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What is the water
cycle? The
constant movement of
water between the
atmosphere, the land
surface, and
underground is
called the water
cycle, or hydrologic
cycle. This movement
supplies all the
water needed to
support life.
The water cycle
is made up of five
processes —
condensation,
precipitation,
infiltration, runoff, and evaporation.
Water vapor
condenses to form
clouds, which result
in precipitation
when the conditions
are suitable.
Precipitation falls
to the surface and
infiltrates the soil
or flows to the
rivers and streams
as runoff. Surface
water evaporates,
returning moisture
to the atmosphere,
which will again
condense to form
clouds.
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Water on
Earth is
recycled
over and
over
again.
In fact,
there is
about
the same
amount
of water
on the
Earth
now as
there
was when
the
dinosaurs
roamed
our
world. |
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Eighty-eight
percent
of all
water
entering
the
atmosphere
originates
from the
ocean
between
60
degrees
north
and 60
degrees
south
latitude.
Most of
the
water
evaporated
from the
ocean
returns
directly
back to
the
ocean.
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Condensation is the
process of water
changing from a
vapor to a liquid.
As the water vapor
rises and cools,
tiny water particles
collect and form
clouds.
You can see
condensation
whenever you take a
cold soda from the
refrigerator and set
it in a room. Notice
how the outside of
the soda can
“sweats?” The water
doesn’t come from
inside the can; it
comes from the water
vapor in the air. As
the air cools around
the can, water
droplets form.
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Precipitation is
water being released
from clouds as rain,
sleet, snow, or hail.
Precipitation begins
after water vapor,
which has condensed
in the atmosphere,
becomes too heavy
and falls.
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A portion of the
precipitation that
reaches the Earth’s
surface seeps into
the ground through a
process called
infiltration. The
amount of water that
infiltrates the soil
varies with the
degree of the land
slope; the amount
and type of
vegetation, soil and
rock; and whether the
soil is already
saturated by water.
The more porous, or
loose, the soil at
the surface is, the
more infiltration
occurs.
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 Precipitation
that reaches the
Earth’s surface but
does not infiltrate
the soil is called
runoff. Runoff also
comes from melted
snow and ice. Runoff
flows to the streams
and rivers, and
eventually to the
ocean.
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Evaporation
occurs when radiant
energy from the sun
heats water, causing
the water molecules
to become so active
that some of them
rise into the
atmosphere as vapor.
On a lake such as
Lake Conroe,
evaporation can
cause a water loss
of up to 180 million
gallons on a hot,
summer day.
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