Wind is the result of horizontal differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure. The unequal heating of Earth’s surface generates pressure differences. Solar radiation is the ultimate energy source for most wind.

Three factors combine to control wind: pressure differences, the Coriolis effect, and friction.

Factors Affecting Wind: Pressure Differences

Isobars: lines on a map that connect places of equal air pressure.

Closely spaced isobars mean a steep pressure gradient and high winds.

Widely spaced isobars

mean a weak pressure gradient and light winds.

Wind:

Wind happens when air flows from areas of high pressure to areas of low pressure.

Pressure Gradient Force (PGF):

The pressure gradient points toward low pressure and away from high pressure.

Pressure balances with Coriolis and wind travels parallel to isobars

Friction causes surface winds move across isobars

Jet streams are fast-moving rivers of air that travel between 120 and 240 kilometers per hour in a west-to-east direction.

Friction slows air movement and can even change wind direction.

Winds Aloft Winds aloft blow parallel to the pressure isobars.

Winds on the Ground Winds on the ground blow at about 45º to the winds aloft. This is due to friction with the ground.

Factors Affecting Wind: FRICTION

Coriolis Effect: If the Earth did not turn, a wind from the north would move along a straight path to the south.

Because the Earth turns, a wind from the north appears to veer to the right as it moves south.

In the Northern Hemisphere winds veer to the right, in the Southern Hemisphere to the left. This is called the Coriolis Effect. It is caused by the Earth's rotation and curvature.

Air Circulation About Highs and LowsThe coriolis effect causes winds to blow counterclockwise around a center of low pressure and clockwise around a center of high pressure. In the Southern Hemisphere winds blow in opposite directions.

Air Circulation About Highs and Lows

Airflow Associated with Highs and Lows

Lows: (also called cyclones) have winds that blow inward and counterclockwise. Air rises upward causing clouds.

Highs: (also called anticyclones) have winds that blow outward and clockwise. Air descends in a high leading to clear skies.

Local Winds: All winds are produced by unequal heating of the Earth's surface which generates pressure differences. Local winds are produced by a local pressure gradient.

Valley Breezes: blows toward the mountains during the day. In the day, air in the mountains is heated more intensely. It rises up and draws more air up the valley, this generates a valley breeze.

LOCAL WINDS

Mountain Breeze: blows from the mountains after sunset. After dark, cooling of the air in the mountains can result in a cool air drainage into the valley. This generates a mountain breeze.

LOCAL WINDS

Chinook winds: warm, dry, gusty winds that descend down the leeward side of mountain slopes.

Sea and Land Breezes: The land heats up quickly in the day and cools fast at night. This generates sea breezes in the day and land breezes at night.

LOCAL WINDS

1. Why doesn’t your table collapse under the great amount of air pressure above it?

2. If the height of mercury in a mercury barometer is decreasing, what is happening?

3. What three things affect wind?

4. What is the energy source for most winds?

5. When isobars are close together, how is wind affected?

6. Why do jet streams move parallel to isobars?