Evaporation occurs when heat energy is available at the waters surface in order to change the liquid water into a vapour. Most often this heat is provided through the radiant heat of the sun, but in some cases the heat can also be generated through the moving of water molecules. In addition to heat, the air must not already be saturated with water and able to absorb and retain the water from the body of water being heated. The air near the surface of water usually carries a larger number of vapour molecules, and thus has a higher pressure. Water that is further away from the surface of the water carries as much smaller number of vapour molecules, and thus us of a lower pressure. As we have already seen, high pressure areas move into low pressure areas, and in this case it moves upward explaining the movement of water from the larger body upward into the air. The attached video is a crude illustration of the movement of water, when heated, turning into a vapour and rising up.
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