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What causes evaporation?
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| It would be difficult to find a more important process in our weather and climate
system than evaporation. Evaporation is the change of water from a liquid to a gas. Water is
continuously evaporating from the surface of the Earth, literally pumping more and more
water
vapor into the atmosphere. Averaged across the entire Earth, water evaporates from the surface
at a rate of about 3 millimeters per day (about 1/8 of an inch). Over the tropical oceans, the
value is much larger; over cold surfaces, it is much lower; and over deserts it is almost zero,
since there is little or no water to evaporate. This change of phase (from a liquid to a gas)
requires heat, called the "latent heat of condensation". When water evaporates, it removes heat,
lowering the temperature of the surface. For both water and land surface, most of this
heat energy comes from the surface, not from the air. Evaporative cooling, along with
convection, helps keeps the surface
of the Earth from getting too hot.
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| Interesting facts:
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AVERAGED OVER THE WHOLE EARTH, EVAPORATION EQUALS PRECIPITATION: The only thing that keeps all of this evaporation
from filling up the atmosphere with water vapor is precipitation. Rain and snow are continually
forming within clouds and falling back to the surface, completing one element of the Earth's
hydrologic cycle. The latent heat of condensation,
absorbed when the water evaporated, is released to the atmosphere when the water vapor condenses
back into cloud water (or cloud ice). This source of heat helps drive clouds and most other
weather systems. For instance, evaporation from the ocean surface is the primary source of
energy for hurricanes.
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