Geoscience Reference
In-Depth Information
radius would fall 42km before evaporating. On
average, clouds contain only 4 percent of the total
water in the atmosphere at any one time but they
are a crucial element in the hydrological cycle.
300
200
Coalescence
2 Cloud types
The great variety of cloud forms necessitates a
classification for purposes of weather reporting.
The internationally adopted system is based upon
(1) the general shape, structure and vertical extent
of the clouds, and (2) their altitude. This approach
was originally developed by Luke Howard in 1803.
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 generally fibrous appearance.
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
considerable range for of any of these types and
varies with latitude. The approximate limits in
thousands of meters for different latitudes are
shown in
Table 5.1
.
Following taxonomic practice, the classifica-
tion subdivides the major groups into species
and varieties with Latin names according to
their appearance. The
International Cloud Atlas
and
Plates 5.1-5.15
provide illustrations.
Clouds may also be grouped according to their
mode of origin. A genetic grouping can be made
100
Condensation
20
0
0
20
40
60
Time (min)
Figure 5.9
Droplet growth by condensation and
coalescence.
Source: Jonas (1994a). Reprinted from Weather by permis-
sion of the Royal Meteorological Society. Crown copyright ©.
Supersaturation in clouds rarely exceeds 1
percent and, because the saturation vapor pressure
is greater over a curved droplet surface than over
a plane water surface, minute droplets (<0.1
m
radius) are readily evaporated (see
Figure 5.8
).
Initially, the nucleus size is important; for
supersaturation of 0.05 percent, a droplet of 1
μ
m
radius with a salt nucleus of mass 10
-13
g reaches
10
μ
m in 30 minutes, whereas one with a salt
nucleus of 10
-14
g would take 45 minutes. Later,
when the dissolved salt has ceased to have
significant effect, the radial growth rate slows due
to decreasing supersaturation.
Figure 5.9
illustrates the very slow growth of
water droplets by condensation - in this case at 0.2
percent super saturation from an initial radius of
10μm. As there is an immense size difference
between cloud droplets (<1 to 50
μ
m radius) and
raindrops (>1mm diameter), it is apparent that
the gradual process of condensation cannot
explain the rates of formation of raindrops that are
often observed. For example, in most clouds
precipitation develops within an hour. The
alternative coalescence mechanism illustrated in
Figure 5.9
is described below (p. 128). It must be
remembered too that falling raindrops undergo
evaporation in the unsaturated air below the cloud
base. A droplet of 0.1mm radius evaporates after
falling only 150m at a temperature of 5
μ
Table 5.1
Cloud base height (in 000 m)
Tropics
Mid-
High
latitudes
latitudes
High cloud
6-18
5-13
3-8
Medium cloud
2-8
2-7
2-4
C and 90
percent relative humidity, but a drop of 1mm
°
Low cloud
Below 3
Below 2
Below 2
