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Chapter 8: Air Masses,
Fronts, and Middlelatitude Cyclones
By the end of this chapter you
should:


Know the different air masses


Understand cold, warm, stationary, and
occluded fronts

Have an understanding on how storms
form


Source Regions

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?
• Because air sinks in high pressure systems, air stays
in contact with the surface and acquires its temperature
and moisture characteristics.
Source Regions

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?
Source Regions
Classification



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)
Table 8-1, p. 205
cP (Continental Polar) and cA
(Continental Arctic) Air
Masses
Continental polar and Continental arctic
 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

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

Pacific Air is modified mP as it moves over mountains
mT (Maritime Tropical Air
Masses)



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

Front – a boundary between air masses of differing
densities (another defintion?)
Stationary Fronts

Stationary front – a front with basically no movement
 Alternating blue triangles and red semicircles
 Winds blow
parallel to front
Cold Fronts
Cold front – region where cold, dry air is replacing warm,
moist conditionally unstable air
 Drawn with triangles pointing to the direction of
movement
Cold fronts can be diagnosed in a number of ways

Sharp
temperature changes
Change
Shift
in air’s moisture content
in wind direction
Pressure
Clouds
changes
or precipitation
Cold Fronts
Warm Fronts
Warm front – boundary where warm air replaces cold air
 Drawn as red semicircles pointing the direction of
movement
 Overrunning – warm air moving above cold air well
ahead of the front
 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
Polar Front Theory


Polar Front Theory – describes the life cycle of a midlatitude 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

sector – region of warm air between the warm
front and the cold front (partly cloudy)
Warm
Energy
for the storm comes from condenstation and
wind speeding towards the low center
Polar Front Theory


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
Where Do Mid-latitude
Cyclones Tend to Form?



Cyclogenesis
 Any strengthening
of a mid-latitude storm
Lee-side low
 East of Rockies
Storms that move up
the east coast are
nor’easters