The heat energy radiated from the sun changes water into water vapour.
The term humidity refers to the amount of water vapour present in a given air.
Humidity of the air is mainly expressed in the following two ways –
 Absolute humidity
 Relative humidity
 Absolute Humidity
Absolute humidity is the ratio of the mass of water vapour actually in the air to a unit mass of air, including the water vapour.
It is expressed in gram per cubic metre of air.
For example, if the absolute humidity of air is 10 grams it means that one cubic metre of that air holds 10 grams of moisture in the form of water vapour.
Absolute humidity is variable and changes from place to place and with change in time.
The ability of an air to hold water vapour depends entirely on its temperature.
The capacity of holding water vapour of an air increases with the increase in its temperature.
For example, at 10°C, one cubic metre of an air can hold 11.4 grams of water vapour.
If the temperature of the same air increases to 21°C, the same volume of air can hold 22.2 grams of water vapour.
Change in temperature and pressure conditions of an air results in the change of its volume and consequently there is change in its absolute humidity.
 Relative Humidity
It is the ratio of the amount of water vapor actually in a volume occupied by air to the amount the space could contain at saturation.
The temperature at which a given sample of air becomes fully saturated is called the dew point or saturation point.
The relative humidity of an air at saturation point is hundred percent.
If the relative humidity of air is less than 100 percent, the air is said to be unsaturated.
The relative humidity increases when the temperature of the air goes down or when more moist air is added to it.
The relative humidity decreases when the temperature of the air increases or when less moist air is added to it.
In order to make it clear that relative humidity is a better measurement of water vapour in atmosphere than absolute humidity, an example can be cited.
Suppose, there is a tumbler containing 250 grams of water, one cannot tell how much portion of the tumbler is filled with water till one knows its maximum water containing capacity.
When one comes to know that the tumbler can contain maximum of 500 grams of water, one can immediately tell that the tumbler is half filled with water.
Likewise, when one measures relative humidity of an air, one not only needs to know about its actual water vapour content but also its total capacity to contain water vapour at that temperature.
Relative humidity is more useful in making predictions about atmospheric conditions.