Geoscience Reference
In-Depth Information
When geographers consider the humidity characteristics
of a particular region, they typically combine the processes
of evaporation and transpiration into the singular concept of
evapotranspiration
. Evapotranspiration rates are dependent on
many factors, including:
KEY CONCEPTS TO REMEMBER ABOUT
ATMOSPHERIC HUMIDITY
1.
Three kinds of humidity exist: maximum, specific,
and relative. Maximum humidity refers to how much
vapor a parcel of air can hold. This variable depends
on temperature, with warm air having a higher maxi-
mum humidity than colder air. Specific humidity
measures how much water vapor is in the air. Rela-
tive humidity is the ratio of specific humidity to max-
imum humidity.
•
Net Radiation:
Heating of plants or ground surface.
Evaporation rates are higher when a lot of energy
(sunlight) exists.
•
Air Temperature:
Warm air can hold more moisture than
cold air.
•
Relative Humidity:
How much moisture the air is already
holding relative to its capacity to contain water vapor.
Evaporation rates are higher when the air is dry because
plenty of room exists for more water vapor molecules in
the air.
2.
When the relative humidity is 100%, the air is saturated
and can hold no more water vapor.
3.
Although the specific humidity may not change on any
given day, the relative humidity can change dramati-
cally due to rising and falling temperature. In this case,
the warmer the temperature, the lower the relative
humidity.
You have previously seen that air flows along a pressure
gradient from high pressure to low pressure. Similarly, water
vapor flows along a vapor pressure gradient from areas of high
vapor pressure to areas of low vapor pressure (Figure 7.13).
Close to the water surface, the air contains more molecules of
water vapor. As you move higher above the water, however, the
air contains fewer water vapor molecules. Water vapor flows
upward from the areas with more water vapor to the areas with
less water vapor.
A final factor that influences evapotranspiration rates is
wind speed. Wind moves moist air away from the source of the
water vapor. Evaporation rates are higher in windy conditions
when the air containing water vapor molecules can be moved
4.
The dew-point temperature is the temperature at which
a mass of air becomes saturated. This is the temper-
ature at which condensation occurs in that air mass.
As specific humidity increases, so does dew-point
temperature.
5.
Once the dew-point temperature is reached, the
specific humidity begins to decrease as water vapor
condenses to liquid form. This decrease in specific
humidity continues as long as the process of cooling
does.
Evaporation
Lower density of
water vapor
Where does water vapor in the atmosphere come from? Liquid
water is transformed into water vapor through two processes,
evaporation and transpiration. As described earlier in the chapter,
the term
evaporation
is used when water is lost from a surface
such as soil, water, or pavement. Liquid water is also transformed
into vapor when it flows through leaf pores in plants to the at-
mosphere. This process of water loss is called
transpiration
and
can result in the transformation of tremendous amounts of wa-
ter. In tropical areas, for example, some trees can transpire over
300 liters (L) (
∙
80 gal) of water a day! You can indirectly see
the process of transpiration by noting the rigid upright posture
of plants. This posture is maintained by water flowing upward
through the plant by capillary action toward the leaf surface. A
plant wilts when the amount of water in the soil declines and
therefore less flows upward through the plant.
Higher density of
water vapor
Figure 7.13 Water vapor gradient.
Water vapor flows
from areas of high-density toward areas of low-density va-
por content. This flow creates room for additional water to be
evaporated from the source area.
The passage of water from leaf pores to the
The combined processes of evaporation
Transpiration
Evapotranspiration
atmosphere.
and transpiration.
