Geoscience Reference
In-Depth Information
cloud formation begins. Once cloud formation is underway and water vapor is
being converted to liquid water or ice, the latent heat so released further
increases  the temperature of the parcel helping it to remain buoyant and thus
aiding further ascent and cooling. The cloud created in this way is called
convective cloud .
A second important way that vertical movement and associated cooling can
give rise to condensation and cloud is as a result of large-scale atmospheric
movement (Fig. 10.1b). Moving masses of air may have different temperatures and
different buoyancy. Thus, a more buoyant warm air mass moving horizontally
may, for example, impinge on and 'float' over a cooler air mass, or a less buoyant
cold air mass moving horizontally may impinge on a warmer air mass and 'sink'
beneath it, pushing it upward. Thirdly, air moving horizontally may sometimes
come up against topography and be forced upward. In these examples it is the
large-scale horizontal movement of the atmosphere that is resulting in ascent and
cooling of the air, the net result being that unsaturated air moves upward, cools
and becomes saturated to give cloud.
Another way that wisps of cloud can be formed is very different because it
doesn't involve ascent. If two portions of air that are both close to saturation but
with different temperatures are mixed, the resulting air mixture will have the
average temperature and average water content. However, because the saturated
vapor pressure curve shown in Fig. 2.2 is not linear, the resulting water content of
the mixed air may be greater than that of saturated air at the new average
temperature. Consequently, water condenses out (Fig. 10.1c). This mechanism is
not a major source of cloud but is the reason why we see sea frets and wisps of
cloud over wet forest after rain.
These different mechanisms for atmospheric condensation result in very
different types of cloud, which are subsequently associated with different patterns
of rainfall, as follows.
Convective cloud : Because the mechanism involved in cloud formation is
associated with strong vertical ascent over fairly small horizontal areas, the
resulting cloud can be quite tall with horizontal dimensions typically on the
order of a few hundred to a few thousand meters. Moreover, being associated
with local rapid ascent, any precipitation associated with clouds of this type
tends to be heavy and local.
Frontal cloud : The nature of the processes involved in the formation of frontal
clouds means the resulting cloud cover tends to have wide spatial coverage,
typically over areas on the order of a few kilometers to several tens of
kilometers or greater. Being associated with gradual uplift, the associated
precipitation is often widespread and steady but is usually lighter than that
for convective clouds.
Surface mixing : The wisps of clouds produced by mixing saturated air with
different temperatures have very limited extent and are not usually associated
with precipitation.
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