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atmospheric turbulence might bring warm and
cold cloud droplets into close conjunction. The
supersaturation of the air with reference to the
cold droplets and the undersaturation with
reference to the warm droplets would cause the
latter to evaporate and cold droplets to develop at
their expense. However, except perhaps in some
tropical clouds, the temperature of cloud droplets
is too low for this differential mechanism to
operate.
Figure 2.4
shows that, below about
-10
100
Dec-Feb
Jun-Aug
90
80
70
60
50
40
C, the slope of the saturation vapor pressure
curve is low. Another theory was that raindrops
grow around exceptionally large condensation
nuclei (observed in some tropical storms). Large
nuclei do experience a more rapid rate of initial
condensation, but after this stage they are subject
to the same limiting rates of growth that apply to
all cloud drops.
Current theories for the rapid growth of
raindrops involve either the growth of ice crystals
at the expense of water drops, or the coalescence
of small droplets by the sweeping action of falling
drops.
°
30
90°N
60°N
30°N
0
Latitude
30°S
60°S
90°S
Figure 5.12
Average zonal distribution of total
cloud amount (%), derived from surface obser-
vations over the total global surface (i.e., land plus
water) for the months of December-February and
June-August during the period 1971-1981.
Source: From London et al. (1989). Courtesy of Cospar and
Elsevier.
This may be associated with higher atmospheric
sulfate concentrations due to increased coal
burning. The relationship with temperature is
unclear.
1 Bergeron-Findeisen theory
This widely accepted theory is based on the fact
that at subzero temperatures the atmospheric
vapor pressure decreases more rapidly over an
ice surface than over water (
Figure 2.14
). The
saturation vapor pressure over water becomes
greater than over ice, especially between temp-
eratures of -5 and -25
E FORMATION OF
PRECIPITATION
The puzzle of raindrop formation has already
been noted. The simple growth of cloud droplets
through condensation is apparently an inadequate
mechanism and more complex processes have to
be envisaged.
Various early theories of raindrop growth can
be discounted. Proposals were that differently
charged droplets could coalesce through electrical
attraction, but it appears that distances between
drops are too great and the difference between the
electrical charges too small for this to happen. It
was suggested that large drops might grow at the
expense of small ones. However, observations
show that the distribution of droplet size in a
cloud tends to maintain a regular pattern; the
average radius is between 10 and 15μm, and few
are larger than 40
C, where the difference
exceeds 0.2mb. If ice crystals and supercooled
water droplets exist together in a cloud, the drops
tend to evaporate and direct deposition takes place
from the vapor on to the ice crystals.
Freezing nuclei
are necessary before ice particles
can form - usually at temperatures of about -15
to -25
°
C. Small water droplets can, in fact, be
supercooled in pure air to -40°C before sponta-
neous freezing occurs. But ice crystals generally
predominate in clouds where temperatures are
below about -22
°
C. Freezing nuclei are far less
numerous than condensation nuclei; there may be
°
μ
m. A further idea was that
