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Air Pressure
High and Low Pressure Areas
• High pressure
causes air to sink
• Usually results in
several days of clear
sunny skies
• Air rises in low
pressure areas and
forms water
droplets
• Usually results in
rain and storms
Barometer – tool used to measure air pressure.
High pressure brings mild winds, stable temperatures,
and clear weather.
H
Low pressure brings strong winds, dropping
temperatures and precipitation.
L
Barometric Pressure – tells us when weather will
change.
when it falls - low pressure moving in
Air Pressure
L
Warm air=
expanding or
rising air= leaves
behind L
pressure
Cold
Air=sinking
air= leaves an
area of H
pressure
Air Pressure
when it rises- high pressure moving in
H
Global Circulation and Wind
Systems
Global Air Masses
• North America gets 5 types of air
masses
• Defined by areas of origination
• Highly variable in terms of temperature
and moisture
The Jet Stream guides
the movement of
different air masse
s across the
United States.
 A jet stream forms high in the upper troposphere over the
boundary between two air masses of different temperature.
 The greater the temperature difference between the air
masses, the greater the air pressure difference, and the faster
the wind blows in the jet stream.
 This river of air has wind speeds which often exceed 100 mph,
and sometimes peak over 200 mph.
 Jet streams usually form in the winter, when there is a greater
contrast in temperature between cold continental air masses
and warm oceanic air masses.
Wind is caused by air flowing from high pressure to low pressure. Since
the Earth is rotating, however, the air does not flow directly from high to low
pressure, but it is deflected to the right (in the Northern Hemisphere; to the left in
the Southern Hemisphere), so that the wind flows around the high and low
pressure areas.
The closer the high and low pressure areas are together, the stronger the
"pressure gradient", and the stronger the winds. On weather maps, lines of
constant pressure are drawn (as in the example, above) which are called
"isobars". The closer these lines are together, the stronger the wind.
Air Masses
Air Mass
a huge “bubble” of air that shares the same
temperature and pressure.
(can be big enough to cover several states!)
Air Masses are huge clumps of air that move
around the Earth, bringing changes in…
weather.
Air masses are like the surface they form over.
The air mass shown above formed over land
in northern Canada.
Therefore the air mass will be…
cold and dry.
A polar air mass brings cold air
Cold Air Mass – this is an air mass made up of
cool temperatures and high pressure.
Usually does not hold much moisture
Forms over cold water or land.
H
The pink air mass formed over the
ocean near the equator.
The air mass will be…
warm and moist.
Warm Air Mass – this is an air mass made up
of warm air that has a low pressure.
Usually holds a lot of moisture.
L
Forms over hot land or water.
A tropical air mass brings warm air
Fronts
Where air masses meet and do not mix
When two air masses
meet, they form a…
front.
During a cold front,
a cold air mass
catches up to a
warm air mass
and…
pushes the
warm air up
quickly.
Weather associated
with cold fronts is…
Thunderstorms
See A Cold Front Forming…. Click Here!
The map symbol for a
cold front is a curved
line with small…
blue spikes
When cold air catches up to warm air
and pushes it up and away, what is
formed?...
cold front
From the map above, you can
predict that the weather in the
southern U.S. will be getting…
colder
See A Warm Front Forming…. Click Here!
The map symbol for a
warm front is a curved
line with small…
rounded knobs
When warm air catches up to a cold
air mass, what is formed?...
warm front
When a warm air mass
catches up with a cold
air mass, it forms a…
warm front
The warm air…
slides over the top
of the cold air,
before …
pushing it away.
Weather associated with
warm fronts are….
Clouds and long
periods of steady
rain
From the map above, you can predict
that the weather over the Great Lakes
will be getting …
warmer
Warm
or cold front
stops moving.
Light wind and
precipitation may
occur across the
front boundary
• One front overtakes another
• Cold front typically faster moving
• Can have warm and cold occluded fronts
• Creates Precipitation
Name the Weather Fronts