
can be enough to initiate storms, and if the atmospheric
and oceanic
conditions are sufficient, tropical cyclones can develop that way as well.
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The map below shows
the regions throughout the world where tropical cyclones originate. Tropical
cyclones are more commonly found in the northern hemisphere, but the Pacific
and Indian Oceans both produce hurricanes in the southern hemisphere. However,
in other parts of the world, hurricanes are called by
different names.
At the equator, ocean surface
temperatures are warm enough to produce hurricanes, but none form.
This is because there is not enough coriolis force
to create spin and induce a potential hurricane.
CISK
how thunderstorms become hurricanes
CISK, or
"Convective Instability of the Second Kind", is a popular theory that
explains how thunderstorms
can evolve and organize into hurricanes.
CISK is a positive feedback mechanism, meaning that once a process starts, it
causes events which enhance the original process, and the whole cycle repeats
itself over and over.
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The surface air that spirals
into the center of a low pressure
system creates convergence
(green horizontal arrows) and forces air to rise in the center (green
vertical arrow). This air cools and moisture condenses
which releases latent heat into the air. It is this latent heat that provides
the energy to fuel these storms.
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Latent heat is simply
heat released or absorbed by a substance (in this case, water vapor) as it
changes its state. When water vapor condenses into liquid, it releases this
heat into the surrounding atmosphere. The atmosphere around this condensation
then warms.
Since warm air is less
dense than cooler air, the warmer air takes up more space. This expansion of
this air (red arrows) forces more air outside away from the center of the storm
and the surface pressure (which is the weight of the air above the surface)
decreases.
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When the surface pressure decreases, a larger pressure
gradient is formed, and more air converges
towards the center of the storm. This creates more surface convergence and
causes more warm moist surface air to rise above the surface. This air, as it
cools, condenses
into clouds. While it does this, it releases even more latent heat.
This cycle
continuously repeats itself each time intensifying the storm until other
factors, such as cool water
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Stages of Development
from tropical depression to hurricane
Hurricanes
evolve through a life cycle of stages from birth to death. A tropical
disturbance in time can grow to a more intense stage by attaining a specified
sustained wind speed. The progression of tropical disturbances can be seen in
the three images below.
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Hurricanes can often
live for a long period of time -- as much as two to three weeks. They may
initiate as a cluster of thunderstorms
over the tropical ocean waters. Once a disturbance has become a tropical
depression, the amount of time it takes to achieve the next stage, tropical storm,
can take as little as half a day to as much as a couple of days. It may not
happen at all. The same may occur for the amount of time a tropical storm needs
to intensify into a hurricane.
Atmospheric
and oceanic
conditions play major roles in determining these events.
Below, in this satellite
image from 1995, we can see different tropical disturbances in each stage are
evident. At the far left, Tropical storm Jerry is over Florida, while
Hurricanes Iris and Humberto are further east, amongst a couple of tropical
depressions.
Tropical Depression
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Once a group of thunderstorms has come together under the
right atmospheric conditions for a long enough time, they may organize into a
tropical depression. Winds near the center are constantly between 20 and 34
knots (23 - 39 mph).
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A tropical depression
is designated when the first appearance of a lowered pressure and organized
circulation in the center of the thunderstorm
complex occurs. A surface pressure chart will reveal at least one closed isobar
to reflect this lowering.
When viewed from a
satellite, tropical depressions appear to have little organization. However,
the slightest amount of rotation can usually be perceived when looking at a
series of satellite images. Instead of a round appearance similar to
hurricanes, tropical depressions look like individual thunderstorms that are
grouped together. One such tropical depression is shown here.
Tropical Storms
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Once a tropical
depression has intensified to the point where its maximum
sustained winds are between 35-64 knots (39-73 mph), it becomes a tropical
storm. It is at this time that it is assigned a name. During this time, the
storm itself becomes more organized and begins to become more circular in
shape -- resembling a hurricane.
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The rotation of a
tropical storm is more recognizable than for a tropical
depression. Tropical storms can cause a lot of problems even without
becoming a hurricane.
However, most of the problems a tropical storm cause stem from heavy rainfall.
The above satellite image is of tropical storm
Charlie (1998). Many cities in southern Texas
reported heavy rainfall between 5-10 inches. Included in these was Del Rio, where more than
17 inches fell in just one day, forcing people from their homes and killing
half a dozen.
Hurricanes
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As surface pressures continue to drop, a tropical storm
becomes a hurricane when sustained wind speeds reach 64 knots (74 mph). A
pronounced rotation develops around the central core.
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Hurricanes are Earth's
strongest tropical cyclones. A distinctive feature seen on many hurricanes and
are unique to them is the dark spot found in the middle of the hurricane. This
is called the eye.
