Geoscience Reference
In-Depth Information
Adiabatic Processes
In an effort to integrate these concepts into an animated format,
go to the
Geo Media Library
and select
Adiabatic Processes.
This animation reviews all the concepts that have been dis-
cussed in this section of the text. As you watch the animation,
pay particularly close attention to when the DAR and WAR are
used. Also, watch what happens to the air parcel as it descends
and warms at the DAR. When this process occurs, the effect is to
produce a warmer air mass at the surface than the original parcel
of air. After you complete the animation, be sure to answer the
questions at the end to test your understanding of this concept.
The Wet Adiabatic Lapse Rate
The DAR applies when air is not saturated and the relative
humidity is less than 100%. How does temperature change
once the dew-point temperature is reached, saturation occurs,
and the relative humidity is 100%? The altitude at which the
saturation point is reached is known as the
level of condensa-
tion
, which varies depending on the particular temperature and
humidity characteristics in any given air parcel. Air parcels that
have relatively high humidity when they begin to lift reach the
level of condensation at a lower elevation than air parcels that
are comparatively dry.
Once the level of condensation is reached, water droplets
begin to form if the air continues to cool. In this fashion, the
air is analogous to a saturated sponge that releases water as
you squeeze it at the same time that you lift it. Give it a try.
All you have to do is take a sponge and let it absorb a small
amount of water in the sink. Starting at the surface of the sink,
slowly raise the sponge and squeeze it slowly as you lift it. At
some point, the sponge will begin to release water. After you
reach this point, start over but let the sponge absorb more water
before you begin. Raise the sponge as before and try to exert
the same amount of force as you did previously. You should
notice that the sponge begins to release water at a lower eleva-
tion than before. This occurs because the sponge was closer to
being saturated the second time you ran this simple experiment.
The atmosphere operates on much the same principle as this
sponge test.
If the air parcel continues to rise and cool, increasing
amounts of water vapor condense into liquid water. During
this process, another principle comes into effect—the release
of latent heat of condensation that was described earlier in this
chapter. This heat energy has the effect of warming the air at the
same time that it is cooling due to the reduction in atmospheric
pressure and expansion. The cooling effect is stronger overall,
however, so the overall temperature continues to decrease if the
air continues to rise. Nevertheless, the rate of cooling in this
saturated state is less than in the unsaturated state because of
the release of latent heat.
KEY CONCEPTS TO REMEMBER ABOUT
ADIABATIC PROCESSES
1.
Adiabatic processes refer to the temperature changes
that occur in an air parcel due solely to changes in air
pressure. When air pressure increases, a parcel of air is
compressed and warms adiabatically. In contrast, when
air pressure decreases, a parcel of air expands and
thus cools internally because air molecules are spaced
farther apart.
2.
The level of condensation is the elevation at which conden-
sation occurs, that is, when the relative humidity is 100%.
3.
Air cools at the dry adiabatic lapse rate (DAR) until con-
densation occurs. This rate is 10°C/1000 m (5.5°F/1000 ft).
When air warms adiabatically, it always does so at the DAR
because condensation is not occurring.
4.
Once the level of condensation is reached, the air temper-
ature begins to decrease at the wet adiabatic lapse rate
(WAR), which is 5°C/1000 m (2.7°F/1000 ft). This lesser
rate of cooling occurs because latent heat of condensa-
tion is released when water changes from vapor to liquid.
Look at Figure 7.16 again and this time focus on the area
above the condensation level to see how this change appears
graphically. The cooling rate of saturated air is called the
wet
adiabatic lapse rate (WAR)
. It is also known as the moist or
saturated lapse rate. Although the WAR varies with moisture
content and temperature, the average rate is about 5°C/1000 m
(2.7°F/1000 ft); this value will be used hereafter in this text.
Level of condensation
The altitude at which water changes
from the vapor to liquid phase.
Wet adiabatic lapse rate (WAR)
The rate at which a satu-
rated body of air cools as it lifts. The average rate is about
5°C/1000 m (2.7°F/1000 ft).
