Geoscience Reference
In-Depth Information
Supersaturation in clouds rarely exceeds 1 per cent
and, because the saturation vapour pressure is greater
over a curved droplet surface than over a plane water
surface, minute droplets (<0.1 µm radius) are readily
evaporated (see Figure 5.8). Initially, the nucleus size is
important; for supersaturation of 0.05 per cent, a droplet
of 1 µm radius with a salt nucleus of mass 10
-13
g reaches
10 µm in 30 minutes, whereas one with a salt nucleus
of 10
-14
g would take 45 minutes. Later, when the dis-
solved salt has ceased to have significant effect, the
radial growth rate slows due to decreasing super-
saturation.
Figure 5.9 illustrates the very slow growth of water
droplets by condensation - in this case, at 0.2 per cent
supersaturation from an initial radius of 10 µm. As there
is an immense size difference between cloud droplets
(<1 to 50 µm radius) and raindrops (>1 mm 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 pre-
cipitation develops within an hour. The alternative
coalescence mechanism illustrated in Figure 5.9 is
described below (p. 102). 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 150 m at a
temperature of 5°C and 90 per cent relative humidity,
but a drop of 1 mm radius would fall 42 km before evap-
orating. On average, clouds contain only 4 per cent of
the total water in the atmosphere at any one time but
they are a crucial element in the hydrological cycle.
2 Cloud types
Figure 5.8
Kohler curves showing the variation of equilibrium
relative humidity or supersaturation (per cent) with droplet radius
for pure water and NaCl solution droplets. The numbers show
the mass of sodium chloride (a similar family of curves is obtained
for sulphate solutions). The pure water droplet line illustrates the
curvature effect.
The wide 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.
Figure 5.9
Droplet growth by conden-
sation and coalescence.
Source
: Jonas (1994). Reprinted from
Weather
, by permission of the Royal
Meteorological Society. Crown copyright
©.
