Geoscience Reference
In-Depth Information
12.0
40.0
Cirrus
Family A -
High clouds
Cirrocumulus
(Veil)
(Anvil head)
(Halo)
Cirrostratus
6.0
20.0
Family B - Middle cl
ouds
Altocumulus
Altost
ra
t
us
3.0
10.0
Family D - clouds with
vertical development
Family C - Low clouds
Cumulonimbus
Cumulus
N
imb
ostra
tu
s
S
tratus
Cumulus
of
fair weather
Stratocumulus
1.5
5.0
(Ground)
0.0
0.0
Figure 5.10
The ten basic cloud groups classified according to height and form.
Source: Modified after Strahler (1965).
based on the mechanism of vertical motion that
produces condensation. Four categories are:
others to produce a high overcast. Group 3
includes fog, stratus or stratocumulus and is
important whenever air near the surface is cooled
to dew-point by conduction or night-time
radiation and the air is stirred by irregularities
of the ground. The final group (4) includes
stratiform or cumulus clouds produced by forced
uplift of air over mountains. Hill fog is simply
stratiform cloud enveloping high ground. A
special and important category is the wave
(lenticular) cloud, which develops when air flows
over hills, setting up a wave motion in the air
current downwind of the ridge (see Chapter 6C.2).
Clouds form in the crest of these waves if the air
reaches its condensation level.
Operational weather satellites provide infor-
mation on global cloudiness, and on cloud
patterns in relation to weather systems. They
supply direct-readout imagery and information
not obtainable by ground observations. Special
classifications of cloud elements and patterns have
been devised in order to analyze satellite imagery.
A common pattern seen on satellite photographs
is cellular, or honeycomb-like, with a typical
diameter of 30km. This develops from the
movement of cold air over a warmer sea surface.
1
gradual uplift of air over a wide area in
association with a low pressure system;
2
thermal convection (on the local cumulus
scale);
3
uplift by mechanical turbulence (
forced
convection
);
4
ascent over an orographic barrier.
Group 1 includes a wide range of cloud types and
is discussed more fully in Chapter 9D.2. With
cumuliform clouds (group 2), upward convection
currents (thermals) form plumes of warm air that,
as they rise, expand and are carried downwind.
Towers in cumulus and other clouds are caused
not by thermals of surface origin, but by those
set up
within
the cloud as a result of the release
of latent heat through condensation. Thermals
gradually lose their impetus as mixing of cooler,
drier air from the surroundings dilutes the more
buoyant warm air. Cumulus towers also tend
to evaporate as updrafts diminish, leaving a
shallow, oval-shaped 'shelf' cloud (
stratocumulus
cumulogenitus
), which may amalgamate with
