Perspiration consists of water charged with waste products. This water is evaporated from the skin by the air. If the air is saturated with moisture, as it often is during the summer, water does not evaporate quickly and consequently perspiration does not evaporate at its usual rate. As a result we sweat or perspire very freely. When we fan ourselves we create a small breeze which quickly evaporates or absorbs the perspiration.

Moisture is, however, readily taken up by dry air, and a consequent cooling results. But if the atmosphere has a humidity of 100 per cent, as it has just before or after rain, the perspiration cannot be evaporated since the air already has all the moisture it can hold. Everyone has noticed that when the sun shines on a hot day just after a rainfall, the heat is almost unbearable.