Wind Zones – Notes

Force & Motion
Earth & Environment


Earth & Environment
Notes & Review Ch. 26 – 28
Our Changing Atmosphere
Wind Zones – Notes
Notes & Review Ch. 29 – 30
Storms – Natural Disasters
Notes & Review Ch. 31
Weather & Climate Review

Wind Zones – Notes & Quiz

  The wind zones are created by the heating (and rising) and cooling (and sinking) of air masses at the equator and the poles. Pressure systems produced by the rising and sinking of these air masses lead to our wind zones. The circulation of our atmosphere is complicated by the Earth's rotation and tilt.

  The Earth's axis is tilted 23.5° from being perpendicular to the plane of the ecliptic, the plane of its orbit around the sun. Due to this tilt, rays of the Sun strike vertically at 23.5° N. latitude, called the Tropic of Cancer, at summer solstice in late June. At winter solstice, in late December, the rays strike vertically at 23.5° S. latitude, the Tropic of Capricorn. In the Northern Hemisphere, the summer solstice day has the most hours of daylight, and the winter solstice has the fewest hours of daylight each year. In the Southern Hemisphere, this is reversed. The tilt of the axis allows unequal heating of the Earth's surface, causing wind.

  On a planetary scale, air warmed by the Sun rises at the Equator, cools as it moves toward the poles, descends as cold air over the poles, and warms again as it moves over the surface of the Earth toward the Equator. This simple pattern of atmospheric convection is complicated by the rotation of the Earth, which introduces the Coriolis Effect. Thus, a wind traveling north from the equator will maintain the sideways velocity acquired at the equator while the Earth under it is moving sideways slower. This effect accounts for the winds turning toward the right as it moves northward across the Earth's surface.

  Winds blow between areas of different atmospheric pressures, from high pressure areas to low pressure areas. The Coriolis Effect also causes these winds to turn right in the Northern Hemisphere, and left in the Southern hemisphere. In the zone between about 30° N. and 30° S., the surface air flows toward the Equator, called the trade winds. A low-pressure nearly windless zone at the Equator is known as the doldrums. Most of the world’s deserts lay outside of the two trade wind belts. Around 30° N. and S., the air flowing towards the poles in the upper atmosphere begins to descend toward the surface in high-pressure nearly windless belts. The sinking air is relatively dry because its moisture has already been released as it rose, cooled, and condensed above the Equator, creating the tropical rain forests. This high-pressure nearly windless zone of descending air is called the Horse Latitudes. The name for this area is believed to have been given by sailors, whose ships stalled at these latitudes while crossing the oceans (with horses as cargo), and were forced to throw a few horses overboard to conserve water.

  The surface air that flows from these subtropical high–pressure belts toward the Equator is deflected toward the west by the Coriolis Effect. Because winds are named for the direction from which the wind is blowing, the trade winds are called easterlies. The trade winds meet at the doldrums. Surface winds known as prevailing westerlies flow from the Horse Latitudes toward the poles. This is our wind zone, so most of our weather comes from the west, specifically from the southwest to the northeast. The "westerlies" meet "easterlies" from the polar regions at about 50–60° N. and S, causing a windless zone called the subpolar lows. Since surface winds “drive” the ocean currents, both air and water circulation are controlled by all these factors.


Windy Recap

bullet Doldrums: windless zone at the equator caused by warm rising air.
bullet Horse Latitudes: a.k.a. subtropical high, windless zones caused by sinking air at 30 degrees north and south of the equator.
bullet Subpolar Low: windless zones produced where the winds of the polar easterlies and prevailing westerlies meet, leading to the upward motion of the air at approximately 60 degrees north and south of the equator.
bullet Polar High: windless zone produced by sinking air above both poles.
bullet Trade Winds: easterly horizontal surface winds that move between the windless horse latitudes towards the windless doldrums.
bullet Prevailing Westerlies: westerly horizontal surface winds that move between the horse latitudes and the subpolar lows.
bullet Polar Easterlies: easterly horizontal surface winds that move between the subpolar lows and the poles.
bullet Jet Stream: area of swiftly flowing upper level winds located at the convergence of the main wind belts. Flow in a west to east direction at speeds exceeding 200 mph and heights of 5–7 miles.
bullet Coriolis Effect: turning of moving bodies due to the rotation of the earth on its axis. Turns to the right in the northern hemisphere and the left in the southern.
When you are done studying the wind zones, ask Mrs. Downs for the quiz over them.

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