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GEU 0047: Meteorology
Lecture 11: Air Masses and Fronts
Air Masses
• the air with similar properties of temperature and moisture
which covers a large area
• Semi-permanent structures in the atmosphere
Air Mass Classifications
• Moisture content
– Dry
– Moist
Continental (c)
Marine (m)
• Temperature
– Cold
– Warm
Polar
(P),
Tropical (T),
Arctic (A)
• Results in 5 basic types
– cA
– cP
– cT
–
Continental Arctic
Continental Polar
Continental Tropical
mP
mT
Marine Polar
Marine Tropical
Global air mass source regions
Air Mass Source Regions
Source Region Profiles
Arctic Air (cA)
• Arctic Air Mass
– Bitterly Cold and Usually Very Dry
– Develops over the snow or ice covered regions of continents
– Similar to polar air masses but usually develops in regions
north of 60N where there is little or no insolation during the
winter.
Continental Polar (cP)
• Continental Polar Air Mass
– Cold, Dry
– Develops over the interior of
• North America -- Central Canada -- Siberia
Marine Polar (mP)
• Marine Polar Air Mass
– Cold, Moist
– Develops over the cold ocean waters of the North
Pacific and North Atlantic
• Usually associated with conversion of cP siberian air to mP
which is moistened and warmed from the Japanese “Gulf
Stream Current”
• Moves across the N. Pacific to affect N. California and the
Pacific NW
• Responsible for fueling “Nor-easters”
Tropical (T)
• Continental Tropical (cT)
– Hot, Dry
– Develops over the deserts of Mexico and
the Southwestern United States
– Very unstable because of heat and
convection, but cloudless because of lack
of moisture.
• Marine Tropical (mT)
– Warm, Humid
– Develops over the tropical and subtropical
oceans and the Gulf of Mexico
Example Air Masses
cP
mT
Air Mass Modification
• Air masses do not always remain over their source
regions.
• If the air moves over a region that is different from
where it originated, the air mass will be modified, or
changed, by the land/ocean surface that the air is
travelling over.
• Changes include: warming, cooling, adding or
reducing moisture content
Air Mass Modification
cP
The cP air mass will
be warmed by the
warmer land that it
passes over.
Warmer
Land
Air Mass Invasion
Air Mass Invasion
Air Mass Invasion
Changes in Stability
Cold air mass moves over a warmer surface.
cP
The lower layers are
warmed and may
become unstable.
Warm, Moist
Stability Changes
Dry Adiabat
Moist Adiabat
Environment
Cold air mass moving over a warmer surface.
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Temperature (C)
Sounding Before Modification:
Conditionally Unstable
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Temperature (C)
After modification:
Absolutely Unstable
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Stability Changes
Warm air mass moves over a colder surface.
mT
Warm, Moist
The lower layers are
cooled and tend to
stablize.
Cold
Stability Changes
Dry Adiabat
Moist Adiabat
Environment
Warm air mass moves over a colder surface.
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Temperature (C)
Sounding Before Modification:
Conditionally Unstable
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Temperature (C)
After modification:
Absolutely stable
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(m)
Frontal Boundary Symbols
Fronts
Fronts
• Front
– A narrow transition zone between air
masses of differing densities.
– The density differences usually arise
from temperature differences.
– Density differences may be a result of
humidity differences (summer).
• A front is the boundary or transition zone
between different air masses.
Air Mass Invasion
• The transition zone is usually narrow (15 to 200 km
in width).
• Frontal Boundaries may be over 1000 km long.
• On maps, we draw the frontal boundary on the
warm side of the transition zone.
Idealized Cold Front
• Cold Front
– A boundary that moves in such a way
that the colder (more dense) air
advances and displaces the warmer
(less dense) air.
– The largest temperature differences
are normally associated with cold
fronts.
Idealized Cold Front
Cross-sectional Characteristics
Warm Air
Cold Air
Strong Lifting near the
frontal boundary can
produce convective
clouds.
Idealized Cold Front
Idealized Cold Front
• Precipitation:
– Located on either side of the front.
– Convective, showery in nature
– If warm air is stable
• Nimbostratus, Altostratus
– If warm air is unstable
• Cumulus, Cumulonimbus, Thunderstorms
Weather conditions associated with a cold front
Weather
Phenomenon
Prior to the Passing of the Contact with the
Front
Front
After the Passing of the
Front
Temperature
Warm
Cooling suddenly
Cold and getting colder
Atmospheric
Pressure
Decreasing steadily
Leveling off then
increasing
Increasing steadily
Winds
South to southeast
Variable and gusty
West to northwest
Precipitation
Showers
Heavy rain or snow,
Showers then clearing
hail sometimes
Clouds
Cirrus and cirrostratus
changing later to cumulus
and cumulonimbus
Cumulus and
cumulonimbus
Cumulus
Idealized Warm Front
• Warm Front
– A boundary that moves in such a way that
the colder (more dense) air retreats and is
replaced by the warmer (less dense) air.
– The warm fronts tend to have weaker
temperature gradients.
Idealized Warm Front
Weather conditions associated with a warm front
Weather
Phenomenon
Prior to the Passing of the Contact with the
Front
Front
After the Passing of the
Front
Temperature
Cool
Warming suddenly
Warmer then leveling off
Atmospheric
Pressure
Decreasing steadily
Leveling off
Slight rise followed by a
decrease
Winds
South to southeast
Variable
South to southwest
Precipitation
Showers, snow, sleet or
drizzle
Light drizzle
None
Clouds
Cirrus, cirrostratus,
altostratus, nimbostratus,
and then stratus
Stratus, sometimes
cumulonimbus
Clearing with scattered
stratus, sometimes
scattered cumulonimbus
Occluded Front
• Cold fronts typically move faster than warm fronts.
• Cold fronts can catch up and “overtake” a warm
front.
• Depending on the different air masses, there are two
types of occlusions:
– Cold type occlusion
– Warm type occlusion
Cold-type Occlusion
when the air behind the front is colder than the air ahead of the front.
Warm Air
Cold Air
Cool Air
Cold type occlusions are common on the east coast of continents and generally form
when the continental polar or arctic air collides with maritime subtropic air.
Warm-type Occlusion
When the air behind the front is warmer than the air ahead of the front
Warm Air
Cool Air
Cold Air
Warm type occlusions are common on the west coast of continents and generally form
when maritime polar air collides with continental polar or arctic air.