Geoscience Reference
In-Depth Information
Cold clouds
Dominant in the
higher atmosphere
and at high latitudes
Mixed clouds
Warm clouds
Dominant in the
lower atmosphere
and at low latitudes
Supercooled droplets
Water droplets
Ice particle
−
50
C
−
40
C
−
30
C
−
20
C
−
10
C
0
C
10
C
Figure 10.3
Temperature ranges and constituents for different cloud types.
to have a terminal velocity such that they fall through the cloud despite uplift,
colliding with and capturing water from the smaller particles as they do so. The
resulting large cloud particles so formed which fall out of the cloud may then
be big enough to reach the ground as rain or snow before they can evaporate.
In strongly convective conditions, uplift in the cloud can become so rapid that it is
hard for particles to fall against it. As a result, they continue to grow until they
eventually become much larger and can fall out of the cloud to reach the ground,
usually as heavy rain.
Ice in clouds
Broadly speaking there are three types of clouds which are distinguished by the air
temperature in which they exist, i.e.,
cold clouds
when the air temperature is -40°C
or less, made up almost entirely of ice particles;
warm clouds
when the air
temperature is greater than 0°C, made up almost entirely of water droplets; and
mixed clouds
between these two temperatures which are a mixture of ice particles
and water droplets, see Fig. 10.3.
The ice particles present in mixed and cold clouds can be formed or grow in
different ways, the most important being:
Spontaneous nucleation
: at low temperatures (
-40°C) a chance aggregation
of enough water molecules can spontaneously freeze into an ice particle;
<
●
Heterogeneous nucleation
: ice grows around a pre-existing ice nucleus
inside a water droplet;
●
Contact nucleation
: a pre-existing supercooled water droplet makes contact
with a pre-existing ice particle and is captured to increase the size of the ice
particle; and
●
Bergeron-Findeison process
: water evaporated from water droplets freezes
directly on to pre-existing ice particle.
●
