ABSTRACT
From the point of view of life on the planet, precipitation is arguably more important than temperature, as the droughts in the Sahel in the early 1970s and in the UK in 1976 graphically illustrated. Before considering man's effects upon precipitation in detail, a brief review of precipitation types and mechanisms may be of value. The main precipitation forms are rain, snow and hail. All these forms of precipitation ultimately owe their existence to the cooling of air, usually by adiabatic expansion in uplift, so that the water vapour condenses or turns into ice. Man's conscious attempts to modify precipitation are of two main types: to increase amounts of rain and snow for agricultural and water-supply purposes; and to decrease the amounts of hail which cause so much damage in many agricultural areas. In both cases the modification is concerned with both the macro- and micro-physics of the clouds, particularly the latter.
