Humidity refers to the amount of water vapour present within the atmosphere. The actual amount of water vapour per unit volume is called absolute humidity. Relative humidity is the amount of water vapour in a given volume of air.

Heat transfer which occurs during evaporation, condensation, freezing and melting.

The layer of water that cover the earth from a maximum depth of more then 11km in the oceans to shallower and less extensive bodies of water in lakes and river.