The process of condensation: The temperature of the air falls in two ways. Firstly cooling occurs around very small particles of freely floating air when it comes in contact with some colder object. Secondly, loss in air temperature takes place on a massive scale due to rising air to higher altitudes. The condensation takes place around smoke, salt and dust particles which attract water vapour to condense around them. When the relative humidity of an air is high a slight cooling is required to bring the temperature down below dew point but when the relative humidity is low and temperature of the air is high a lot of the cooling of the air will be necessary to bring the temperature down below the dew point. In this way the process of condensation takes place.
The forms of condensation: The forms of condensation may be classified in two groups.
(i) Frost, snow and clouds are formed when the dew point is below freezing point.
(ii) Dew, fog, smog and some clouds are formed when the dew point is above freezing point.
1. Dew: When the atmospheric moisture is condensed and deposited in the form of droplets on the cooler surface of solid objects such as grass, leaves of plants, trees and stones it is termed as dew.
2. Frost: When the dew point is below freezing point, under above mentioned conditions the condensation of extra moisture takes place in the form of very tiny particles of ice crystals. It is called frost.
3. Most 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 kilometre and less than two kilometres but when the visibility is reduced to less than one kilometre it is called fog. The ideal conditions for the formation of mist and fog are clear sky, calm and cold winter nights.
4. Smog: Smog is a fog that has been polluted and discoloured by smoke, dust, carbon monoxide, sulphur dioxide and other fumes.
5. Cloud: Clouds are visible aggregates of water droplets, ice particles or a mixture of both along with varying amounts of dust particles. A typical cloud contains billions of droplets having dieter on the order 060.01 to 0.02 mm.
