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Cold air sinks because it is denser than warm air. As cold air sinks, it compresses and warms. Warm air rises. It expands and cools as it rises. Warm air can hold more water vapor than cold air can. Therefore, when warm air cools, the water vapor it contains may condense into liquid water to form rain, snow, and fog. Solar energy heats the ground, which warms the air above it. Because the earth rotates, and because different latitudes receive different amount of solar energy. This determines earth's precipitation pattern. The intense solar energy striking earth's surface at the equator causes the surface as well as the air above the equator to become very warm. The warm air can hold large amounts of water that evaporate from the equatorial oceans and land. As the warm air rises it cools, which reduces some of its ability to hold water. Areas near the equator receive large amounts of rain. Cool water over the equator cannot sink because hot air is rising below the cool air. The cool air rises and is forces away from the equator toward the north and south poles. Some of this cool air sinks back down to the earth's surface. The air becomes warmer as it descends. The warm dry air moves across the surface of the earth and causes water to evaporate from the land below, which creates dry conditions. Air moving toward the poles warms while it is near the earth's surface. When this rising air reaching the top of the troposphere, a small amount of the air returns back to the circulation patters. Most of this uplifted air is forces toward the poles. Cold, dry air descends at the poles, which are essentially very cold deserts.