Transcript All About the Water Cycle - Mercer Island School District

Are you drinking the
same water a dinosaur
drank?
Could you be drinking
the same water a saber
tooth tiger lapped up?
Water on earth moves in a
continuous cycle. This is called
THE WATER CYCLE. There
is about the same amount of
water on earth now that there
was when the dinosaurs
roamed our planet.
What do you think? Is
the water we have on
earth today the same
water that was here
millions of years ago?
The cycle starts when the sun's heat provides
energy to evaporate water from the surface.
Then, winds lift the water vapor from the ocean
over the lands into the atmosphere. The water
vapor cools and condenses into droplets to form
clouds, and when the conditions are just right, the
clouds release water as rain or snow. About 80%
of the rain falls in the oceans, but the rest falls
onto land. Rivers and streams collect water from
the ground and return it to the ocean so the whole
cycle can start all over again. The water cycle
never ends because the salty ocean water
constantly supplies fresh water to the continents.
One process which transfers water from the ground back to
the atmosphere is evaporation. Evaporation is when water
passes from a liquid phase to a gas phase. The gas phase of
water is water vapor. Water vapor is invisible. Rates of
evaporation of water depend on factors such as solar
radiation, the temperature, humidity, and wind.
Water that is held in lakes and rivers evaporates directly into
the atmosphere, but some of the water in the ground may
also be returned to the atmosphere by way of
evaporation through the
Evaporation
soil surface. Of course,
the ocean is the greatest
source for water
evaporated into the
atmosphere.
Condensation is the change of water from its gaseous form
(water vapor) into liquid water. Condensation generally
occurs in the atmosphere when warm air rises, cools and
looses its capacity to hold water vapor. As a result, excess
water vapor condenses to form cloud droplets.
Precipitation is the main way for transporting water from
the atmosphere to the surface of the earth. There are
several forms of precipitation, including rain, hail, snow,
sleet, and freezing rain.
Groundwater is all the water that has gone through the
earth's surface and is found in one of two soil layers. The
one nearest the surface is the "zone of aeration", where
gaps between soil are filled with both air and water. Below
this layer is the "zone of saturation", where the gaps are
filled with water.
Transpiration is the process by which plants return
water to the atmosphere. After absorbing water from
the ground, plants release water through their leaves.
Transpiration helps plants stay cool, in the same way
perspiration keeps humans and animals cool.
Runoff is the movement of land water to the oceans, mostly
in the form of rivers, lakes, and streams. Runoff consists of
precipitation that hasn’t evaporated, transpired or
penetrated the surface to become groundwater. Even the
smallest streams are connected to larger rivers that carry
billions of gallons of water into oceans worldwide.
The Sun's heat
provides energy to
evaporate water from
the Earth's surface
(oceans, lakes, etc.).
Evaporation occurs when
heat acts on water until
the temperature becomes
warm enough to change
water into a gas.
Condensation is
the cooling of
water vapor until
it becomes a
liquid. As the dew
point is reached,
water vapor
forms tiny visible
water droplets.
When these
droplets form in
the sky and other
atmospheric
conditions are
present, clouds
will form.
When the clouds meet cool air over land, precipitation
(rain, sleet, or snow) is triggered, and water returns to the
land (or sea).
A long, long, long time later, two very bored drops of
water emerge from the bottom of the glacier.
Most of the water flows downhill as runoff (above ground
or underground), eventually returning to the seas as
slightly salty water.