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“air has a lower capacity to hold water vapor” is an easy way to
say that condensation exceeds evaporation, therefore decreasing
the amount of water vapor in the atmosphere.

Humidity
The amount of water vapor is referred to generally as humidity.
Damp, moist conditions are more likely to have condensation
than evaporation, and this is said to be a high humidity. Dry
Evaporation Condensation Air saturation conditions are more likely to have evaporation than condensa-
tion, on the other hand, and this is said to be a low humidity. A
Key:
= leaving measurement of the amount of water vapor in the atmosphere
= returning at a particular time is called the absolute humidity ( Figure 22.15).
At room temperature, for example, humid air might contain
FIGURE 22.14 Evaporation and condensation are occurring all
15 grams of water vapor in each cubic meter. At the same
the time. If the number of molecules leaving the liquid state exceeds temperature, air of low humidity might have an absolute
the number returning, the water is evaporating. If the number of 3
humidity of only 2 g/cm . The absolute humidity can range
molecules returning to the liquid state exceeds the number leaving,
the water vapor is condensing. If both rates are equal, saturation has from near zero at temperatures well below freezing, up to a
occurred; that is, the relative humidity is 100 percent. maximum that is determined by the temperature at the time.
The relationship between the actual absolute humidity at
a particular temperature and the maximum absolute humidity
decreases in the temperature mean increases or decreases in that can occur at that temperature is called the relative
the kinetic energy of water vapor molecules. Water vapor mol- humidity. Relative humidity is a ratio between (1) the amount
ecules usually undergo condensation when attractive forces be- of water vapor and (2) the amount of water vapor needed to
tween the molecules can pull them together into the liquid state. reach saturation at that temperature. The relationship is
Lower temperature means lower kinetic energies, and slow- absolute humidity at present temperature
moving water vapor molecules spend more time close to one _____ ×

100%
another and close to the surface of liquid water. Spending more maximum absolute humidity at present temperature
time close together means an increased likelihood of attractive
relative
=

forces pulling the molecules together. On the other hand, higher humidity
temperature means higher kinetic energies, and molecules with
For example, suppose a measurement of the water vapor at 10°C
higher kinetic energy are less likely to be pulled together. As 3
(50°F) finds an absolute humidity of 5.0 g/m . According to
the temperature increases, there is therefore less tendency for
Figure 22.15, the maximum amount of water vapor when the tem-
water molecules to return to the liquid state. If the temperature 3
perature is 10°C is about 10 g/m . The relative humidity is then
is increased in an equilibrium condition, more water vapor
3
must be added to the air to maintain the saturated condition. 5.0 g ∕ m
_
× 100% = 50%

A warm atmosphere can therefore hold more water vapor than 3
10 g ∕ m
a cooler atmosphere. In fact, a warm atmosphere on a typical
3
summer day can hold five times as much water vapor as a cold If the absolute humidity had been 10 g/m , then the relative
atmosphere on a cold winter day. humidity would be 100 percent. A humidity of 100 percent
The rate of evaporation or condensation depends on the means that saturation has occurred at the present temperature.
temperature and some variables concerning the surface of the The important thing to understand about relative humidity
solid, liquid, or gaseous water vapor. Temperature, however, is that the maximum amount of water vapor in the atmosphere
is the important variable, with evaporation increasing with changes with the temperature. As the atmosphere becomes
warming and condensation increasing with cooling. It is the colder, the evaporation rate decreases faster than the condensa-
temperature of the water or water vapor that is important, but tion rate, with the result that condensation will occur at a cer-
weather broadcasters often refer to the air temperature when tain temperature. This temperature is called the dew point. The
discussing humidity. The air temperature is nearly the same as relative humidity increases as the atmosphere cools, reaching
the water vapor temperature, and the important thing to re- saturation and 100 percent relative humidity at the dew point.
member is that you are actually referring to the waver vapor The evaporation rate increases with temperature increases
(not air) temperature when discussing humidity. because the water vapor molecules become more energized.
Also, broadcasters sometimes refer to the air’s ability “to Warming can thus reduce the relative humidity since the
hold water vapor.” Here we understand that air has nothing to do amount of water vapor that can be present is increased. With
with how much water vapor can be present in the atmosphere. the same amount of water vapor in the room, this will decrease
Saying that “air can hold more water vapor” is an easy way to say the relative humidity. For example, warming of a room can
that evaporation exceeds condensation, therefore increasing the reduce the relative humidity from 50 percent to 3 percent. Lower
amount of water vapor in the atmosphere. Likewise, saying that relative humidity results because warming the air increases the
554 CHAPTER 22 The Atmosphere of Earth 22-14