Wind:  the horizontal movement of air

Useful in many ways:

  • 1. moderates surface temperatures
  • 2. distributes moisture
  • 3. cleanses the atmosphere

-Wind is constantly blowing across Earth’s surface!

What sets the air into motion?

*Differences in air pressure!

-Air wants to flow from an area of high-pressure to an area of low-pressure.

*For example: When you pop your bike tire, air rushes out b/c it is under high-pressure inside the tire, and wants to move to an area of low-pressure outside the tire.

-The greater the difference in air pressure between two areas, the stronger the winds between them.

*Pressure differences and winds are ultimately caused by unequal heating of Earth’s surface!

*For example, land tends to absorb more of the sun’s radiation than water.  As the air over the land is heated, it becomes less dense than the air over the water.  As the hot, less dense air rises, a sea breeze blows in to replace it, moving from an area of higher pressure to one of lower pressure.

Winds are named for the direction from which they come or originate.  For example, a wind that blows from west to east is called a west wind, or westerly.

Factors Affecting Winds & Patterns

*Key Idea: Wind direction depends upon the Coriolis effect, the pressure gradient, and friction.

Coriolis Effect: when the Earth’s rotation causes an object moving freely over Earth’s surface to curve away from its path of travel

*Remember, the Earth is always spinning beneath us!  As objects move in the atmosphere, the Earth is turning under them!

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-In the Northern Hemisphere, objects are deflected to their right.

-In the Southern Hemisphere, objects are deflected to their left.

-The effect is only noticeable for objects traveling great distances, such as airplanes and winds.

-Because of the Coriolis Effect, winds in the N. Hemisphere blow clockwise out of areas of high pressure and counterclockwise into areas of low pressure.

Friction:  resistance to motion

-Friction between the air and the ground slows surface winds and decreases the Coriolis effect.

-More friction = less Coriolis effect = winds tend to blow perpendicular across isobars from areas of high pressure to areas of low pressure

-Less friction = more Coriolis effect = winds tend to blow parallel along isobars because deflection is greater

-Near the top of the troposphere, there is no friction with the ground, so isobars at this level appear as wavy lines that stretch across entire weather maps!

-Bands of quickly moving winds form here, creating the jet stream!

-Weather in the U.S. is affected by the polar-front jet stream, which forms when cool air from the north meets warm air from the south.

Global Wind Patterns

*Key Idea: Many factors affect winds, including the temperature difference between equatorial and polar regions, the rotation of Earth, the location of continents, the time of year, and local topography.

Three-Celled Circulation Model

*The N. and S. Hemispheres each have 3 circulation cells of wind:

  • -Between the equator and 30° latitude
  • -Between 30° and 60° latitude
  • -Between 60° latitude and the pole
Air Pressure and Wind: Why does air pressure change?

*Warm temps. near the equator result in rising air and, thus, a low pressure area.

-As this air moves away from the equator, it cools and sinks near 30° latitude, resulting in a high-pressure area.

**This is where most of the world’s deserts are located!!

-As the air flows between 30° and 60° latitude, it is warmed by the Earth’s surface and rises up near 60° latitude, thus creating another low-pressure area.

-As this warmer air flows from 60° latitude toward the poles, it is again cooled and sinks at the poles, resulting in another high-pressure area.

Intertropical Convergence Zone (ITCZ): the low-pressure area around the equator where surface winds from both hemispheres come together

-Nickname: the doldrums– due to the lack of wind in this area

-Sailors would often get stuck in the doldrums because there was little wind to push them along!

Horse Latitudes: dry, high-pressure area around 30° latitude

-Little to no wind, so sailors transporting horses would often throw the horses overboard to lighten the load!

Trade Winds:  Located between the ITCZ and the horse latitudes, used as trade routes because they are warm

and relatively steady

Prevailing Winds:  Winds that usually blow from the same direction at all times

Ex- trade winds; westerlies; polar easterlies

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