HUMIDITY

Water vapour is amongst the most important atmo­
spheric gases. The atmosphere gathers moisture by the process of evaporation, while it loses moisture through
condensation and precipitation. Humidity, which refers to the condition of the air with regard to water vapour, plays a decisive role in evaporation, condensation and precipita­tion.
HUMIDITY
It is the degree of water vapour present in the air. For any specified temperature, there is a definite limit to the maximum quantity of moisture that can be held by the air. This limit is known as the saturation point. Humidity can be measured by- a hygrometer or sling psychrometer.

RELATIVE HUMIDITY is the ratio between the amount
of water vapour actually present in an air mass and the maximum amount that the air mass can hold at that temperature. It is expressed as a percentage. At the
saturation point, relative humidity is 100 per cent. It varies inversely with temperature, given a fixed amount of water vapour. Secondly, if an exposed water surface is present the relative humidity can be increased by evaporation.

ABSOLUTE HUMIDITY is the actual amount of mois­ture present in air. The amount of water vapour per unit volume of air is usually expressed in grams per cubic metre. It is a measure of the quantity of water that can be extracted from the atmosphere as precipitation. As the absolute humidity cannot remain a constant figure for the same body of air, modem meteorology makes use of another measure of moisture content-specific humidity.

SPECIFIC HUMIDITY is the ratio of the weight of water vapour to the weight of moist air. Expressed in units of grams of water vapour per kilogram of moist air, specific
humidity is often used to describe the moisture character­istic of a large mass of air.