Transcript Fronts and Air Masses

Air masses
An air mass is a relatively
homogenous large mass of air in
terms of temperature and moisture
characteristics.
Four air masses commonly affect our
region:
cP: continental polar
cT: continental tropical
mP: maritime polar
mT: maritime tropical
What do these regions have in
common? Why?
Air mass modification
Continental polar (cP)
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Only occurs in the northern hemisphere at
middle and high latitudes
More dominant in the winter
Can be extremely cold and dry
Generally stable (cold days, bright blue
skies)
– However, where cP air masses meet moist air
masses, the weather conditions can become
severe
Maritime polar (mP)
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Source areas are over the Pacific and
Atlantic Oceans in the mid and high
latitudes
Generally produce cool and unstable
conditions
Maritime tropical (mT)
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Originates in the tropics in the Atlantic
and Pacific Oceans
Pacific Ocean mT air mass contains
less moisture than the Atlantic mT air
mass
– Also more stable
Air mass modification
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Since air masses move, they can migrate
over areas and inherit characteristics from
those areas
– e.g. the Great Lakes
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In winter cold cP or cA air can move over
the lakes and pick up moisture
The moisture is precipitated in the lee of
the lakes as snow
– Lake effect snow
Types of fronts
WARM
STATIONARY
COLD
OCCLUDED
A front separates two different air masses.
Cold fronts
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Occurs between advancing cP or mP air
and warmer air masses
Warm air is forced to rise upwards against
the colder denser air, forming cumulus and
cumulonimbus clouds
Cold front
symbol
Cloud types with a cold
front
Stratocumulus These cloud types
Cirrus
usually occur ahead of a
cold front.
In which order would
you expect them to
occur as a front
approaches?
Cumulonimbus
Altocumulus
Identifying a cold front’s
passage
Precipitation generally falls in the
vicinity of the cold front.
Before
Temperature Warm
Dew Point
High
Winds
SSW
Pressure
Fall
During
Sharp fall
Sharp fall
Shift
Lowest
After
Slight fall
Slight fall
W to N
Rise
Warm fronts
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Warm front symbol
Occurs when warm air advances over
colder air
Cloud types with a warm
front
Nimbostratus
These cloud types
usually occur
ahead of a warm
front.
Cirrus
In which order
would you expect
them to occur as
a front
approaches?
Altostratus
Cirrostratus
Identifying a warm front’s
passage
Precipitation generally falls ahead of
the warm front.
Before
Temperature Rising
Dew Point
Rising
Winds
E to SE
Pressure
Fall
During
Rising
Rising
Shift
Lowest
After
Warm
High
S to SW
Rise
Cold Fronts and Warm
Fronts
Dry Line
Mid-latitude cyclones
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Migrating low-pressure cells
Contain converging, rising air that
spirals towards the center of the
system, in a counterclockwise direction
Also called wave cyclones
L
Life cycle of a mid-latitude
cyclone
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Takes 3-10 days
Goes through 4 stages
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Cyclogenesis
Open stage
Occluded stage
Dissolving stage
Cyclogenesis
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Occurs where cold
air masses and
warmer air masses
collide
Where air
converges at
ground level it must
diverge aloft
Common at the
polar front, the gulf
coast, eastern areas
of mountain
Open stage
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Air to the south of
the cold air forces
it’s way northward
along a warm front
Air from the
north/west forces
its way southward/
eastward along a
cold front
Occluded stage
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Cold air “catches up
on itself”
When this happens
it forces warm air
aloft with only cold
air below
Most occluded
fronts run from the
north to the south
Dissolving stage
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Usually all warm air
is now aloft and all
cold air is at ground
level
This will eventually
dissipate causing air
to mix
Open stage
cyclogenesis
Occluded stage