Evaporation is the process in which a liquid changes to the gaseous state at the free surface, below the boiling point through the the transfer of heat energy. Consider a body of water in a pond.
The molecules of water are in constant motion with a wide range of instantaneous velocities. An addition of heat causes this range and average speed to increase.
When some molecules possess sufficient kinetic energy, they may cross over the water surface. Similarly, the atmosphere in the immediate neighbourhood of the water surface contains water molecules within the water vapour in motion and some of them may penetrate the water surface.
The net escape of water molecules from the liquid state to the gaseous state constitutes evaporation.
Evaporation is a cooling process in that the latent heat of vaporization (at about 585 cal/g of evaporated water) must be provided by the water body.
The rate of evaporation is dependent on the
(i) vapour pressures at the water surface and air above,
(ii) air and water temperatures,
(iii) wind speed,
(iv) atmospheric pressure,
(v) quality of water and
(vi) size of the water body.
The rate of evcporation is proportional to the difference between the saturation vapour pressure at the water temperature, ew and the actual vapour pressure in the air,ea
EL = C(ew —ea)
where EL = rate of evaporation (mm I day) and C = a constant; ew and ea are in mm of mercury.
The above equation is known as Dalton’ s law of evaporation after John Dalton (1802) who first recognised this law.
Evaporation continues till ew = ea. If ew > ea condensation takes place.
Other factors remaining same, the rate of evaporation increases with an increase in the water temperature.
Regarding air temperature, although there is a general in crease in the evaporation rate with increasing temperature, a high correlation between evaporation rate and air temperature does not exist.
Thus for the same mean monthly temperature it is possible to have evaporation to different degrees in a lake in different months.
Wind aids in removing the evaporated water vapour from the zone of evaporation and consequently creates greater scope for evaporation.
However, if the wind velocity is large enough to remove all the evaporated water vapour, any further increase in wind velocity does not influence the evaporation.
Thus the rate of evaporation increases with the wind speed up to a critical speed beyond which any further increase in the wind speed has no influence on the evaporation rate.
This critical wind-speed value is a function of the size of the water surface. For large water bodies high-speed turbulent winds are needed to cause maximum rate of evaporation.
Other factors remaining same, a decrease in the barometric pressure, as in high altitudes, increases evaporation.
When a solute is dissolved in water, the vapour pressure of the solution is less than that of pure water and hence causes reduction in the rate of evaporation.
The percent reduction in evaporation approximately corresponds to the percentage increase in the specific gravity. Thus, for example, under identical conditions evaporation from sea water is about 2-3% less than that from fresh water.
Heat Storage in Water Bodies
Deep water bodies have more heat storage than shallow ones. A deep lake may store radiation energy received in summer and release it in winter causing less evaporation in summer and more evaporation in winter compared to a shallow exposed to a similar situation.
However, the effect of heat storage is essentially to change the seasonal evaporation rates and the annual evaporation rate is seldom affected.