Geoscience Reference
In-Depth Information
Table 10.2
Typical values for the radius, concentration and terminal velocity of the
airborne entities present in clouds.
Concentration
(per m
3
)
Terminal
velocity (m s
-
1
)
Airborne entity
Radius (
m
m)
Atmospheric aerosols
0-2
10
12
Various but small
Typical cloud
condensation nuclei
0.1
10
9
10
−7
Typical cloud droplet
10
10
9
10
−2
Large cloud droplet
50
10
6
∼
0.27
Cloud/rain borderline
droplet
100
-
0.7
∼
Raindrop
1000
10
3
7
∼
The form of Equation (10.1) means that the saturated vapor pressure at which
condensation on to a surface can occur is less for surfaces with larger radius than
it is for surfaces with smaller radius.
Cloud droplet size, concentration and terminal velocity
Because saturated vapor pressure is higher over curved surfaces, condensation in
clouds is preferentially onto the larger aerosols, i.e., onto the fraction of aerosols that
are most rare, particularly if they are hydrophilic. Because of this, the cloud droplet
density in clouds is typically a thousand times less than the density of atmospheric
aerosols, i.e., about 10
9
per cubic meter rather than 10
12
per cubic meter.
The ratio of the area of a cloud particle to its volume is greater for smaller
particles than for larger particles. As a result, if growth is only by condensation
(as opposed to by collisions), smaller drops increase their diameter more quickly
than the larger drops. Hence, not only does the mean value of the particle diameter
increase with time in a cloud, but also the spectrum of diameters present in the
cloud narrows because the smaller diameter particles grow quicker.
Table 10.2 shows typical values for the radius of aerosol particles, cloud
condensation nuclei, cloud droplets and raindrops in a cloud, their likely
concentration, and terminal velocity. For comparison, the order of magnitude of
uplift in developing clouds is 1 m s
−1
.
How do clouds work? Clouds occur when atmospheric uplift causes air to cool,
saturate, and condense water vapor onto cloud condensation nuclei. The smallest
cloud droplets or ice particles are swept upward by the uplift while still growing.
When they have grown larger they can hang, apparently stationary, in the rising air
because their terminal velocity is similar to the uplift. A fraction of these water
droplets or ice particles may become large enough to fall out of the cloud but these
quickly evaporate. Eventually, the cloud particles become larger still, large enough
