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ORIGIN OF PRECIPITATION
F.H. SCHMIDT
Royal Netherlands Meteorological Institute, De Bilt
In Western Europe, precipitation is normal phenomenon; it is of importance to
all aspects of society, particularly to agriculture, in cattle breeding and, of course, it is a
subject of hydrological research. Precipitation is an essential part in the hydrological
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a&. How disastrous local conditions may be when precipitation holds off was obvious
recently in the Sahel territory, where for several years precipitation, which had previously
been marginal, stayed well under normally expected quantities. This introductory paper
aims at conclusions that may be drawn with respect to the distribution of rainfall, when
judged from a scrutiny of the mechanism that is responsible for the very origin of precipitation.
(See Fletcher (1 962) and Mason (197 1)).
Precipitation is exclusively caused by clouds, and the origin of clouds is in itself some-
thing special. The condensation of water vapour, which results into clouds, cannot take
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place in pure air. For, -- relative humidities of 300 to 40W0 would then be necessary and
-"*-+-.- --
such --...high .-. "L-e humiditiesnc~r
- -,"- occurin the-atmosphere. That is why condensation of water
vapour in the free atmosphere always takes place on small particles, the so-called condensation
nuclei. These condensation nuclei can have different dimensions, generally in the
order of l(rl to lop. It is interesting to note that larger condensation nuclei generally
lead to larger cloud-drops.
Condensation of water vapour generally takes place in the form of
these drops have temperatures below zero degrees centigrade, ice-crystals may be formed.
Generally speaking, in the free atmosphere this does not happen immediately at a temperature
of 0°C but as a rule at lower temperatures. Even at temperatures down to -10 or
-20"~ or, in extreme cases, even -40°C, waterdrops can still exist as such. They are
called super-cooled droplets. .-
For the forming of ice-crystals it is generally that small crystalline particles are present
in the waterdrops; around these crystalline particles, the waterdrops start freezing. So
there are clouds consisting of waterdrops only. There are also clouds at higher levels that
exclusively consist of ice-crystals. Very important for the genesis of precipitation are
those clouds that largely consist of waterdrops but in which ice-crystals are present too.
also peculiar. For there are clouds that do not give precipitation and others that do. This
means that apparently something special must be the matter with a cloud, should it
discharge in terms of rain, hail, or snow.