Condensed moisture occurs in various forms. This is due to variations in the amount of humidity in the air, in its movements and turbulence, and in its temperature and rate of cooling.
Condensation occurs on solid surfaces on the earth’s surface as well as in free air above it. The common forms of condensed moisture include dew, frost, fog and clouds. Fog and clouds being very important weather elements have been described in detail in the next chapter.
Dew consists of tiny droplets of water produced by condensation on surface objects rather than on nuclei in the air above the surface. Any nature observer will find in the morning shining beads of water droplets deposited on leaves and blades of grass.
In fact, dew refers to “water drops deposited by direct condensation of water vapour from the adjacent clear air mainly on horizontal surfaces cooled by nocturnal radiation”. Cole defines dew as the “condensed moisture that forms in place as a consequence of contact cooling”.
Clear skies and calm air are necessary conditions for the formation of dew. Clouds act as blankets which greatly reduce the radiation cooling during night. Movement and turbulence in the lower stratum of atmosphere cause mixing and thereby prohibit the air from reaching its dew point.
That is why windy nights do not favour the formation of dew. In brief, the necessary conditions for dew formation are: cloudless sky, calm weather, and appreciable quantity of moisture in the air.
Frost should not be mistaken to be frozen dew. Frost occurs when the dew point of air falls below the freezing point (°C). When condensation starts with temperature below 0°C, the water vapour in the air passes directly from gaseous to solid state.
Sometimes condensation under such conditions results in the formation of ice-crystals commonly called ‘white frost’.
The United States Weather Bureau classifies frost into two categories on the basis of the amount of deposits: light frost, and heavy or killing frost. The killing frost destroys the staple crops of the locality.
Low places such as valleys without any outlet are the ideal locations for the occurrence of frosts. The cold and heavy air accumulates in such locations, becoming much colder than the surrounding air.
Under such conditions, a temperature inversion is created. Valley floors are affected by the frosts, while the slopes remain unaffected.
However, the conditions necessary for the formation of frost are the same as for dew except that condensation taking place below the freezing point produces frost.
Rime is another unimportant form of condensation which develops at or near the ground. It consists of ice crystals deposited chiefly on vertical surfaces, especially on points and edges of objects, generally in super-cooled fog or mist.
Protection against frost is a great challenge to the fruit-growers in some of the cold countries of the world. Various measures have been adopted to protect the orchards from injury caused by frost.
Attempts are made at reducing the heat loss during night or adding heat to the layer of air touching the ground. Heat conservation methods are also adopted.
This includes covering plants with materials having low thermal conductivity, such as paper or cloth. Sprinklers, air mixing giant-size fans and orchard heaters are frequently used for warming the air.