Geoscience Reference
In-Depth Information
as few as 10 per liter at -30°C and probably rarely
more than 1000. However, some become active at
higher temperatures. Kaolinite, a common clay
mineral, initially becomes active at -9
120
B
°
C and on
A
subsequent occasions at -4
C. The origin of
freezing nuclei has been a subject of much debate
but it is generally considered that very fine soil
particles are a major 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
°
80
Ice particles
40
C.
Tiny ice crystals grow readily by deposition
from vapor, with different hexagonal forms
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 101 minutes. Ice
crystals readily aggregate upon collision due to
their branched (dendritic) shape, and tens of
thousands of crystals may form a single snowflake.
Temperatures between about 0 and -5
°
Cloud droplets
A
B
0
0
5
10
Time (min)
15
20
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% of which were assumed to
be frozen. B: a density of 1000 drops per cc, 0.1%
of which were assumed to be frozen.
Source: Jonas 1994a. Reprinted from Weatherby permission
of the Royal Meteorological Society. Copyright ©.
subfreezing layer (
warm clouds
). A suggested
mechanism 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 experiments is the freezing nucleus. Super-
cooled (water) clouds between -5 and -15
C are
particularly favorable 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 up-currents the
snowflake falls, melting into a raindrop if it falls
about 250m 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 2000m deep and the cloud-top temperature
is 5
°
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 pre-
cipitation. The seeding of some cumulus clouds at
these temperatures probably produces a mean
increase of precipitation of 10-15 percent from
clouds that are already precipitating or that are
'about to precipitate'. Increases of up to 10 percent
have resulted from seeding winter orographic
°
C or more. In mid-latitudes in summer,
precipitation may fall from cumuli that have no
°
