The term humidity is generic for water vapor content of the air.
Water vapor is simply water existing as a gas.
It is present everywhere in the lower atmosphere to some degree. A useful way to examine the amount of any gas in the atmosphere is to consider the contribution it makes to the total pressure of the air. The pressure exerted by any gas is called the partial pressure . The diagram below illustrates the concept of partial pressures for several important gases in the atmosphere. One of the more common ways of expressing humidity is to consider the contribution of water vapor towards the total atmospheric pressure.