Environmental Engineering Reference
In-Depth Information
Plate 1
A cumulonimbus cloud where the convection has reached
the tropopause and spread out into an anvil shape. Convection of
this intensity is associated with unstable air. It is more common in
summer, but the snow on the photograph indicates a winter scene.
Photo: Peter Smithson.
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 1). The groups are split up into
genera
, as shown in the table, 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 1 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 (2000-6000 m) we get a
mixture of water droplets and ice crystals. Above about 6000 m stratiform clouds are
composed mainly of ice crystals and take the name
cirrus
. Some of the clouds may show
signs of convection, even if it is weak. These types have
cumulus
incorporated into their
names, such as cirrocumulus or altocumulus.
The other main group of cloud, cumuliform, is the result of local convection or
instability. Bubbles of warm air, rising beyond 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
