Transcript Air Masses and Fronts

AIR MASSES AND FRONTS
Chapters 8 and 9
What is an Air Mass?
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A huge body of air characterized by homogeneous
physical properties
 Temperature
 Moisture
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Content
As the air moves out of its region of origin, it will
carry these temperatures and moisture conditions
elsewhere, eventually affecting a large portion of a
continent
What is an Air Mass?
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Because of how big air masses are they can span
many degrees of latitude there will be small
differences in temperature and humidity from one
point to another
Air Mass Weather: the region under an air masses’
influence will probably experience constant weather
conditions
Source Regions
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Areas in which air masses originate
Two criteria to be considered a source region:
 Extensive
and Physically uniform area
 Area characterized by air that is stationary or slowmoving
 Most source regions are subtropical or subpolar
 Middle latitudes are where air masses collide
Classifying Air Masses
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Land vs. Water
 Does
the air mass originate over a large continental
land mass or over a large ocean
 Maritime
(m)
 Continental (c)
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Cold vs. Warm
 Which
 Polar
latitude does the air mass originate from:
(P)
 Arctic (A)
 Tropical (T)
North American Air Masses
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cA: continental arctic
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cP: continental polar
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Warm, Dry
mT: maritime tropical
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Cool, Dry
cT: continental tropical
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Very cold, Very dry
Warm, Wet
mP: maritime polar
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Cool, Wet
Continental Polar (cP)
Continental Arctic (cA)
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Cold and Dry
cP originates over the snow covered regions of
Canada and Alaska
cA originates over the Arctic Basin and the
Greenland ice cap
Very little distinction between the two air masses, cA
is much colder
Continental Polar (cP)
Continental Arctic (cA)
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As wintertime approaches the Sun’s angles are
lower and lead to colder temperatures near the
ground.
Also results in a stable air environment
The air mass is pushed to the United States as
winter progresses at a rapid rate due to little
boundaries to slow it down.
Last freeze in spring and first freeze in fall can be
correlated to movement of air mass south.
Maritime Polar (mP)
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Cool to cold and humid. (relatively mild in
comparison to cP and cA)
Originate over oceans at high latitudes
Influence weather in North Pacific and the
northwestern Atlantic from Newfoundland to Cape
Cod
Because of west to east movement the mP over the
N. Pacific is more influential to U.S. weather
patterns
Maritime Polar (mP)
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During the winter the mP air masses begin as cP air
masses over Siberia
As air moves past the Pacific Ocean it picks up
water vapor through evaporation
As it hits the western coast of N. America it is often
accompanied by low clouds and shower activity
Orographic lifting causes this air mass to stay in the
Northwestern part of U.S.
Maritime Polar (mP) – Pacific
Maritime Polar (mP) – Atlantic
Maritime Polar (mP)
Maritime Tropical (mT)
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Warm to Hot and Humid
Unstable air environment.
Originate over the warm waters of Gulf of Mexico
and Caribbean
These air masses are important to the weather
whenever present because they are capable of
contributing significant precipitation.
Maritime Tropical (mT)
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Air masses from Gulf-Carribean-Atlantic source
region greatly affect weather in the U.S. east of the
Rockies
During the winter mT rarely affects weather, but
during the summer mT air masses affect a wide
area and stay around for a long period of time.
Responsible for hot, humid conditions on East Coast
and Central U.S.
Maritime Tropical (mT)
Continental Tropical (cT)
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Warm to Hot, Dry
Only affects N. America during the summer months
and a very small portion of Mexico and
southwestern U.S.
If it persists for too long a drought might occur.
Fronts
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Boundary surfaces
that separate air
masses of different
densities
Fronts
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A cold front occurs when a wedge of
cold air advances toward the warm air
ahead of it.
A warm front represents the boundary
of a warm air mass moving toward a
cold one.
A stationary front differs in that neither
air mass has recently undergone
substantial movement.
Occluded fronts appear at the surface
as the boundary between two polar air
masses, with a colder polar air mass
usually advancing on a slightly warmer
air mass.
Warm Front
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Forms when warm air advances on a cooler air
mass.
Up to 200 miles wide
Moves very slowly
Slope is gradual
Produces light to moderate precipitation over large
area
Lasts 2-3 days
Warm Front
• Warm fronts have gentler
sloping surfaces and do not
have the convex-upward profile
of cold fronts.
• Surface friction decreases with
distance from the ground,
• as indicated by the longer wind
vectors away from the surface
(a).
• This causes the surface of the
front to become less steep
through time (b).
Warm Front
Warm Front Clouds
Cold Front
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Forms when colder/cooler air advances on a
warmer air mass.
Not as wide as warm front
Has a greater temperature difference btw. air
masses
Moves faster than warm fronts
Precipitation is often more intense but shorter in
duration
Greater potential to produce severe weather
Cold Front
Cold Front
Stationary Front
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Forms when neither air mass advances on the other.
Surface position of front does not move.
Weather of a stationary front:
 Thick
clouds and gentle to moderate precipitation as
long as front stays in your area.
Stationary Fronts
• Nonmoving boundaries are called stationary fronts.
• Although they do not move as rapidly as cold or warm
fronts, they are identical to them in terms of the relationship
between their air masses.
• As always, the frontal surface is inclined, sloping over the
cold air.
Occluded Front
• The most complex type of front is an occluded front or an occlusion, which refers
to closure such as the cutting off of a warm air mass from the surface by the
meeting of two fronts.
• When the cold front meets the warm front ahead of it, that segment becomes
occluded, as shown above.
• The warm air does not disappear, but gets lifted upward, away from the surface.
• The occluded front becomes longer as more of the cold front converges with the
warm front.
Occluded Front
• Eventually, the cold front completely overtakes the warm front, as
shown above, and the entire system is occluded.
• In this occlusion, the air behind the original cold front was colder than
that ahead of the warm front. This is an example of a cold-type
occlusion.
Occluded Front
Occluded Front