Air Pressure & Wind
Air Pressure
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Pressure exerted by weight of air above
At sea level it as on average 1kg of air per square centimeter
Air pressure is exerted in all directions
Use barometer to measure air pressure
Invented by Torricelli in 1643
Air pressure increases Hg rises
Air pressure decreases Hg lowers
What causes wind?
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Horizontal differences in air pressure
Flows from high pressure to low pressure
Think about opening a container of tennis balls or coffee
The noise you hear is caused by air rushing from high pressure of container
to lower outside pressure
• Unequal heating of Earth’s surface generates pressure differences
3 Factors affect wind Pressure Difference
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Variations in air pressure are determined by barometric readings
Isobars lines on a map that connect places of equal air pressure
Closely spaced isobars indicate steep pressure gradient & high winds
Widely spaced isobars indicate weak pressure gradient & low winds
Low pressure pressure lower than area surrounding it
High pressure pressure is higher than area surrounding it
Pressure Difference
Isobar Activity
Now it is your turn to draw isobars & analyze the wind patterns.
3 factors affecting wind Coriolis Effect
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Describes how Earth’s rotation affects moving objects
Deflected to the right in the North
Deflected to the left in the South
Affects only direction, not speed
Strong the wind, strong the deflection
Strongest at poles, weakest at equator
3 factors affecting wind Friction
• Only important within a few km of Earth’s surface
• Slow air movement, changes wind direction
Highs & Lows
Low system air rises and cools, forming
clouds, results in precipitation
High systems air sinks and warms, results
in warm dry weather
In Northern Hemisphere low system is
counterclockwise & high system is clockwise
Wind is deflected to the right because of
Coriolis Effect
Global Winds
Warm air rises
Cool air falls
Creates cells of
wind
Westerlies blow
from west to east
Polar Easterlies
polar high towards
polar low
Trade Winds
winds from easterly
direction between 030°
https://www.youtube.com/watch?v=uBqohRu2RRk
Influence of Continents
• Land becomes cold in winter & hot in summer
• Monsoon seasonal changes in wind direction
• India has large monsoons that result in rainy
summers
Local Winds
• Land & Sea Breezes
Local Winds
• Valley & Mountain Breezes
El Niño
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Normal Conditions
Peruvian current flows toward equator
Upwelling off the coast of Peru
At the end of the year warmer currents
El Niño
https://www.youtube.com/watch?v=7FVZrw7bk1w
• http://esminfo.prenhall.com/science/geoanimations/animations/26_Nino
Nina.html
Assessment Questions
• In the winter, large land masses often develop a seasonal
• High-pressure system
• Low-pressure system
• Typhoon
• Trade winds
• Variations in air pressure from place to place are the principal cause of
• Clouds
• Hail
• Lows
• Wind
• High pressure systems are characterized by
• Air sinking
• Air rising
• Air sinking and rising at the same time
• None of the above
• Where is deflection of wind due to the Coriolis Effect the strongest?
• Near the equator
• In the mid-latitudes
• Near the poles
• Near the westerlies
• Write a general statement relating the spacing of isobars to wind speed
• How does the Coriolis Effect modify wind movement in the Northern
Hemisphere?
• The force exerted by the air above is called
• Air pressure
• Convergence
• Divergence
• The Coriolis effect
• Explain how you would determine if an area is high or low pressure based
on the isobars
• Explain how a mercury barometer works.
• Describe the difference between valley and mountain breezes.
• Valley breezes result from the land warming during the day. This warms the
air and it rises. Mountain breezes result from land cooling at night and the
surrounding air cools. It sinks from the mountains down into the valley.
• Explain the difference between normal and El Niño conditions.
• Describe why low pressure systems typically result in clouds and
precipitation.
• Tip high pressure systems are high pressure because air is sinking, creating
more pressure on the surface of earth. Low pressure systems are lower
pressure because air is rising and relieving pressure from the surface of earth.
• What are the three factors that affect wind?
• Does Coriolis effect change direction, speed, or both? What about friction?
• Describe which direction the following winds are moving from and where
they are moving to:
• Polar Easterlies
• Westerlies
• NE Trade Winds
• SE Trade Winds