Study Notes on Humidity

Moisture, or water vapour, is an extremely important constituent of the atmosphere. Water vapour present in the air is known as Humidity. The nature and amount of precipitation, the amount of loss of heat through radiation from the earth's surface, latent heat of atmosphere etc depend on the amount of water vapour present in the atmosphere.Humidity capacity that is content of water vapour in the air is directly positively related with temperature i.e. higher the temperature higher the humidity capacity. Oceanic and coastal areas record higher humidity capacity. It decreases from equator to pole.

Humidity of a place can be expressed in three ways

Absolute Humidity: The measure of water vapour content of the atmosphere which may be expressed as the actual quantity of water vapour present in a given volume of air is called Absolute Humidity. The absolute humidity is measured in terms of grain per cubic metre air. If the absolute humidity of air at a given place is 10 gm/cu m. it means that 10 grams of water vapour are present in a cubic metre of air. Absolute humidity of the air changes from place to place and from time to time. The ability of air to hold water vapour depends entirely on its temperature. Warm air can hold more moisture than the cold air.

Specific Humidity: Another way to express humidity as the weight of water vapour per unit weight of air or the proportion of the mass of water vapour to the total mass of air is called the specific humidity. Specific humidity is not affected by changes in pressure of temperature.

Relative Humidity: A more useful measure of humidity of the atmosphere is called the relative humidity. This is a ratio expressed as a percentage between the actual quantity of water vapour present in the air at a given temperature and the maximum quantity of water vapour that the atmosphere can hold at that temperature. Relative humidity determines the amount and rate of evaporation and hence it is an important climatic factor. With the same quantity of water vapour, relative humidity will decrease with increase of temperature and vice versa.