3. What is the Water Cycle?
Water covers more that 70% of Earth’s surface
and most of it is salt water
Some of Earth’s water is fresh water that is
frozen as glaciers and ice caps
Groundwater is fresh water that is below the
surface
◦ It collects as rain seeps into soil, filling spaces
between soil and rock
Our supply of water never runs out because of
the water cycle
◦ Water cycle: the constant movement of water from
Earth’s surface to the atmosphere and back again
4.
5. Evaporation
Evaporation: the change
from the liquid to gaseous
state
When water is heated,
particles of the liquid
move from the surface of
the water to the air above
◦ These particles are always
crashing into each other
and gain more energy
◦ The warmer water is, the
faster it evaporates
Earth’s ocean is the
largest source of water
for evaporation
6. Condensation
Dew on blades of grass is an example
of condensation
When water vapor in the air cools, it
condenses
◦ Remember that condensation is the
change from a gas to a liquid
In the lower atmosphere, air gets
cooler the higher it goes and as it rises
it condenses into clouds
Clouds often form when air pressure
lowers and warm air rises
◦ Warm humid air near the ground rises
and condenses once it reaches the cool
air above
The water vapor condenses when it
reaches the surface of those tiny bits
of dust in the air
7. Precipitation
Rain, snow, sleet, freezing rain, and
hail are all types of precipitation that
falls from the clouds
◦ The type of precipitation depends on
the temperature of the air between the
clouds and the ground
Rain
◦ Often starts in the clouds as snow
◦ If the snow falls through air that is
above freezing (hotter temperature) on
its way to the ground, it melts
Snow
◦ Snow starts on the clouds as snow
◦ If the snow falls through air that is
below freezing (a colder temperature)
on its way to the ground, it remains
snow
8. Sleet
◦ Starts as snow
◦ The snow melts into rain as it falls
through warmer air below the clouds
◦ If there is a layer of freezing air
between the warm air and the
ground, the rain freezes again
Freezing Rain
◦ Forms almost the same way as sleet
◦ The only difference is the layer of
freezing air near the ground is
thinner
◦ When the rain falls through the thin
layer of freezing air, there isn’t
enough time for it to refreeze
It becomes supercooled water (a liquid
even though its temperature is below
freezing)
9. Hailstones
◦ Round lumps of ice
◦ Start out as small pieces
of ice in storm clouds
◦ Winds in the clouds
bump these ice pieces up
and down and the cloud
droplets freeze onto the
pieces of ice
◦ These pieces become
bigger and bigger and
eventually fall from the
clouds
