Science High School Reviewer | Grade 4 Science | Earth Science | Weather and Climate | Lesson 22: The Water Cycle

Why does water change state?
Water moves from Earth’s surface into the
atmosphere. Then it moves back to the surface.
Water changes state as it moves.

Water cycle diagram

Evaporation
Water seems to disappear when it evaporates
(ee•VAP•uh•rayts). Evaporation is the term for
a liquid changing slowly to a gas. Liquid water
doesn’t really disappear. It just changes to a gas.
Water vapor is water in the gas state. You
cannot see water vapor, but it is part of the air
around you.

Water is always evaporating from oceans,
streams, lakes, rivers, and ponds. The Sun’s heat
causes particles of water at the surface to move
rapidly. The more heat energy they take in, the
faster and farther apart they move. Some of the
particles rise into the air as a gas—water vapor.

Condensation
As particles of water vapor rise
into the air, they cool. The particles
lose energy. They move more slowly.
High in the atmosphere, the water
vapor condenses (kuhn•DEN•sez) to
liquid water. Condensation is when
a gas changes to a liquid.

Dew is a familiar kind of
condensation. Dew forms when
water vapor cools and condenses
onto a surface. Have you ever seen
drops of water cover the grass on a
cool morning? Those drops are dew.
Water vapor can also condense
onto dust particles in the air. The
tiny drops, or droplets, form clouds.
A cloud is a group of water droplets
in the atmosphere. The droplets are
pure water in liquid form.

Precipitation
Inside a cloud, small water
droplets may join together and form
larger ones. If it is very cold, some
droplets freeze into ice. To freeze is
to change from a liquid to a solid.
The droplets and bits of ice
grow larger and heavier. When
they are too heavy, they fall to
Earth’s surface. Precipitation
(pri•sip•i•TAY•shuhn) is the term
for water that falls from clouds
down to Earth.

Where does water go?
By now you know a lot about
water. You know that water can be
found in many places. You know it
has three different states.
Water is always moving from
place to place, in one form or
another. The water cycle is the
movement of water between Earth’s
surface and the air. Evaporation,
condensation, and precipitation help
water move through the cycle. The
diagram shows you how.

In the Air
In the water cycle, water changes
state between liquid, gas, and solid.
The Sun is the energy source for this
cycle. The Sun’s energy causes water
to evaporate from lakes, oceans,
and other bodies of water. Water
also evaporates from the leaves of
plants. This is called transpiration
(trans•puh•RAY•shuhn). As it rises in
the air, the water vapor condenses.
Clouds form. During precipitation,
water falls from the clouds over
land and water.

On and Below the Ground
Precipitation can fall as rain,
snow, sleet, or hail. When it rains,
water flows over Earth’s surface
as runoff. Runoff gathers in lakes,
oceans, rivers, and streams. Over
time, water collects in glaciers and
ice caps.

Rainwater also soaks into the
ground. Plants take up some of the
water from soil. The rest collects in
small cracks and spaces below the
ground. This groundwater can stay
above the bedrock, flow, or slowly
evaporate.

What are some
types of clouds? (diagram with pictures)

Scientists classify clouds into
three main types based on how and
where they form. Clouds give clues
about the weather you can expect.

Cumulus
Cumulus (KYEW•myuh•luhs)
clouds are puffy, white clouds that
look like cotton balls. They often
appear in fair weather.
You may have seen clouds
grow dark before a rainstorm. If a
cumulus cloud becomes dark and
thick, it is called a cumulonimbus
(kyew•myuh•loh•NIM•buhs)
cloud. This kind of cloud causes
precipitation.

Stratus
Stratus (STRAT•uhs) clouds form
in layers. The layers look like sheets
or blankets. Stratus clouds are
often the lowest clouds in the sky.
Dark stratus clouds can produce
precipitation.
Cirrus
Cirrus (SIR•uhs) clouds look thin,
wispy, or feathery. Cirrus clouds
are usually found very high in the
sky. They indicate changes in the
weather.
Observing Clouds
In the diagram to the right, you
can see some other cloud types.
Often, you can observe more than
one cloud type in the sky at one
time.

What are other forms
of precipitation?
Rain is just one form of
precipitation. Water can change
state as it moves through the air.
When this happens, other kinds
of precipitation may fall.
Snow
When water reaches a
temperature below 0ºC (32ºF), it
freezes into ice. Remember, to freeze
is to change from a liquid to a solid.
Bits of ice can collect in a cloud. If
they get too heavy, they fall as snow.
Snow may melt as it falls to the
ground. To melt is to change from
a solid to a liquid. Melting happens
when sunshine or warm air heats
the icy snowflakes. The heat makes
the snow change to rain.