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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.

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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.

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

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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.

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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.

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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.

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.

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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