Why Do People Living In Coastal Areas Sweat Profusely?

Answer 1:

`Sweat' denotes the moisture exuded through the pores of the skin of human beings and other mammals.

The temperature of the human body is maintained at about 37 degrees centigrade. This, irrespective of the temperature of the surroundings is continually generated by the chemical actions (metabolism) which occur in the body. This must be dissipated as fast as it is produced if the temperature is to remain constant. Also, water is a by-product of the chemical actions. This water is, invariably, the source for sweat.

The excess heat passes from the body by convection, by radiation and by evaporation. Convection takes place when a current of air cooler than the body receives heat by contact with it and then passes off carrying heat with it.

Heat is dissipated by radiation when the surroundings are at a lower temperature than the body. Evaporation occurs when the skin is moist and the air is sufficiently dry so that it can readily absorb moisture.

If the air temperature is equal to or above that of the body, no heat can be carried away by convection. If the surroundings, say, the walls of the rooms and the objects in it, are as warm or warmer than the body, no heat can be dissipated by radiation.

The moisture on the skin is in the form of water and that in the air in the form of vapour. In passing from the body to the air, this moisture must absorb the latent heat of vaporisation and, thereby, extract large quantities of heat. But, if the air is already saturated, it cannot absorb any more moisture and, consequently, no heat can be dissipated by evaporation.

Obviously, the sweat that has been exuded cannot escape and hence gets accumulated on the skin surface. This is `profuse sweating.'

Now, consider the atmospheric conditions in the coastal areas, particularly, in the tropics and, more so, during the summer. The air temperature invariably exceeds 37 degrees Centigrade and the air is almost saturated (highly humid).

More so, if the wind direction is from the sea towards the land. Hence, virtually, no heat can be dissipated by convection, by radiation or by evaporation. Consequently, people living in such coastal areas sweat profusely.

Answer 2:

Normally the `relative humidity', in coastal areas will be higher than inland areas. Relative Humidity (RH) is defined as the ratio of the mass of water vapour in a certain volume of moist air at a given temperature to the mass of water vapour in the same volume of saturated air at the same temperature.

The rate of evaporation of moisture depends on the temperature of air and relative humidity. Relative humidity is considered to be a more commonly understood measure of the degree of saturation of air.

Actually the sweat comes out of our body irrespective of the RH, whereas when the RH level is higher we felt it. When the RH is lower, we don't feel the sweat because it gets evaporated quickly, since the evaporation rate is higher when RH is low.

That's why inside an air-conditioned room we don't feel the sweating. The moisture in the air gets condensed over an air-conditioner's low temperature evaporator coil and the RH inside the room drops.

Published in The Hindu on May 2, 2002.