Geoscience Reference
In-Depth Information
water to produce clouds occurs over warm-water surfaces
such as the Great Lakes in the autumn, or over the Arctic
Ocean, where water will evaporate from the relatively
warm sea surface and rapidly condense into the cold air.
Almost calm conditions are needed to avoid the saturated
air being mixed with drier air above, so it is usually mist
or fog that is formed.
Radiational or contact cooling at a cold ground surface
may also be sufficient to produce saturation, but as these
are ground-based processes the resulting condensation is
Cloud types
SYSTEMS
Clouds are a vital element of Earth's energy budget. They reflect and absorb some of the incoming solar radiation
and trap much of the outgoing long-wave radiation. Such is their importance in climate control that when models of
the atmosphere are used to predict future climate change the results can vary widely according to the assumptions
about the nature of the cloud systems. Cloud top height, thickness, density and spatial distribution are of vital
significance in affecting where energy can be absorbed and where it is lost.
Despite this importance, cloud features are one of the least well observed climate variables. Most climate stations
will observe the amount of cloud only once a day; it is noted as the proportion of the sky covered by cloud expressed
in oktas (eighths) in Europe or in tenths in the United States. Few stations record the type of cloud or its height;
observations at night are not easy.
Because clouds develop in an infinite variety of forms and shapes attempts have been made to classify them. The
easiest and most widely used way is on the basis of their appearance, a system largely devised by Luke Howard in
1803. Genetic systems based upon the origin of the cloud have been suggested. As it is not always clear exactly
how a cloud formed they have been less successful.
The basic division is between clouds which are predominantly layered, known as stratiform, and those where the
vertical extent of the cloud is important. These are known as cumuliform (
Plate 4.1
).
The groups are split up into
genera, as shown in
Table 4.1
,
and then into species, using Latin names in a similar manner to plant classification.
Thus we can have altocumulus lenticularis, which means a mid-level cloud showing some signs of vertical
development (altocumulus) which in detail is in the shape of a lens or almond, often elongated and usually with well
defined outlines (lenticularis). These clouds are usually associated with flow over hills; within them are some moister
layers which reach condensation when forced to rise over the hill. There are a large number of these species
descriptions because of the variety of cloud forms.
The stratiform types are shown in
Figure 4.6
and subdivided according to the height of their formation. In stratiform
types of cloud the rate of upward motion is slow, but it may take place over hundreds or even thousands of square
kilometres. At low levels these clouds are composed of water droplets, but at higher levels (2,000-6,000 m) we get
a mixture of water droplets and ice crystals. Above about 6,000 m stratiform clouds are composed mainly of ice
The other main group of cloud, cumuliform, is the result of local convection or instability. Bubbles of warm air, rising
above the condensation level (if the air is unstable), are seen as cumulus clouds. The precise shape and form of the
cloud will depend upon the degree of instability, the water vapour content of the air and the strength of the horizontal
wind (
Figure 4.7
).
There are many different types of cumulus cloud, subdivided on the basis of their appearance.
Some cumuliform cloud may grow larger and taller. The sharp outlines of the cauliflower-like cumulus become more
diffuse and ragged as the upper part of the cloud becomes a fibrous mass of ice crystals. The cumulonimbus stage
has then been reached. At this stage of development precipitation is usually occurring, sometimes accompanied by
lightning and thunder. As the mass of ice crystals develops, it is often blown downwind by the strong winds of the
high troposphere to form an anvil shape, characteristic of cumulonimbus clouds. Convection may initiate other clouds
near by or on the flanks of the parent cloud as it gradually decays and evaporates.