Surrounding the eye is the region of most intense winds and rainfall called the
eye wall.
Large bands of clouds and precipitation spiral from the eye wall and are thusly
called spiral rain bands.
Hurricanes are easily
spotted from the previous features as well as a pronounced rotation around the
eye in satellite or radar animations. Hurricanes are also rated according to
their wind speed on the Saffir-Simpson
scale. This scale ranges from categories 1 to 5, with 5 being the
most devastating. Under the right atmospheric conditions, hurricanes can
sustain themselves for as long as a couple of weeks. Upon reaching cooler water
or land, hurricanes rapidly lose intensity.
The Eye
the center of the storm
The most recognizable
feature found within a hurricane
is the eye. They are found at the center and are between 20-50km in diameter.
The eye is the focus of the hurricane, the point about which the rest of the
storm rotates and where the lowest surface
pressures are found in the storm. The image below is of a hurricane
(called cyclone
in the Southern Hemisphere). Note the eye at the center.
Skies are often clear
above the eye and winds are relatively light. It is actually the calmest
section of any hurricane.
The eye is so calm
because the now strong surface winds that converge towards the center never
reach it. The coriolis
force deflects the wind slightly away from the center, causing the wind to
rotate around the center of the hurricane (the eye wall),
leaving the exact center (the eye) calm.
An eye becomes visible
when some of the rising air in the eye wall
is forced towards the center of the storm instead of outward -- where most of
it goes. This air is coming inward towards the center from all directions. This
convergence
causes the air to actually sink in the eye. This sinking creates a warmer
environment and the clouds evaporate
leaving a clear area in the center.
The Eye Wall
a hurricane's most devastating region
Located just outside
of the eye is
the eye wall. This is the location within a hurricane
where the most damaging
winds and intense rainfall
is found. The image below is of a hurricane (called cyclone
in the Southern Hemisphere).
Eye walls are called
as such because oftentimes the eye is
surrounded by a vertical wall of clouds. The eye wall can be seen in the
picture above as the thick ring surrounding the eye.
At the surface, the
winds are rushing towards the center of a hurricane -- forcing air upwards at
the center. The coriolis
force acts on these surface winds, and in the Northern Hemisphere, the
deflection is to the right. The convergence
at the eye wall is so strong here that the air is being lifted faster and with
more force here than any other location of the hurricane.
Thus, the moisture transport from the ocean and subsequent latent heat
production is maximized.
Spiral Bands
where more rain is found
Radiating outward from
the eye wall
one can see a banded structure within the clouds. These clouds are called
either spiral rain bands (or spiral bands). The image below is of a hurricane (called cyclone
in the Southern Hemisphere).
There are sometimes
gaps in between these bands where no rain is found. In fact, if one were to
travel between the outer edge of the hurricane
to its center, one would normally progress from light rain to dry back to
slightly more intense rain again over and over with each period of rainfall
being more intense and lasting longer until reaching the eye. Upon exiting the
eye and moving towards the edge of the hurricane, one would see the same events
as they did going in, but in opposite order.
A schematic of this
banding feature can be seen in the diagram above. The thunderstorms
are now organized into regions of rising and sinking air. Most of the air is
rising, but there is a small amount found in between the thunderstorms that is
sinking.
Movement of Hurricanes
steered by the global winds
The global wind
pattern is also known as the "general circulation" and the surface
winds of each hemisphere are divided into three wind belts:
- Polar Easterlies: From 60-90 degrees latitude.
- Prevailing Westerlies: From 30-60 degrees latitude (aka Westerlies).
- Tropical Easterlies: From 0-30 degrees latitude (aka Trade Winds).
The easterly trade
winds of both hemispheres converge at an area near the equator called the
"Intertropical Convergence Zone (ITCZ)", producing a narrow band of
clouds and thunderstorms that encircle portions of the globe.
The path of a hurricane
greatly depends upon the wind belt in which it is located. A hurricane
originating in the eastern tropical Atlantic,
for example, is driven westward by easterly trade winds in the tropics.
Eventually, these storms turn northwestward around the subtropical high
and migrate into higher latitudes. As a result, the Gulf of Mexico and East
Coast of the United States
are at risk to experience one or more hurricanes each year.
In time, hurricanes
move into the middle latitudes and are driven northeastward by the westerlies,
occasionally merging with midlatitude frontal systems. Hurricanes draw their
energy from the warm surface water of the tropics, which explains why
hurricanes dissipate rapidly once they move over cold water or large land
masses.
How They Are Named
differently in different parts of the world
Hurricane-like storms
are called by different names in the different regions of the world. For
example, the name "hurricane" is given to systems that develop over
the Atlantic or the eastern Pacific
Oceans. In the western
North Pacific and Philippines,
these systems are called "typhoons" while in the Indian and South Pacific Ocean, they are called
"cyclones".
