Environmental Engineering Reference
In-Depth Information
doubt the processes producing precipitation cadn be very effective when conditions are
favourable. But how do these minute cloud droplets (Plate 4.4) grow large enough to fall
as rain within as little as twenty minutes of the cloudy air reaching saturation?
To answer this question, we must delve inside a cloud and see what is happening
there. In a cloud made up entirely of water droplets there will be a variety of droplet
sizes. The air will be rising within the cloud, perhaps at the rate of 10-20 cm per second,
though much more rapidly in cumulonimbus clouds. As it rises so the drops get larger
through collision and coalescence; some will reach drizzle size. When the uplift is
stronger, say 50 cm per second, the downward movement of the drops will be reduced, so
there will be more time for them to grow. If the cloud is about 1 km deep, small raindrops
of 700 µm diameter may be formed.
When temperatures fall below 0° C, because of their small size the droplets do not
freeze immediately but may remain unfrozen in what is said to be a supercooled state.
With further cooling to −10° C, ice crystals may start to develop among the water
droplets. This mixture of water and ice would not be particularly important but
Plate 4.4
A typical sample of cloud droplets, caught on an
oiled slide and photographed under a microscope in the
aircraft. The largest droplet has a diameter of about 30 µm.
Photo: After B. J. Mason.
for a peculiar property of water. The saturation vapour pressure curve of ice (Figure 4.2)
is slightly different from that of water. The air can be saturated for ice when it is not
saturated for water. Thus at −10° C, air saturated with respect to liquid water is super-
saturated relative to ice by 10 per cent and at −20° by 21 per cent. As a result the ice
crystals in the cloud tend to grow and become heavier at the expense of the water
droplets.
As the ice crystals sink into lower layers of the cloud where temperatures are only just
below freezing, they have a tendency to stick together to form snowflakes. This is
brought about by the supercooled droplets of water in the cloud acting as an adhesive.
