The amount of water vapour present in the air is called Humidity.
Humidity is a measure of the dampness of the atmosphere which varies greatly from place to place at different times of the day.
The maximum amount of moisture that can be hold by the air in the particular temperature is called Humidity Capacity.
Humidity is of three types:
It is the weight of the actual amount of water vapour present in a unit volume of air and is expressed as grams/cubic meter.
The absolute humidity differs from place to place on the surface of the earth. It is less frequently used by the meteorologists since volume changes as an air mass ascends or subsides and as a result, the absolute humidity varies with contraction or expansion of air. The differences in a unit of measurement between the numerators and denominators also make it highly inadequate for mathematical manipulation.
The weight of water vapour present in per unit weight of the air and is expressed as grams/kilograms.
Relative Humidity is expressed in percentage.
Relative Humidity is a ratio between the total capacity of the air for holding moisture under a given temperature and the actual amount of moisture being carried by it.
For Example- the temperature of a sample of air is 30°C. It can hold 25 grams of moisture per kilogram, but it is carrying only 5 grams of moisture. Its relative humidity is calculated as follows-
5/25 X 100 = 20. Thus its relative humidity is 20 percent.
With the increase of temperature, relative humidity decreases.
Relative Humidity is greater over the oceans and least over the continents.
The temperature at which relative humidity becomes 100% is called dew point temperature and the air is said to be saturated. In other words, dew point temperature is the temperature of the air at which condensation of moisture starts when the air is cooled. This temperature is equal to the saturation temperature of water vapour at its partial pressure.
Instrument for Measuring Humidity:
The humidity of a place can be found out with the help of an instrument called a Hygrometer.
A psychrometer is a hygrometer with one dry bulb thermometer and one wet bulb thermometer. The difference between the two readings gives the humidity.