Persperation – the evaporation of moisture from the skin – is an important factor in controlling body temperature. The rate at which evaporation takes place depends on three factors:
the amount of water vapour present in the air
the rate of movement of air over the skin
When the air is still and the water vapour content in the air is close to saturation the rate of evaporation from the skin slows. Even moderate exercise can then produce a large amount of respiration but the water can not evaporate and the skin remains moist and clammy. Conditions are described as being close or humid. The rate of evaporation can be increased with the presence of a breeze. If there is no breeze, often an artificial one can be created by a fan and a fan is often present in frequently humid rooms just for this purpose.
The rate of evaporation can also be increased if the air is dry. In hot continental climateswhere the air tends to be drier, high temperatures are more easily tolerated than in tropical climates, where heavy rainful often leads to near saturation. For this reason, North Africa is often more easily tolerated than India during the monsoon season.
As far as personal comfort is concerned and for some industrial purposes, the quantity of water vapour presentis not as important as how close to saturation it is. Nearness to saturation is expressed as relative humidit.
There are instruments available to measure relative humidity called hygrometers.