Geoscience Reference
In-Depth Information
10
Formation
of Clouds
Introduction
On average, about 60% of the Earth is cloud-covered. Clouds are extremely
important in hydrometeorology because when they move in the atmosphere they
transport substantial amounts of water from one location to another. If present,
they also have a major impact on the absorption of solar radiation and modify the
surface energy balance and thereby the input of water and energy into the
atmosphere from below. Tall cumulus and stratocumulus clouds tend to shade and
inhibit solar radiation reaching the ground, but high altitude cirrus cloud tends to
blanket and inhibit the loss of longwave radiation. In seeking to predict climate
change, one of the biggest challenges is to predict how any additional water
evaporated into the atmosphere will be expressed in terms of modified cloud
cover - will that cloud reduce solar radiation input or increase the retention of
outgoing longwave radiation?
Three things are required for clouds to form. One is the presence of moisture
in the air in sufficient quantity to result in cloud if the air is cooled. This is often
the limiting constraint on cloud formation in arid and semi-arid regions such as
the southwestern US and northern Mexico. It is difficult to condense water from
air unless there is something for the water to condense on to, so a second
possible constraint on cloud formation is a lack of
cloud condensation nuclei
(CCN), i.e., entities in the atmosphere around which condensation can begin.
The third important need for cloud development is a mechanism by means of
which air can be cooled sufficiently to allow condensation of water vapor. In
practice, the cooling required is usually associated with atmospheric movement,
as discussed next.
