Geoscience Reference
In-Depth Information
enough mass to overcome uplifting forces within a
cloud. So there are three conditions that need to be
met prior to precipitation forming:
expended in using silver iodide particles, dropped
from planes, to act as condensation nuclei. However,
more recent work has suggested that other salts such
as potassium chloride are better nuclei. There is
much controversy over the value of cloud seeding.
Some studies support its effectiveness (e.g. Gagin
and Neumann, 1981; Ben-Zvi, 1988); other authors
query the results (e.g. Rangno and Hobbs, 1995),
while others suggest that it only works in certain
atmospheric conditions and with certain cloud types
(e.g. Changnon
et al
., 1995). More recent work in
South Africa has concentrated on using hygroscopic
flares to release chloride salts into the base of
convective storms, with some success (Mather
et al
.,
1997). Interestingly, this approach was first noticed
through the discovery of extra heavy rainfall
occurring over a paper mill in South Africa that was
emitting potassium chloride from its chimney stack
(Mather, 1991).
1
Cooling of the atmosphere
2
Condensation onto nuclei
3
Growth of the water/ice droplets.
Atmospheric cooling
Cooling of the atmosphere may take place through
several different mechanisms occurring independ-
ently or simultaneously. The most common form
of cooling is from the uplift of air through the atmos-
phere. As air rises the pressure decreases; Boyle's
Law states that this will lead to a corresponding
cooling in temperature. The cooler temperature leads
to less water vapour being retained by the air and
conditions becoming favourable for
condensation
.
The actual uplift of air may be caused by heating
from the earth's surface (leading to
convective
precipitation
), an air mass being forced to rise
over an obstruction such as a mountain range (this
leads to
orographic precipitation
), or from a low
pressure weather system where the air is constantly
being forced upwards (this leads to
cyclonic pre-
cipitation
). Other mechanisms whereby the
atmosphere cools include a warm air mass meeting
a cooler air mass, and the warm air meeting a cooler
object such as the sea or land.
Water droplet growth
Water or ice droplets formed around condensation
nuclei are normally too small to fall directly to the
ground; that is, the forces from the upward draught
within a cloud are greater than the gravitational
forces pulling the microscopic droplet downwards.
In order to overcome the upward draughts it is
necessary for the droplets to grow from an initial
size of 1 micron to around 3,000 microns (3 mm).
The vapour pressure difference between a droplet
and the surrounding air will cause it to grow
through condensation, albeit rather slowly. When
the water droplet is ice the vapour pressure
difference with the surrounding air becomes greater
and the water vapour sublimates onto the ice
droplet. This will create a precipitation droplet
faster than condensation onto a water droplet, but
is still a slow process. The main mechanism by
which raindrops grow within a cloud is through
collision and coalescence
. Two raindrops collide and join
together (coalesce) to form a larger droplet that
may then collide with many more before falling
towards the surface as rainfall or another form of
precipitation.
Condensation nuclei
Condensation nuclei are minute particles floating
in the atmosphere which provide a surface for the
water vapour to condense into liquid water upon.
They are commonly less than a micron (i.e. one-
millionth of a metre) in diameter. There are many
different substances that make condensation nuclei,
including small dust particles, sea salts and smoke
particles.
Research into generating artificial rainfall has
concentrated on the provision of condensation nuclei
into clouds, a technique called
cloud seeding
.
During the 1950s and 1960s much effort was
