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 The
total amount of water on the earth and in its atmosphere does
not change, but we have all seen rain and rivers flow, so
there must be a motion that transfers water in a never-ending
cycle. This cycle is called the 'water cycle' or the 'hydrological
cycle'.
Most of the earth's
water is in the sea, about 97.5 % of the total. The rest is
on land or in the air. To start the cycle, the water must
get from the sea to the earth's atmosphere. This happens by
'evaporation', the water becomes 'water vapour' which is a
gas. It is evaporation that dries clothes and puddles.
 Plants
also play an important role in converting water into water
vapour. Plants transport water from the soil to their leaves
to supply nutrients. Water evaporating into the atmosphere
from their leaves is called 'transpiration'.
Once in the air, the
water vapour is blown by winds and distributed through the
atmosphere. Generally as air rises it cools, as it does so
it loses its capacity to hold as much water. The water vapour
condenses to form the water droplets of clouds. When there
is enough water in the air, it rains, and the water returns
to earth. The water does not always come to earth as rain
though - sometimes it is snow, hail or sleet.
Back on the earth's
surface, the water continues its journey. If the water falls
on land, it could make its way back to the sea as 'runoff'
flowing over the land. If the water is soaked up by the ground,
then it can flow underground back to the sea. The water could
also fall onto a glacier as snow or ice. Once there it will
slowly melt and eventually return to the sea.
 All
this does not have to happen quickly of course. Water can
stay under the ground for 10,000 years, and in a glacier for
40 years. So the water cycle can be more of a slow amble than
a sprint.
The distribution of
water throughout the earth is not uniform. Some places have
far more rainfall than others. There are many reasons for
this, such as where the mountains are and where the prevailing
winds blow. This rainfall distribution partly explains the
differences in vegetation, why some areas are deserts, and
some rainforests…and why the UK is neither.
Other features in
the Weather Basics series
Air Pressure
Clouds
Day and Night
Fog and Mist
Fronts
Frost
High Pressure
Humidity
Jet Streams
Lightning
Low
Pressure
Mirages
Northern Lights
Rain or Showers?
Thunder
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