Why Are Deserts so Dry

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Why are deserts so dry? Three rules of atmospheric aerodynamics: First, hot air rises and cool air sinks. Second, rising air expands and cools, while sinking air compresses and becomes warmer. Third, warmer air can hold more water vapour than cooler air. These three things plus the sun's heat determine where rain falls on the planet. The sun shines almost vertically on the equator year round, but it shines on the poles at a steep angle. There are two consequences. A beam of sunlight ten square feet shines on about ten square feet of Earth's surface at the equator, but it covers more than twice that area near the poles. The sun's light and heat are less concentrated at higher latitudes. In addition, at the equator the sunlight travels straight down through the atmosphere, but near the poles it travels through a thicker layer air where more of the light is reflected, absorbed, or scattered and less reaches the ground. This is why the equator is hot and the poles are cold. Because of the great quantity of heat delivered to the equator it is a zone of warm, rising air. It absorbs much water vapour from the oceans and land vegetation through evaporation. As this air rises it cools. Eventually the water vapour condenses into clouds and it then creates precipitation. So the equatorial region is both hot and wet. The air rises, then spreads horizontally to the north and south. Eventually the now cool air sinks and flows along the surface to replace the rising air at the equator, forming a circulation cell or a Hadley cell. It tends to sink at about 30° north and 30° south latitude. As the air sinks it warms and dries. Not only can sinking air not produce rain, but when it reaches the ground it absorbs water from the soil and vegetation, creating even more arid conditions. We find deserts where this air descends. In fact, at about 30° North, hold the Sahara, the
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