The water vapour is the major radiative and dynamic parameter in the atmosphere. Its concentrations vary highly in space and time, with the tropospheric water vapor being determined by the hydrological cycle processes, namely the evaporation, condensation and precipitation and by large-scale transport processes. Specific humidity decreases rapidly with pressure (following an exponential function) and with latitude. In particular, the variability of the H2O concentration shows a bimodal distribution: it’s very small in the equatorial region and poleward, relatively small in stratosphere and shows a maximum in the subtropics of both hemispheres. The concentration of H2O in the lower stratosphere is controlled by the temperature of the tropical tropopause, and by the formation and dissipation of cirrus. The water vapor can condense into water droplets when it has a particle to condense upon. The atmosphere continuously contains aerosol particles ranging in size from ∼10−3 to ∼20 μm. These aerosols are known to be produced by natural processes (volcanic dust, smoke from forest fires, particles from sea spray, windblown dust, and small particles produced by the chemical reactions of natural gases) as well as by human activity (particles directly emitted during combustion processes and particles formed from gases emitted during combustion). Some aerosols are effective condensation and ice nuclei upon which cloud particles may form. For the hygroscopic type, the size of the aerosol depends on relative humidity. Thin layers of aerosols are observed to persist for a long period of time in some altitudes of the stratosphere.
Clouds are global in nature and regularly cover more than 50% of the sky. There are various types of clouds. Cirrus in the tropics and stratus in the Arctic, and near the coastal areas are climatologically persistent. The microphysical composition of clouds in terms of particle size distribution and cloud thickness varies significantly with cloud type. Clouds can also generate precipitation, an event generally associated with midlatitude weather disturbances and tropical cumulus convection.