Geoscience Reference
In-Depth Information
Cloud Formation and
Classification
As noted in the preceding discussion on adiabatic processes,
once air reaches the dew-point temperature, through cooling at
the DAR, water vapor begins to condense into liquid droplets.
We see the results of this process through the development of
clouds, with the base of any particular cloud representing the
level of condensation in the atmosphere.
Clouds are visible masses of suspended, minute water drop-
lets or ice crystals. Recall from Chapter 4 that clouds have a very
important role in regulating the way solar radiation interacts with
Earth. Some insolation is reflected immediately back into space
from bright cloud tops. At the same time, clouds cause insolation
to be scattered and absorbed. They also absorb longwave radia-
tion from the Earth's surface. In addition, the character of clouds
is a clue to atmospheric dynamics at any given point in time.
Figure 7.17 shows the close relationship between atmo-
spheric water vapor and the geographic distribution of clouds in
the United States on April 24, 2005. Notice in part (a) that high
concentrations of vapor (shown in white) are present in the west-
ern part of the country, as well as over the Great Lakes region.
These concentrations of vapor are closely associated with the
band of clouds in part (b) that cover the same geographic areas.
Two conditions are necessary in order for clouds to form:
(a)
1. The air must be saturated. Air can become saturated two
different ways: when air cools below the dew-point tem-
perature or when more water vapor is added to the air.
2. There must be a substantial quantity of small particles, such
as dust or pollutants, about which water vapor can collect or
condense. These particles are called
condensation nuclei
.
(b)
Figure 7.17 Satellite images showing atmospheric moisture
on April 24, 2005.
(a) GOES water vapor image. Notice the par-
ticularly high concentrations of water vapor (in white) in the west
and northeast. (b) GOES visible image. Note how the concentra-
tions of clouds are closely associated with the vapor patterns in (a).
Condensation nuclei
Microscopic dust particles around
which atmospheric water coalesces to form raindrops.
VISUAL CONCEPT CHECK
7.2
Water vapor is produced in the exhaust of jet engines,
such as those on large airliners. As the vapor cools in
the air behind the plane, it often forms condensation
trails, or contrails. What conditions must be present for
the contrail to form?
a)
Air must be close to saturation.
b)
Condensation nuclei (dust) must be present.
c)
The plane must be above the condensation
level of the air.
d)
All of the above.
