Geoscience Reference
In-Depth Information
Table 5.1
Cloud base height (in 000s m).
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 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 (Plate 7), which develops when air flows over
hills, setting up a wave motion in the air current down-
wind of the ridge (see Chapter 6C.3). Clouds form in
the crest of these waves if the air reaches its conden-
sation level.
Operational weather satellites provide information
on cloudiness over the oceans, 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 analyse
satellite imagery. A common pattern seen on satellite
photographs is cellular, or honeycomb-like, with a
typical diameter of 30 km. This develops from the
movement of cold air over a warmer sea surface. An
open cellular pattern, where cumulus clouds are along
the cell sides, forms where there is a large air-sea
temperature difference, whereas closed polygonal cells
occur if this difference is small. In both cases there is
subsidence above the cloud layer. Open (closed) cells
are more common over warm (cool) ocean currents to
the east (west) of the continents. The honeycomb pattern
has been attributed to mesoscale convective mixing,
but the cells have a width-depth ratio of about 30:1,
whereas laboratory thermal convection cells have
a corresponding ratio of only 3:1. Thus the true
explanation may be more complicated. Less common
is a radiating cellular pattern (Plate 8). Another common
pattern over oceans and uniform terrain is provided by
linear cumulus cloud 'streets'. Helical motion in these
two-dimensional cloud cells develops with surface
heating, particularly when outbreaks of polar air move
over warm seas (see Plate 9) and there is a capping
inversion.
Tropics
Middle
High
latitudes
latitudes
High cloud
Above 6
Above 5
Above 3
Medium cloud
2-7.5
2.7
2-4
Low cloud
Below 2
Below 2
Below 2
These primary characteristics are used to define
the ten basic groups (or genera) as shown in Figure
5.10. High cirriform cloud is composed of ice crystals,
giving a fibrous appearance (Plate 17). Stratiform clouds
are in layers, while cumuliform clouds have a heaped
appearance and usually show progressive vertical
development. Other prefixes are
alto
- for middle-level
(medium) clouds and
nimbo
- for thick, low clouds
which appear dark grey and from which continuous rain
is falling.
The height of the cloud base may show a consid-
erable range for any of these types and varies with
latitude. The approximate limits in thousands of metres
for different latitudes are shown in Table 5.1.
Following taxonomic practice, the classification
subdivides the major groups into species and varieties
with Latin names according to their appearance.
The
International Cloud Atlas
(WMO 1956) provides
illustrations.
Clouds can also be grouped in their mode of origin.
A genetic grouping can be made based on the mecha-
nism of vertical motion that produces condensation.
Four categories are:
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 (Plates 4 and 6) are caused not by thermals of
surface origin, but by ones set up
within
the cloud as
a result of the release of latent heat by condensation.
