Hydrological Cycle
The water of the universe always changes from one state to another in a cyclic order under the
influence of the sun. The water from the surface source like lakes, rivers, ocean etc. converts to vapour by evaporation. These vapours get condensed due to the sudden fall in temperature and
pressure and thus clouds are formed. These clouds again causes precipitation (i.e. rainfall). Some of
the vapours are converted to ice at the peak of the mountains. The ice again melts in summer and
flows as rivers to meet the sea or ocean. These processes of evaporation, precipitation and melting of
ice go on continuously and thus a balance is maintained in the atmosphere. This earth's water circulatory
system (from the stage of evaporation from ocean to the final return to the ocean) is called hydrological
cycle.
If E is the evaporation, P is precipitation and R is run-off, then the hydrological cycle is expressed
by the relation,
p= E+ R
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