Condensation by cooling is produced by four different processes: (a) cooling by expansion, (b) conduction of heat from the air to a cold surface, (c) direct radiation-cooling of the air, and (d) mixing of warm and cold masses of saturated air. Generally these processes work in combination. These processes may be classified into two broad categories: (1) Adiabatic processes and (2) Non-adiabatic processes.(1) Adiabatic processes:
Cooling by expansion is far more important than any other process so far as condensation in free air is concerned. This type of cooling is due to lifting of the air. Since an ascending air mass undergoes ever decreasing pressure exerted on it, it expands and cools.
Such temperature changes are brought about without any heat 6etng added to or subtracted from the rising air. These temperature changes are, therefore, called adiabatic. Unsaturated air cools at the dry adiabatic rate of 10°C per km.
However, after passing beyond the condensation level, the latent heat of condensation lowers the rate of cooling. This modified rate of cooling is called the wet or moist adiabatic rate.
The average rate of this cooling is about 6°C per km, but the actual values vary with pressure. Adiabatic cooling may be accomplished due to convection, convergence of different air masses as along the fronts, or orographic uplifting.
(2) Non-adiabatic processes:
Non-adiabatic processes include cooling by radiation, conduction or mixing with colder air. The air may be cooled due to loss of heat by radiation. In case there is direct radiation from the moist air, the cooling produces fog or clouds provided hygroscopic nuclei are present in the air.
Cooling may also be produced by conduction or advection of warm air across a cold surface. Cooling by contact with a cold surface produces dew, frost or fog depending on other atmospheric conditions. Sometimes the air is cooled due to its mixing with colder air.
It is noteworthy that the effect of cooling produced by radiation, conduction and mixing is confined to a thin layer of the atmosphere. The non-adiabatic processes of cooling produce only dew, fog or frost. They are incapable of producing a substantial amount of precipitation.
The only process capable of reducing the temperature of deep and extensive air masses, so that cloud formation and appreciable precipitation may be possible, is the expansion associated with rising air currents or the adiabatic cooling.