To persist in the form of a gas, water must achieve much higher energy content
than the liquid or solid states. In the dynamic setting of the free atmosphere water
readily changes state to and from its gaseous phase. When vapor changes phase to
liquid, this process is called condensation and the water molecules organize them-
selves into chains because of loss of energy from the vapor. Conversely, when
water evaporates and changes its phase from liquid to vapor, this necessitates the
addition of substantial amounts of energy to break the chains of liquid molecules.
Less well known to most of us is that water vapor can lose enough energy at once
to change directly from gas to the regular crystalline arrangement of ice mole-
cules; this process is called deposition and the result can be witnessed as the accu-
mulation of frost on an automobile on a clear winter night. In reverse, ice can
directly change to water vapor by gaining enough energy; this process is called
sublimation and can be subtly observed by noting the decrease in an ice or snow
cover after windy conditions; even whentemperaturesare below 0°C, ice can
sublimate into water vapor.
The energy inherent to water vapor that allows water vapor to remain as water
vapor is known as latent heat. This name implies that the heat is “hidden” and
not directly detectable by thermometer. At sea level, the conversion of ice to water
vapor requires about 2,850 joules. This is energy lost from the water’s surroundings
and converted into hidden, latent form. The latent heat remains with the water
vapor until condensation or evaporation takes place. These processes may take
place many thousands of kilometers from where the vapor was added to the air.
The latent heat effect can be quite dramatic. When water changes phases from
vapor to liquid or solid, the release of latent heat can warm the atmosphere. If this
phase change is ongoing while air is rising latent heat release retards the adiabatic
cooling, making the air rise to much greater heights than it would without this
phase change. Latent heat provides the huge amounts of energy needed for thun-
derstorms and hurricanes.
When the atmosphere is holding all the water vapor it can hold, the condition is
known as saturation. This does not mean that there will beprecipitation, but sat-
uration is a necessary precondition for precipitation. The saturation condition of
the atmosphere is a cubic function of air temperature. That is, several times more
water vapor is needed to saturate the air at 40°Cthenat0°C. This explains why
precipitation amounts can be much more out of cold air than out of warm air.
In that water vapor is so essential to the functioning of world weather, several
methods are used to describe atmospheric water vapor. The following are mea-
sures highlighting various facets of humidity. As one of the gases in the atmos-
pheric mix, water vapor exerts a partial pressure subsumed as part of barometric
pressure. In the polar regions, vapor pressure is frequently less than 5 mb at sea
level while it averages over 25 mb over tropical oceans. Compared to the average
Humidity 175