Dew, Frost, Smog, Mist and Fog:
Condensation takes place in two situations, firstly, when dew point is below freezing point or below 0° C and secondly, when it is above freezing point.
In this way, the forms of condensation may be classified into two groups:
(1) Frost, snow and some clouds are formed when dew point is below freezing point.
(2) Dew, mist, fog, smog and some clouds are formed when dew point is above freezing point.
When the atmospheric moisture is condensed and deposited in the form of water droplets on cooler surface of solid objects such as grass blades, leaves of plants and trees and stones, it is termed as dew.
Condensation in dew form occurs when there is clear sky, little or no wind, high relative humidity and cold long nights.
These conditions lead to greater terrestrial radiation and the solid objects become cold enough to bring the temperature of air down below dew point.
In this process the extra moisture of the air gets deposited on these objects.
Dew is formed when dew point is above freezing point.
Dew formation can be seen if the water is poured into a glass from the bottle kept in a refrigerator.
The outer cold surface of the glass brings the temperature of the air in contact with the surface down below dew point and extra moisture gets deposited on the outer wall of the glass.
When the dew point is below freezing point, under above mentioned conditions, the condensation of extra moisture takes place in the form of very minute particles of ice crystals.
It is called frost.
In this process, the air moisture condenses directly in the form of tiny crystal of ice.
This form of condensation is disastrous for standing crops such as potato, peas, pulses, grams, etc.
It also creates problems for road transport system.
Mist and Fog –
When condensation takes place in the air near the earth’s surface in the form of tiny droplets of water hanging and floating in the air, it is called mist.
In mist the visibility is more than one kilometer and less than two kilometers.
But when the visibility is reduced to less than one kilometer, it is called fog.
Ideal conditions for the formation of mist and fog are clear sky, calm and cold winter nights.
Smog is a fog that has been polluted and discoloured by smoke, dust, carbon monoxide, sulphur dioxide and other fumes.
Smog frequently occurs in large cities and industrial centres.
It causes respiratory illness.