Water on the ground is exposed to dry air, so it evaporates. Water vapor is less dense than the surrounding air, so it collects in the upper parts of the sky, forming clouds. When the clouds reach a saturation point (too much water vapor) or when they are blown by the wind into a cooler body of air (cold air can hold less water vapor than warm air), the water vapor condenses back into liquid water and falls out of the sky, landing on the ground and starting the cycle over again.
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