What is Evaporation ?

Evaporation is the process by which water is changed to vapours (the gaseous state) at the free surface, below the boiling point of water. 

The vapours go to the atmosphere and form clouds. 

Evaporation occurs when molecules of water attain high kinetic energy to eject themselves from the water surface into the atmosphere. 

The amount of energy used by a unit mass of water from the liquid state to the vapour state at constant temperature is known as the latent heat of evaporation, which is about 585 calories per gram. 

If no external source of energy is available, heat energy is extracted from water body and, consequently, it results in lowering of the water temperature.


The evaporation losses from a water surface depend upon the following factors: 

(1) The vapour pressure at the water surface and air above : 

Evaporation rate is proportional to the difference between the saturation vapour pressure at the water temperature and the actual vapour pressure in the air. 

(2) Air and Water temperatures :

Other factors remaining same, the rate of evaporation increases with an increase in the water temperature. 

Regarding air temperature, although there is a general increase in the evaporation rate with increasing temperature.

Thus, in summer season or in hot countries, the evaporation will be more as compared to that in the winter season or in cold countries.

(3) Wind speed :

The increase in wind velocity increases evaporation. Wind aids in removing the evaporated water vapour from the zone of evaporation and consequently creates greater scope for evaporation. 

There is always a critical velocity of wind beyond which evaporation does not increase. Wind velocity near the ground and water level is effective and influence on evaporation is more if the wind velocity is turbulent. 

(4) Atmospheric pressure :

If the atmospheric pressure is more, naturally there will be lesser evaporation. 

Thus, decrease in atmospheric pressure can increase evaporation.

At higher altitude, atmospheric pressure is low, hence evaporation should normally be higher.

(5) Quality of water :

It also affects the rate of evaporation. If water contains dissolved salts, it reduces the saturated vapour pressure and hence evaporation decreases.

(6) Size of the water body :

The amount of evaporation is directly proportional to the area of evaporation.

If the exposed area is large, the evaporation will be more, and vice-versa. But there is always a difference of evaporation rate between smaller and larger water surface although total evaporation from larger area will be more.

(7) Depth of water in the water body : 

The depth of water influences the evaporation considerably.. If depth of water is more, it reduces the summer evaporation, and increases the winter evaporation.

(8) Humidity :

If the humidity of the atmosphere is more, the evaporation will be less. If humidity is more, water holding capacity of air is less, so less evaporation.

If water content is less in air, more evaporation will take place.

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