• Precipitation is water in liquid or solid forms, falling to the earth.
• It always precedes condensation or sublimation or a combination of the two and is primarily associated with raising air.
• In the same way that isotherms and isobars are used to show temperature and pressure distribution respectively, isohyets indicate rainfall distribution.
• An isohyet is a line connecting points with equal values of rainfall.
• Change of state from water vapour to liquid water is
• When moist air comes in contact with cool surfaces, it may be cooled to the point where its capacity to hold water vapour is exceeded by the actual amount in the air.
• Part of the water vapour then condenses into liquid form on the cool surface, produce dew.
• When this happens, the latent heat of vaporisation, in this process, called the latent heat of condensation is released.
• At temperatures below freezing, water may bypass the liquid form in its change of state.
• When dry air with a temperature well below freezing comes in contact with ice, molecules of ice (H₂O) pass directly into the vapour state by the processes of sublimation.
• Under proper weather conditions, water vapour condenses over nuclei to form tiny water droplets of sizes less than 0.1 mm in dia.
• The nuclei are usually salt particles or products of combustion and are normally available in plenty.
• Wind speed facilitates movement of clouds while its turbulence retains water droplets in suspension.
• Precipitation results when water droplets come together and coalesce to form larger drops that can drop down.
• Considerable part of this precipitation gets evaporated back to the atmosphere.