Chapter 8: Air Masses,
Fronts, and Middlelatitude Cyclones
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Air masses
Fronts
Middle-latitude cyclones
Source Regions
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Air mass – a large body of air with
relatively similar qualities of air. Air is
same temperature in the horizontal
Source regions – regions where air
masses obtain their characteristics
The best way to obtain its characteristic is for
the air to remain over an area for a long time
 Thus, what type of weather system is best for
a source region?
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• Because air sinks in high pressure systems, air stays
in contact with the surface and acquires its temperature
and moisture characteristics.
Source Regions
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High pressure systems are best at source
regions
Air is stagnant, and remains for a long time
 What areas of Earth have good source
regions then?
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Source Regions
Source Regions
Table 8-1, p. 205
Classification
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Air masses classified by temperature and
humidity (cold air masses and dry air
masses
Air masses originating over land start with
c (continental)…over water start with m
(maritime)
Air masses that are Arctic start with (A),
Polar (P), or Tropical (T)
cP (Continental Polar) and cA
(Continental Arctic) Air
Masses
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Continental polar and Continental arctic
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Bitterly cold, dry air masses that originate over
northern Canada and Alaska
Clear, nights and snow covered lands create very
cold air
Moves into the United States by air aloft
Can create lake effect snow when the cold dry
air passes over a “warm” lake. Water
evaporates and creates snow downwind of the
lake
cP (Continental Polar) and cA
(Continental Arctic) Air
Masses
mP (Maritime polar) Air
Masses
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mP Air mass may originate from northern
Asia and become modified as it cross the
Pacific. Ocean adds warmth and moisture
mP (Maritime polar) Air
Masses
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Pacific Air is modified mP as it moves over
mountains
mT (Maritime Tropical Air
Masses)
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Much of California’s winter precipitation
comes from mT air masses
Subtropical air – air originating from the
subtropics
Bermuda high – on the East coast, flow
around the Bermuda high produces mT air
that flows in the Southeast
• mT air brings hot, muggy air to the eastern
US in summer.
mT (Maritime Tropical Air
Masses)
mT (Maritime Tropical Air
Masses)
Stationary Fronts
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Front – a boundary between air masses of
differing densities (another defintion?)
Stationary Fronts
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Stationary front – a front with basically no
movement
Alternating blue triangles and red semicircles
 Winds blow
parallel to front
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Cold Fronts
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Cold front – region where cold, dry air is
replacing warm, moist conditionally unstable air
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Drawn with triangles pointing to the direction of
movement
Cold fronts can be diagnosed in a number of
ways
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Sharp temperature changes
Change in air’s moisture content
Shift in wind direction
Pressure changes
Clouds or precipitation
Cold Fronts
Warm Fronts
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Warm front – boundary where warm air replaces
cold air
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Overrunning – warm air moving above cold air
well ahead of the front
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Drawn as red semicircles pointing the direction of
movement
Cause precipitation to form far ahead of the front
Slope of a warm front is much more gentle
Precipitation can change from snow to rain
Changes across the front are less dramatic than
in cold fronts
Warm Fronts
Occluded Fronts
Occluded front – occurs when
a cold front catches a warm front
Marked by semicircles and
triangles pointing in the same
direction
 Cold occlusion – air behind front
is colder than air ahead
 Warm occlusion – air ahead of a
front is colder than air behind it
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Polar Front Theory
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Polar Front Theory – describes the life cycle of a
mid-latitude storm
Wave cyclone – how a storm develops, in a
wavelike manner
Frontal wave – initial formation of a kink in an
isobar (start of a low pressure system)
Open wave – second stage of development
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Warm sector – region of warm air between the warm
front and the cold front (partly cloudy)
Energy for the storm comes from condenstation and
wind speeding towards the low center
Polar Front Theory
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Mature cyclone – stage when occluded front
initially forms (most intense)
Advanced occlusion – occurs when cold air is
on both sides of
the occluded
front. Dying
storm
Stepped Art
Fig. 8-23, p. 223
Where Do Mid-latitude
Cyclones Tend to Form?
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Cyclogenesis
Any strengthening
of a mid-latitude storm
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Lee-side low
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East of Rockies
Storms that move up
the east coast are
nor’easters
Developing Mid-latitude
Cyclones and Anticyclones
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Convergence and divergence
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If the upper-level low is directly above the
surface low, then the storm will weaken
Jet Streams and Developing
Mid-latitude Cyclones
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Jet streams can help with divergence or
convergence
Jet streak – an area of strong wind
• During World War II, the jet stream was used by
the Japanese to carry balloon bombs across the
Pacific Ocean to North America.