Earth & Environment
 

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.

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