Download Chapter 20.1

CHAPTER 20.1
Air Masses

Severe storms can be one of
nature’s most destructive forces.

During spring time there are
tornadoes, which or short, violent
storms.

During late summer and early fall
there are hurricanes – have strong
winds and heavy rains.

Thunderstorms produce heavy rains,
loud thunder, and lightning.

In many places in the US summer will have heat waves, several
days with high temperatures and high humidity. This will be
followed by storms and then cooler weather for a few days.

Winter cold spells have periods of very cold temperatures with
clear skies followed by cloudy, snowy, warm days.
AIR MASSES AND WEATHER

The weather before takes place
because of air masses.

An air mass is a large body of air
that has similar temperatures and
amounts of moisture at any altitude.

Air masses can very large so it may
take several days for an air mass to
move over an area. This gives fairly
constant weather.
AIR MASSES

When an air mass moves out of an
area where it formed it takes the
temperature and moisture it has
with it.

What this means is that as an air
mass moves it will become warmer
as it moves southward.

As an air mass moves the
characteristics and weather in the
air over the air mass changes.
MOVEMENT OF AIR MASSES

The source region is the area where
an air mass forms and where it gets
is temperature and moisture.

Air masses are named according to
their source regions.

Polar air masses form at high
latitudes near the poles. They tend
to be cold.

Tropical air masses form at low
latitudes. They tend to be warm.
CLASSIFYING AIR MASSES

Air masses are also classified by the
surface over which they form.
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Continental air masses form over
land and tend to be dry.

Maritime air masses form over water
and will be humid.

There are 4 basic types of air
masses: continental polar (cP)
which is dry and cool.

Continental tropical (cT) which is
dry and warm or hot.

Maritime polar (mP) which is colder.

Maritime tropical (mT) which is
warmer.

Most of the weather in North
America is influenced by
continental polar and maritime
tropical air masses (especially east
of the Rocky mountains)
WEATHER IN NORTH AMERICA

Continental polar air masses are
usually cold and dry in the winter
and cool and dry in the summer.
They usually do not bring large
amounts of rain. They can bring
snow in areas with large lakes. This is
called lake-effect snow.
CONTINENTAL POLAR AIR MASSES

These air masses usually have warm
air with a lot of moisture that is
unstable.

This air brings a majority of the
precipitation in the eastern United
States.

This type of air mass also gives very
high temperatures and high
humidity in the summer in the
central and eastern United States.
MARITIME TROPICAL AIR MASSES

In the North Pacific we get air
masses that begin in Siberia. This air
moves over the ocean so it
becomes mild, humid, unstable air.
This air will produce heavy rain or
snow.

These air masses also start in the
North Atlantic and move into the
northeastern United States. This air
brings in storms that has snow and
cold temperature – a nor’easter.
MARITIME POLAR AIR MASSES

This type of weather does not affect
much of the United States. It can
cause extremely hot, drought-like
conditions in the Great Plains in the
summer. This also gives what is
called an Indian Summer – where
you have unseasonably warm and
mild weather.
CONTINENTAL TROPICAL AIR MASSES