The gaseous form of water, present in the atmosphere in varying amounts. It is an intermediate stage in the hydrological cycle. Water vapour in the atmosphere represents 0.01% of the total water inventory of the Earth. Water molecules have a short residence time in the atmosphere, on average about nine days, though this varies over a very wide range, from a few minutes upwards. Water enters the atmosphere by evaporation, transpiration, and sublimation at the Earth's surface. The concentration of water vapour decreases with increasing altitude up to over 20 km, where some increase may occur. Measures of water-vapour concentration include vapour pressure, the humidity mixing ratio, and the absolute and relative humidity. The physical significance of water vapour in the atmosphere is in its condensation to produce cloud and precipitation (with release of latent heat of condensation, which often accelerates the uplift of rising air), its absorption and scattering of radiation, etc.