Geoscience Reference
In-Depth Information
11
Formation of
Precipitation
Introduction
As discussed in the last chapter, clouds are made up of cloud droplets, ice particles,
or a mixture of the two, depending on the temperature of the cloud and therefore
on its height and location. It is very difficult for these basic constituents to reach
the ground because they are small in size and so have very low terminal velocity;
there is also often uplift in the cloud that counteracts the tendency they have to
fall. Even droplets or particles that are just large enough to fall out of the cloud are
likely to evaporate before they reach the ground, once they leave the saturated
atmosphere inside the cloud.
Moreover, cloud particles do not grow rapidly enough by simple condensation
to fall as precipitation. Some other growth mechanism is required. As discussed in
more detail below, there are two general ways cloud particles can grow more
quickly. The first is via some form of collision. The name given to growth processes
that involve collision is different depending on the phase of the cloud particles
involved. When water droplets collide with other water droplets and grow as a
result, the process is called
coalescence
. This can occur in both warm clouds and
mixed clouds. When ice particles grow by colliding with other ice particles, the
growth process is called
aggregation
. This can occur in both cold clouds and mixed
clouds. When an ice particle grows by colliding with and freezing the water from
(or
riming
) water droplets, the process is called
accretion
. This process can only
occur in mixed clouds. All of these collision processes require that there is air
motion within the cloud, but this is a common phenomenon in clouds.
A second way that ice particles grow in mixed clouds is by the Bergeron-
Findeisen process. In this process (described in greater detail later) ice particles
grow at the expense of water droplets in the cloud because there is a difference in
the saturated vapor pressure for ice and water at the same temperature. This growth
process does not necessarily require internal movement in the cloud, but it can
