Condensation is the process by which atmospheric water vapour changes into water or ice crystals. It is just reverse of the process of evaporation.
When the temperature of saturated air falls below dew point, the air cannot hold the amount of humidity which it was holding earlier at a higher temperature. This extra amount of humidity changes into water droplets or crystals of ice depending upon the temperature at which condensation takes place.
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 of air to higher altitudes.
The condensation takes place around the smoke, salt and dust particles which attract water vapour to condense around them. They are called hygroscopic nuclei.
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 the temperature of the air is high, a lot of cooling of the air will be necessary to bring the temperature down below dew point. Thus, condensation is directly related to the relative humidity and the rate of cooling.
Forms of condensation
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 –
- Frost, snow and some clouds are formed when dew point is below freezing point.
- Dew, mist, fog, smog and some clouds are formed when dew point is above freezing point.
The forms of condensation may also be classified on the basis of place where it is occurring, for example, on the ground or natural objects such as grass blades and leaves of the plants or trees, in the air close to the earth’s surface or at some height in the troposphere.
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,
- 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.
(iii) 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.
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 diameters on the order 060.01 to 0.02 mm; yet liquid or solid water accounts for less than 10 parts per million of the cloud volume.
Clouds are generally classified on the basis of their general form or appearance and altitude. Combining both these characteristics, clouds may be grouped as under –
Low clouds: The base level of low clouds varies from very near the ground to about 2000m. The basic type of this family is the status, a low, uniform layer resembling fog but not resting on the ground.
Stratocumulus clouds form a low, gray layer composed of globular masses or rolls which are usually arranged in groups, lines, or waves. Clouds with vertical development fall into two principal. Categories: cumulus and cumulonimbus. Cumulus clouds are dense, dome-shaped and have flat bases. They may grow to become cumulonimbus, the extent of vertical development depending upon the force of vertical currents below the clouds as well as upon the amount of latent heat of condensation liberated in the clouds as they form.
To an observer directly beneath, a cumulonimbus cloud may cover the whole sky and have the appearance of Nimbostratus, The word nimbus (or prefix nimbo) applies to a cloud from which rain is falling. It derives from the Latin for “violent rain”.
Medium clouds: These clouds are formed at altitudes between 2000 to 6000 metres. This group of clouds include altocumulus and altostratus.
High clouds: These clouds are formed above the altitude of 6000 metres and include cirrus, cirrostratus and cirrocumulus.
Condensation is a process of changing water vapour into tiny droplets of water or ice cystals.
Bibliography : NIOS Geography Book
Reference : http://www.nios.ac.in