Geoscience Reference
In-Depth Information
Condensation and growth of condensation products
When the air reaches or exceeds saturation, water vapor may start to condense in the form
of liquid drops or ice crystals on the small nuclei, such as dust, smoke and various types of
salt particles, that are invariably present in the atmosphere. Initially, these condensation
products are small enough to be kept afloat in the atmosphere as cloud. Condensation
generates latent heat. Therefore, any further growth of these droplets and ice crystals
depends on the rate of diffusion of water vapor to their surface from the surrounding air
and on the rate of conduction of latent heat away from their surface into the air; in the
case of liquid droplets, there are also hygroscopic effects due to some nuclei, surface
tension and for the larger sizes, accretion. It can be shown (see, for example, Fleagle and
Businger, 1963) that the growth of droplets is primarily controlled by condensation up
to radius sizes of the order of 15
μ
m. Full-grown raindrop development to radii larger
μ
than 20
m requires accretion by collisions and coalescence with neighboring drops of
different sizes and fall velocities; this process, in turn, is affected by the turbulence of
the air, the temperature distribution in the clouds and electrical effects. In contrast, the
growth of ice particles depends only on vapor diffusion and on heat removal, and not
on accretion; however, on account of the lower vapor pressure characteristics of ice, the
water vapor diffusion process is much more effective than in the case of liquid water. If the
particle sizes continue to increase, eventually they become too heavy and precipitation
takes place. The onset of precipitation on the ground depends on a number of conditions;
most obvious among them is that the condensation products must be large enough to
fall down against the entrainment of updrafts, and to overcome evaporation while falling
down to the ground. A rough rule of thumb for the demarcation between precipitation
and cloud particle diameters is around 0.1 mm. Substantial rates of precipitation usually
can take place only when the cloud thickness is 1200 m or more.
Blanchard (1972) has written an enlightening history of the discovery of the main
mechanisms of raindrop and ice crystal formation.
Moisture supply
Calculation of the precipitable water, as defined in Equation (2.11), shows that even
under the most favorable conditions an atmospheric column at rest can hold only a very
limited quantity of water vapor. For instance (List, 1971), for a near-surface temperature
of 20
◦
C and a surface pressure of about
p
0
=
1000 hPa, a saturated atmosphere with a
pseudoadiabatic lapse rate can at most hold an amount, which is equivalent with about 5
cm of liquid water; for 10
◦
C this precipitable water is only about half as much. Heavy
precipitation amounts regularly exceed such values. But even so, it is well known that the
humidity of the air tends to remain relatively constant during precipitation events. This
means that it is not so much the local precipitable water, but the horizontal influx of moist
air into an area, that controls the local intensity and the total amount of precipitation.
The specific nature of this moisture influx depends on the weather system.
Water recycling
In the study of regional water budgets over seasonal or longer time periods it is often of
interest to determine the origin of the water vapor producing the precipitation. Part of this
