Humidity and Relative Humidity
Humidity is the amount of water vapour present in atmosphere. The amount of water vapour in atmosphere changes with time and weather. The air containing water vapour is called humid air. The amount of vapour present per unit volume of air is called the humidity of air. Humidity is generally measured in kg/m3.
The knowledge of humidity helps us to predict weather. When the amount of water vapour in the air is small, the air appears to be dry and the humidity is low. When the amount of water vapour in the air is large, the air appears to be wet and the humidity is high. The degree of wetness of air is expressed in terms of its relative humidity.
The ratio of the mass of water vapour actually present in certain volume of air (m) to the mass of water vapour (M) required to saturate the same volume of air at the same temperature is called relative humidity (R.H)
If the air contains the maximum amount of water vapour its R.H is 100%. In such a case, water on earth cannot vaporate at all. If the relative humidity is less than 100% but still high, the rate of evaporation will be slow and the clothes do not dry up easily in such weather. The relative humidity varies from season to season. During rainy season, as the amount of water vapours in air increases, the relative humidity becomes more (R.H = 100%) More R.H is a permanent feature of coastal areas. Due to more R.H perspiration from our body does not evaporate and we feel sultry.