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Since 1953, the Tropical
Prediction Center has produced lists of names for hurricanes. As a tropical
depression develops into a tropical storm,
it is given the next available name on the list, which is prepared in
alphabetical order and alternates from between male and female names. The list
of storm names for 1999-2004 is given below.
Atlantic Storm
Names for 1999-2004
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1999
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2000
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2001
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2002
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2003
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2004
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Arlene
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Alberto
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Allison
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Arthur
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Ana
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Alex
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Bret
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Beryl
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Barry
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Bertha
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Bill
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Bonnie
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Cindy
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Chris
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Chantal
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Cristobal
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Claudette
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Charley
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Dennis
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Debby
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Dean
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Dolly
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Danny
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Danielle
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Emily
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Ernesto
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Erin
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Edouard
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Erika
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Earl
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Floyd
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Florence
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Felix
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Fay
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Fabian
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Frances
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Gert
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Gordon
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Gabrielle
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Gustav
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Grace
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Gaston
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Harvey
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Helene
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Humberto
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Hanna
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Henri
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Hermine
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Irene
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Isaac
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Iris
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Isidore
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Isabel
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Ivan
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Jose
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Joyce
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Jerry
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Josephine
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Juan
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Jeanne
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Katrina
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Keith
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Karen
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Kyle
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Kate
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Karl
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Lenny
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Leslie
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Lorenzo
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Lili
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Larry
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Lisa
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Maria
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Michael
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Michelle
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Marco
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Mindy
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Matthew
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Nate
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Nadine
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Noel
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Nana
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Nicholas
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Nicole
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Ophelia
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Oscar
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Olga
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Omar
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Odette
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Otto
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Philippe
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Patty
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Pablo
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Paloma
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Peter
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Paula
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Rita
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Rafael
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Rebekah
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Rene
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Rose
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Richard
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Stan
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Sandy
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Sebastien
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Sally
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Sam
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Shary
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Tammy
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Tony
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Tanya
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Teddy
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Teresa
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Tomas
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Vince
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Valerie
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Van
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Vicky
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Victor
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Virginie
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Wilma
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William
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Wendy
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Wilfred
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Wanda
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Walter
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Eastern Pacific
Storm Names for 1999-2004
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1999
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2000
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2001
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2002
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2003
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2004
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Adrian
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Aletta
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Adolph
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Alma
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Andres
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Agatha
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Beatriz
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Bud
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Barbara
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Boris
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Blanca
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Blas
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Calvin
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Carlotta
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Cosme
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Cristina
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Carlos
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Celia
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Dora
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Daniel
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Dalilia
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Douglas
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Dolores
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Darby
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Eugene
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Emilia
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Erick
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Elids
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Enrique
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Estelle
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Fernanda
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Fabio
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Flossie
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Fausto
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Felicia
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Frank
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Greg
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Gilma
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Gil
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Genevieve
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Guillermo
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Georgette
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Hilary
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Hector
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Henriette
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Hernan
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Hilda
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Howard
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Irwin
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Ilerna
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Israel
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Iselle
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Ignacio
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Isis
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Jova
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John
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Juliette
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Julio
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Jimena
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Javier
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Kenneth
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Kristy
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Kiko
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Kenna
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Kevin
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Kay
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Lidia
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Lane
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Lorena
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Lowell
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Linda
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Lester
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Max
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Miriam
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Manuel
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Marie
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Marty
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Madeline
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Norma
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Norman
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Narda
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Norbert
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Nora
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Newton
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Otis
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Olivia
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Octave
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Odile
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Olaf
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Orlene
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Pilar
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Paul
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Priscilla
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Polo
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Patricia
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Paine
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Ramon
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Rosa
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Raymond
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Rachel
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Rick
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Roslyn
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Selma
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Sergio
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Sonia
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Simon
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Sandra
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Seymour
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Todd
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Tara
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Tico
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Trudy
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Terry
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Tina
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Veronica
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Vicente
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Velma
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Vance
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Vivian
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Virgil
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Wiley
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Willa
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Wallis
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Winnie
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Waldo
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Winifred
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Xina
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Xavier
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Xina
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Xavier
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Xina
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Xavier
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York
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Yolanda
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York
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Yolanda
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York
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Yolanda
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Zelda
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Zeke
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Zelda
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Zeke
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Zelda
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Zeke
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Interpreting Surface Observation Symbols
a quick overview
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The value highlighted in yellow located in the
upper left corner is the temperature in degrees Fahrenheit. In
this example, the reported temperature is 64 degrees.
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The weather symbol highlighted in yellow indicates
the type of weather occurring at the time the observation is taken. In this
case, fog was reported. If there were thunderstorms occurring when the
observation was taken, then the symbol for thunderstorms would have appeared
instead.
