a result of condensationpresence of sufficient
moisture, hygroscopic or sublimation nuclei in the
atmosphere, and a cooling process. Moisture is
supplied to the atmosphere by evaporation and is
distributed horizontally and vertically by the winds and
vertical currents. The first task is to consider the
hygroscopic and sublimation nuclei.
Hygroscopic nuclei are particles of any nature on
which condensation of atmospheric moisture occurs. It
can be said that hygroscopic nuclei have an affinity for
water or that they readily absorb and retain water. The
most effective hygroscopic nuclei are the products of
combustion (sulfuric and nitric acids) and salt sprays.
Some dust particles are also hygroscopic, but not
effectively so. The presence of hygroscopic nuclei is a
must; water vapor does not readily condense without
their
presence.
Air
has
been
supersaturated
in
laboratories to over 400 percent before condensation
began when there were no hygroscopic nuclei present.
On the other hand, condensation has been induced with
relative humidity of only 70 percent when there was an
abundance of hygroscopic nuclei.
The condensation, which results when all three
mentioned conditions are fulfilled, is usually in the
form of mist, clouds, or fog. Fogs are merely clouds on
the surface of Earth.
In our industrial cities, where byproducts of
combustion are abundant, the distinction between
smoke, fog, and haze is not easily discernible. A
combination of smoke and fog gives rise to the
existence of the so-called smog characteristic of these
industrial areas.
Little is known about the properties of sublimation
nuclei, although it is believed they are essential for
sublimation to occur at all. It is assumed sublimation
nuclei are very small and very rare, possibly of a quartz
or meteoric dust origin. All cirriform clouds are
composed of ice crystals and are believed to be formed
as a result of direct sublimation. In the atmosphere,
water clouds, water and ice crystal clouds, and pure ice
crystal clouds may coexist at the same time.
Next under consideration is the cooling process
that may induce condensation. There are several
processes by which the air is cooled: convective cooling
by expansion, mechanical cooling by expansion, and
radiation cooling. Any of the three methods may work
in conjunction with another method, making it even
more effective. The methods are as follows:
1.
Convective cooling. The ascent of a limited
mass of air through the atmosphere because of surface
heating is called thermal convection. If a sample of air
is heated, it rises (being less dense than the surrounding
air) and decreases in temperature at the dry adiabatic
lapse rate until the temperature and dew point are the
same. This is the saturation point at which condensation
begins. As the parcel of air continues to rise, it cools at a
lesser ratecalled the moist/saturation adiabatic lapse
rate. The parcel of air continues to rise until the
surrounding air has a temperature equal to, or higher
than, the parcel of air. At this point convection ceases.
Cumuliform clouds are formed in this way. Cloud bases
are at the altitude of saturation and tops are at the point
where the temperature of the surrounding air is the
same as, or greater than, the temperature of the parcel of
air.
2.
Mechanical cooling. Orographic and frontal
processes are considered mechanical means of cooling
which result in cloud formation.
a.
Orographic
processes.
If
air
is
comparatively moist and is lifted over mountains or
hills, clouds may be formed. The type of cloud depends
upon the lapse rate (the rate of decrease in temperature
with increase in height, unless otherwise specified) of
the air. If the lapse rate is weak (that is, a low rate of
cooling with an increase in altitude), the clouds formed
are of the stratiform type. If the lapse rate of the air is
steep (that is, a high rate of cooling with increasing
altitude), the clouds formed are of the cumuliform type.
b.
Frontal processes. In the previous unit, you
learned that, at frontal surfaces, the warmer, less dense
air is forced to rise along the surfaces of the colder air
masses. The lifted air undergoes the same type of
adiabatic cooling as air lifted orographically. The type
of cloud formed depends on the lapse rate and moisture
of the warm air and the amount of lifting. The slope of
the front determines lifting; when the slope is shallow,
the air may not be lifted to its saturation point and
clouds do not form. When the slope steep, as with a
fast-moving cold front, and the warm air is unstable,
towering cumuliform cloud form.
3.
Radiation cooling. At night Earth releases
long-wave radiation, thereby cooling rapidly. The air in
contact with the surface is not heated by the outgoing
radiation, but rather is cooled by contact with the cold
surface. This contact cooling lowers the temperature of
the air near the surface, causing a surface inversion. If
the temperature of the air is cooled to its dew point, fog
and/or low clouds form. Clouds formed in this manner
dissipate during the day because of surface heating.
5-3