Water in Earth’s
Atmosphere
Phase changes
• To evaporate, water must
absorb energy. This energy
goes into changing the liquid
particles that are close
together to a gas where
particles are far apart and
moving fast.
• Water vapor carries this
energy into the atmosphere
with it.
Phase changes
• When water condenses it
releases this energy into
the atmosphere to be used
by other particles which
can use it to increase their
speed and pressure.
Relative Humidity
• At any given time, there is a certain
amount of water vapor in the air.
• Air can hold more water when it is hot
and less when it is cool.
• Relative humidity measures how much
water vapor is in the air compared to
how much water vapor the air can hold
at the given temperature.
Relative Humidity
• Relative humidity is determined by using
a sling psychrometer. One of the
thermometers has a wick covering the
bulb which is wet. When this is swung,
the water evaporates and the wet
thermometer’s reading drops.
Relative Humidity
• Comparing the two temperatures tells
how much water is present in the air
compared to the maximum possible.
• If the readings are far apart, what does
that indicate about the RH?
The Water Cycle
• Energy and water are constantly moving
to and from the atmosphere through
the water cycle.
• http://earthguide.ucsd.edu/earthguide/
diagrams/watercycle/ (animation)
• http://ga.water.usgs.gov/edu/watercycl
e.html (USGS water cycle in different
languages)