The figure above represents a "parcel" or glob of atmospheric air containing
the usual mixture of non-condensing gases plus a certain amount of water
in the gaseous form, called water vapor. Water in the gaseous form
exists in any parcel of air with relative humidity greater than 0%. If the amount
of water vapor (and other environmental conditions) is such that the relative
humidity of the air parcel reaches 100%, meteorologists say that the parcel is
"saturated". Under conditions of saturation, water in the vapor form can change
its phase from vapor to liquid. In plain English, we say that water
vapor condenses into liquid water droplets. When phase changes occur, there
is a readjustment of the forms of energy associated with the water molecules.
In the vapor phase, water has a relatively large amount of "internal" energy
represented by its looser internal molecular structure. When vapor condenses to liquid,
the internal energy of the water molecule in the liquid phase is smaller, owing to its tighter
molecular structure. If liquid freezes into solid water (ice), the internal
energy is reduced further owing to the much tighter packing of water molecules in the solid phase.
Now, when water reduces or increases its molecular internal energy due to phase changes,
that energy must be accounted for somewhere owing to the principle of conservation of energy.
For example, when water vapor condenses to liquid, the reduction of internal energy
of the water molecules is manifested as a release of that energy to the surrounding air
molecules. That energy is absorbed into the molecular internal energy of the air
molecules surrounding the newly formed water droplet. The net effect is that the
condensation of a molecule of water vapor into the liquid phase releases some of
its internal energy to the surrounding air molecules, and these air molecules
experience an increase in their molecular internal energy. The consequent increase
of the molecular internal energy of the surrounding air molecules is manifested
as an increase in the Temperature of the air around the newly formed water droplet.
When many many molecules of water vapor are condensing within a large parcel of air,
the net effect is that a good deal of internal energy, which was latent in the
high internal energy state of vapor molecules, is released to the surrounding
air increasing the internal energy and temperature of all the air molecules in the air parcel.
The transformation of molecular internal energy of vapor molecules to molecular
internal energy of the surrounding air is given the phrase-name "release of
latent heat".
In summary, when water vapor condenses in the atmosphere, it warms the surrounding air
leading to an increase in air temperature where the condensation is occuring. Similarly,
if water droplets freeze into ice crystals or snow flakes, the freezing process releases
latent heat to the surrounding air, once again causing an increae in air temperature.
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