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The value highlighted in yellow located in the
lower left corner is the dew point temperature in degrees Fahrenheit.
In this example, the reported dew point temperature is 58 degrees.
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The symbol highlighted in yellow indicates the
amount of cloud cover observed at the time the
observation is taken. In this case, broken clouds were reported.
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The value highlighted in yellow located in the
upper right corner represents the last three digits of the sea level pressure reading in millibars
(mb).
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The symbol highlighted in yellow is known as a
wind barb. The wind barb indicates wind
direction and wind speed.
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Wind Barbs
determining wind direction
Wind barbs point in the direction
"from" which the wind is blowing. In the case of the diagram below,
the orientation of the wind barb indicates winds from the Northeast.
The term easterly means that the winds are from
the east. In the example above, the winds are out of the northeast, or
northeasterly. On the otherhand, the term "eastward" means that the
winds are blowing towards the east.
Wind Barbs
determining wind speed
Wind speed is given here in the units of
"knots" (knt). A "Knot" is a nautical mile per hour.
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1
Knot = 1.15 Miles Per Hour (MPH)
1 Knot = 1.9 Kilometers Per Hour
(KM/HR)
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Each short barb represents 5 knots, each long
barb 10 knots. A long barb and a short barb is 15 knots, simply by adding the
value of each barb together (10 knots + 5 knots = 15 knots). If only a station
circle is plotted, the winds are calm.
Pennants are 50 knots. Therefore, the last wind example in the chart below has a wind speed of 65 knots. (50 knots + 10 knots + 5 knots).
QUESTIONS
& ANSWERS :
Q:
Air Masses
Characteristics of
Air Masses:
1) The diagram below
depicts two types of air masses that commonly influence weather in the United States.
For each air mass, identify the following characteristics.
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Type of Air Mass:
Source Region: Relative Temperature: Wind Direction: Moisture Content: |
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Find the Air
Masses:
2) One way of
identifying a tropical air mass on the weather map below is to look for a
region of higher temperatures.
To find a polar air mass, look for a region of colder temperatures. The image
below is a map of surface
observations and for this part of the activity, use the temperature
field to draw two lines; a red line to outline the edge of a tropical air mass
and a blue line to identify a polar air mass. Here is an example.
3) Now examine the
regions you have outlined. Look particularly close at the wind barbs
for wind direction
and also examine the reports of dew point
temperature. In question #1, you determined typical wind direction
and dew point temperatures associated with a tropical air mass
and a polar air mass.
Use this additional information to again identify the tropical and the polar
air masses in the diagram above. Label the edge of a tropical air mass with a
red line and use a blue line to indicate the outer edge of a polar air mass.
A:
Air Masses
1)
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Type:
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continental polar air
mass
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maritime tropical air
mass
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Origin:
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snow covered regions
of northern Canada
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warm waters of the
tropics and Gulf of Mexico
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Temperature:
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cold temperatures
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warm temperatures
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Winds:
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from the
north-northwest
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from the
south-southwest
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Moisture:
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dry, little moisture
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moist, air is
typically rich in moisture
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2) and 3)
Q:
Midlatitude Cyclones
Common Characteristics of Cyclones:
1) Complete the following sentence: A cyclone is also is known as a _______________.
1) Complete the following sentence: A cyclone is also is known as a _______________.
2) How is the center of a
cyclone labeled on a weather map?
3) Describe the weather conditions
that typically accompany a cyclone.
4) Describe how a midlatitude
cyclone appears on a satellite image.
Associated Air
Masses and Fronts:
5) The diagram below
depicts a model cyclone with associated fronts and air masses. Answer the
following questions by labeling the diagram itself.
- Circle the center of the cyclone
- Label air mass #1 and air mass #2 (as was done for air mass #0)
- Label the types of fronts represented by front #3 and front #4
6) Describe the general wind pattern
associated with cyclones.
A:
Midlatitude Cyclones
1) "a low pressure center."
2) By a red "L".
3) Cloudy with possible precipitation.
4) Comma-shaped configuration of the cloud bands. The clouds resemble a
giant comma, and the spin counterclockwise around the cyclone center (in the
northern hemisphere).
5)
6) Winds flow generally in a counterclockwise direction around the center of low pressure (in the northern hemisphere).
Q:
Interpreting Weather Symbols
Location of Weather
Symbol:
1) Fill in the blanks of the diagram to indicate
what type of meteorological
data is represented by each position. Then circle the position of
the weather symbol.
Common Weather Symbols:
2) For the following
table of common weather
symbols, fill in the blanks labeled #1 through #7.
A:
Interpreting Weather Symbols
1)
2)
Categories:
Klimatologi
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