Water vapor is the currency of earth's hydrological cycle. Water freely evaporates from earth's oceans, especially the tropical latitudes, where ample solar energy is absorbed by liquid water. The heat energy that causes water to evaporate is termed latent heat. Water molecules, when liberated as vapor, act as 'mobile solar collectors,' transporting the latent heat of evaporation wherever the winds blow. When those winds converge just above the surface (perhaps locally, perhaps thousands of miles away), water vapor-laden air is forced upward, cooling as it expands and becoming saturated. Condensation not only forms drops of rain, but releases the latent heat that was initially obtained during evaporation, warming the interior of clouds. Hence, the excessive warmth of the earth's oceans is conveyed vertically into the deeper layers of the atmosphere, through the agency of water vapor.
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