Geoscience Reference
In-Depth Information
source. Biogenic aerosols emitted by decaying plant
litter, in the form of complex chemical compounds, also
serve as freezing nuclei. In the presence of certain
associated bacteria, ice nucleation can take place at only
-2 to -5°C.
Tiny ice crystals grow readily by deposition from
vapour, with different hexagonal forms (Plate 10)
developing at different temperature ranges. The number
of ice crystals also tends to increase progressively
because small splinters become detached by air currents
during growth and act as fresh nuclei. The freezing of
supercooled water drops may also produce ice splinters
(see F, this chapter). Figure 5.13 shows that a low
density of ice particles is capable of rapid growth in an
environment of cloud water droplets. This results in a
slower decrease in the average size of the much larger
number of cloud droplets although this still takes
place on a time scale of 10
1
minutes. Ice crystals
readily aggregate upon collision, due to their branched
(dendritic) shape, and groups of ten crystals may form
a single snowflake. Temperatures between about 0 and
-5°C are particularly favourable to aggregation, because
fine films of water on the crystal surfaces freeze when
two crystals touch, binding them together. When the fall
speed of the growing ice mass exceeds the existing
velocities of the air upcurrents the snowflake falls,
melting into a raindrop if it falls about 250 m below the
freezing level.
This theory can account for most precipitation in
middle and higher latitudes, yet it is not completely
satisfactory. Cumulus clouds over tropical oceans can
give rain when they are only some 2000 m deep and the
cloud-top temperature is 5°C or more. In mid-latitudes
in summer, precipitation may fall from cumuli that have
no subfreezing layer (
warm clouds
). A suggested mech-
anism in such cases is that of 'droplet coalescence',
discussed below.
Practical
rainmaking
has been based on the Bergeron
theory with some success. The basis of such experi-
ments is the freezing nucleus. Supercooled (water)
clouds between -5 and -15°C are
seeded
with especially
effective materials, such as silver iodide or 'dry ice'
(CO
2
) from aircraft or ground-based silver iodide
generators, promoting the growth of ice crystals and
encouraging precipitation. The seeding of some cumu-
lus clouds at these temperatures probably produces
a mean increase of precipitation of 10 to 15 per cent
from clouds that are already precipitating or are 'about
to precipitate'. Increases of up to 10 per cent have
resulted from seeding winter orographic storms.
However, it appears likely that clouds with an abun-
dance of natural ice crystals, or with above-freezing
temperatures throughout, are not susceptible to rain-
making. Premature release of precipitation may destroy
the updrafts and cause dissipation of the cloud. This
explains why some seeding experiments have actually
decreased
the rainfall! In other instances, cloud growth
and precipitation have been achieved by such methods
in Australia and the United States. Programmes aimed
at increasing winter snowfall on the western slopes of
the Sierra Nevada and Rocky Mountains by seeding
cyclonic storms have been carried out for a number of
years with mixed results. Their success depends on
the presence of suitable supercooled clouds. When
several cloud layers are present in the atmosphere,
natural seeding may be important. For example, if ice
crystals fall from high-level cirrostratus or altostratus
(a 'releaser' cloud) into nimbostratus (a 'spender' cloud)
Figure 5.13
The effect of a small proportion of initially frozen
droplets on the relative increase/decrease in the sizes of cloud ice
and water particles. The initial droplets were at a temperature of
-10°C and at water saturation. (A) A density of 100 drops per cc,
1 per cent of which were assumed to be frozen. (B) A density
of 1000 drops per cc, 0.1 per cent of which were assumed to
be frozen.
Source
: Jonas (1994). Reprinted from
Weather
, by permission of the
Royal Meteorological Society. Crown copyright ©.
