Humidity is a general term referring to the water vapour content of air at any one time and place. This term is not used to describe liquid droplets of fog, cloud, or rain. Since water vapour is dry like -any other gas, it has nothing to do with dryness or wetness of the atmosphere.
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Although there is spatial as well as temporal variation in the quantity of water vapour present in the atmosphere, the total amount of atmospheric water remains nearly constant.
Of the various components of atmosphere, water vapour constitutes only a small fraction varying from nearly zero per cent to about 4 per cent by volume. However, the meteorological significance of even this very small percentage of water in the air cannot be over-emphasized.
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In fact, in the heat budget as well as in day- to-day weather changes that we observe, atmospheric moisture plays a very important role.