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Chapter 19: Air Pressure and Wind
What is Air Pressure?
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Air pressure is the pressure exerted on
all objects by the weight of the air above.
Air pressure is dependent on altitude,
because the higher the altitude, the less
air is above, and as a result, the air
pressure will be lower.
We can measure air pressure by using a
device called a barometer.
Air Pressure and Barometers
How Does a Barometer Work?
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A mercury barometer is a glass tube filled
with mercury that is turned upside down in
a dish or bowl filled with mercury.
When air pressure increases, the air pushes
down on the mercury in the dish, which
raises the height of the mercury in the tube.
A newer barometer called an aneroid
barometer has the advantage that it is
more portable and can record changes in air
pressure.
Barometers
How Does Air Pressure Affect Wind?
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Air pressure is the main cause of wind.
When one area has a higher pressure
than another area, the area with the
higher pressure will blow wind to the area
of lower pressure.
Unequal heating of the earth's surface
causes the differences in the pressure.
The difference in air pressure between
two areas is called the pressure
gradient.
Pressure Gradient and the Wind
Air Pressure and Pressure
Gradients
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Meteorologists use pressure gradients to
predict wind speeds and directions.
As a general rule, the greater and
steeper the pressure gradient, the faster
the wind.
The lesser and shallower the pressure
gradient, the calmer the wind will be.
Pressure Gradient and the Wind
Pressure Gradients and Isobars
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Meteorologists use lines drawn on a map
called isobars to connect areas of equal
pressure.
When isobars are drawn close together,
the pressure gradient is steep.
When the pressure gradient is steep, the
winds will be faster. When the pressure
gradient is shallow, the winds will be
calm.
Isobars and Pressure Gradients
What is the Coriolis Effect?
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The earth rotates on its axis. The equator
spins around at a much faster pace than the
poles.
Because of this difference in motion, the air
at the equator experiences more resistance
to movement due to momentum and
friction.
This difference in motion causes the air at
the equator to be deflected in the opposite
direction as the earth's rotation.
This is what's known as the Coriolis effect.
The Coriolis Effect
Anticyclones and Cyclones
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Areas of low pressures and high
pressures have a central area
where the pressure is the lowest
and the highest.
The center of a low pressure is a
cyclone.
The center of a high pressure is an
anticyclone.
Cyclones
Anticyclones and Cyclones
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In a cyclone, winds blow in towards the
center like a vacuum.
In an anticyclone, winds blow away
from the center.
Because of the Coriolis effect, in the
Northern hemisphere, the winds also
blow counterclockwise in a cyclone.
In the N. hemisphere, the winds blow
clockwise away from an anticyclone.
Cyclones and Anticyclones
How do Anticyclones and Cyclones
Affect Weather Patterns?
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Meteorologists use information from
barometers to determine the locations
and movements of cyclones and
anticyclones.
A cyclone produces cloudy and rainy
weather because air is moving in and
rising.
Rising air produces clouds from
adiabatic cooling and air reaching the
dew point.
Cyclones and Anticyclones
How do Anticyclones and Cyclones
Affect Weather Patterns?
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In an anticyclone, air is sinking to the
ground.
When air descends, it warms, which
causes the relative humidity to lower.
As a result, anticyclones produce clear
beautiful skies with almost no clouds
and rain.
Cyclones and Anticyclones
Global Wind Patterns
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At the earth's equator there is a general
pattern of cyclones to occur called the
equatorial low.
Lots of rainfall and tropical rainforests are
found in the equatorial low region.
At about 30 degrees North and South of
the equator, there is an area that produces
anticyclones called the subtropical high.
Hot, arid conditions produce deserts near
the subtropical high.
Global Wind Patterns
Global Wind Patterns
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The Coriolis effect affects the global wind
patterns as well.
From the subtropical high to the equatorial
low, the winds blow from east to west.
These winds are called the trade winds.
From about 30 degrees North and South of
the equator to about 60 degrees North and
South, the winds blow from West to East.
These winds are known as the Westerlies.
Trade Winds and Westerlies
Global Wind Patterns
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At about 60 degrees North and South of
the equator, there is another cyclone
region known as the subpolar low.
At the subpolar low, cold air masses
from the North and warm air masses
from the South come together to
produce frontal wedging, which creates
cloudy, wet weather.
This area of cloudy wet weather is
known as the polar front.
Polar Front
Polar Front
Global Wind Patterns
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The winds North and South of the 60
degree latitude line generally blow from
West to East to produce winds called the
Polar Easterlies.
At the poles there is another anticyclone
zone called the polar high.
The polar high is generally cold and dry.
Polar Easterlies
Local Wind Patterns
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Winds can also be controlled by variations of
the local topography and surface
composition.
Land and Sea Breezes – Areas located by
large bodies of water can be influenced by
land and sea breezes.
Land changes temperatures faster than
water.
Land heats up faster in the day. The heat
creates rising air. This creates sea breezes
that blow from the water to the land.
Land and Sea Breezes
Local Wind Patterns
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At night, the larger bodies of water stay
warm as the land cools off more quickly.
This causes the air to rise over the water
and the air over land to move out to sea.
This causes the land breezes.
Valley and Mountain Breezes – valley
and mountain Breezes work similarly to
land and sea breezes. Mountainous terrain
changes temperature quicker than the
valley floors.
Local Wind Patterns
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In the heat of the day, the mountains
heat up quickly, causing air to rise. This
draws air up the valleys (updrafts) to
the mountains.
This causes a valley breeze.
At night the mountains cool off more
quickly than the valleys. This creates
downdrafts which flow down from the
mountains.
This causes a mountain breeze.
Valley and Mountain Breezes
How Wind is Measured
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Wind direction can be determined by a
wind vane. The direction the wind
travels most of the time is referred to as
the prevailing wind.
Wind speed can be determined by using
a device known as a wind
anemometer.
Anemometer
El Nino Weather Patterns
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At irregular intervals of 3-7 years, warm
countercurrents in the Pacific Ocean
become unusually strong and replace
normally cold waters with warm
equatorial waters.
This pattern known as El Nino effects
both local and global weather patterns.
During years of El Nino, fishing is poor
and farming is good near Ecuador and
Peru. Winters are wetter and warmer
than average in the U.S.
Normal Wind and Weather
Patterns
El Nino Wind and Weather
Patterns