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Liquid water consists of water molecules that are held near each other
by attractive forces but are still free to move around among one another.
Also, water molecules have thermal energy, in the form of extremely fast
vibrations. Water molecules at a water surface occasionally vibrate so
strongly that they fly out into the air to become vapor. At the same time,
water molecules in the air occasionally crash back onto the water surface
to join the liquid water. These motions are occurring all the time at
the water surface.
When the concentration of water molecules in the air is less than the
maximum the air can hold, the relative humidity is less than one hundred
percent. Then the number of molecules that go from the liquid to the air
is greater than the number that goes from the air to the liquid. That
condition is known as evaporation. Evaporation causes cooling of the remaining
liquid water. The reason is that the water molecules that fly out into
the air are, on average, the ones with unusually high thermal energy.
What is left behind has a little less thermal energy-in other words, it
has a slightly lower temperature.
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© Michael Collier
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Stratocumulus clouds over Monument Valley in Arizona indicate a stable air mass.
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Condensation is the opposite of evaporation. If the air has even slightly
more water vapor than it can hold, the number of water molecules passing
from the air into the liquid water is greater than the number passing
from the liquid water into the air. When you fill a glass with ice water
on a warm and humid day, soon the glass has drops of water on the outside
of the glass. That water has not leaked through the glass. It has condensed
from the air. Cold air can hold less water vapor than warm air. The cold
glass chills the air next to it, causing condensation. Dew on the grass
on a chilly morning forms in the same way, when the ground surface is
chilled by radiating its heat out to space on a clear night.
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