Download Air Masses

Chapter 12
Air masses and fronts
Why are cold fronts
usually associated
with showery
weathers?
How ca warm fronts
cause freezing rain and
sleet to form over a
vast area during the
winter?
And how can one read
the story of an
approaching warm
front by observing its
clouds?
Air Masses
• Definition: Large body of air with similar
temperature and moisture characteristics in
any horizontal direction at any given altitude.
• Cover many 1000's of
square kilometers
• Part of weather
forecasting is
determining air mass
characteristics, how
they may be modified,
and their movement.
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Air Mass Source Regions
• Regions where air mass originate are know as Source
Regions.
• Source regions:
– flat, uniform composition
– light winds
• So, where are the
good source regions?
– Arctic
– Tropics
• Air masses tend to
clash and interact in
the middle latitudes.
Air mass source regions and their paths.
Air Mass Classification
•
•
•
•
Four general categories according to source region (see table)
Extremely cold cP air is sometimes denoted at cA
Extremely hot, humid mT air is sometime denoted by mE
Air Masses on the move:
– if an air mass is colder than the surface over which it is moving, "k" is
added
– if an air mass is warmer than the surface over which it is moving, "w" is
added
– Example - a cP air mass moving over the great lakes in December
becomes cPk
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Air Masses of North America
1. Continental Polar/Arctic
(cP/cA) air mass
2. Maritime Polar air
mass
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1
2
2
4
3
3. Maritime Tropical
(mT) air mass
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3
4. Continental
Tropical (cT) air mass
Continental Polar/Arctic (cP/cA) air
mass
• Stable, cold, dry air masses originating over N.
Canada and Alaska
• Eventually plunge southward to interior of U.S. as
a Shallow dome of high pressure - why?
• Can reach Gulf of Mexico and Florida - freeze
crops
• Usually do not move west of Rocky mountains mountains confine cold air to the east. Upslope
precipitation is common east of the rockies as the
cP air mass slides to the south.
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Average upper-level wind flow (heavy arrows) and surface
position of anticyclones (H) associated with two extremely
cold outbreaks of arctic air during December. Numbers on
the map represent minimum temperatures (°F) measured
during each cold snap.
•During the summer, cP air mass can bring relief to hot,
humid regions.
Typical vertical
temperature profile
over land for a
summer and a winter
cP air mass.
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•Produce lake effect snows as they move over the great lakes.
Areas shaded purple show regions that
experience heavy lake-effect snows.
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When an air mass moves over a large body of water ,
its original properties may change considerably.
Cloud Street
mP Air Mass:
• West Coast:
– Originate over Asia
as cP
– Tends to be unstable
– Heavy rains as cool
moist air flows over
mountains along west
coast
– mP is modified
(how?) by time it
reaches interior of
US, though is milder
than cP
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After crossing several mountain ranges, cool moist mP air
from off the Pacific Ocean descends the eastern side of the
Rockies as modified, relatively dry Pacific air.
Cold surface
• East Coast:
– not as common
as west coast
mP
– colder than
west coast mP
– usually brought
onshore by
high pressure
to the north of
US and/or low
pressure to the
south moving
up the coast.
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• mT Air Mass:
– largely originates in Gulf of Mexico, western Atlantic affecting eastern 2/3 of country
– also originates in tropical eastern pacific (SW monsoons in
summer)
– warm, moist, unstable
– confined to southern US in winter
– important source of moisture feeding storms all year round
A constant supply of mT air from the Golf of Mexico can
bring record-breaking maximum temperatures to the
eastern half of the country!
Weather conditions during an unseasonably hot spell in the eastern portion of
the United States that occurred between the 15th and 20th of April, 1976.
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• cT Air Mass:
– originates over Mexican Plateau region and desert SW
– hot, dry, unstable at low levels, stable at upper levels
– boundary between cT and mT is often called the dryline
• The dryline is often seen in surface and satellite data and is a
favored location for storm initiation:
Atmospheric Fronts - Introduction
• Front - boundary, transition zone between
two different air masses
• The two air masses have different
densities. Frequently, they are
characterized by different temperatures
and moisture contents
• Front has horizontal and vertical extent
• Frontal boundary/zone can be 1-100 km
wide!
• The upward extension of a front is referred
to as a frontal surface, or a frontal zone.
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• Types of synoptic-scale fronts:
–
–
–
–
stationary fronts
cold fronts
warm fronts
occluded fronts
Stationary Fronts
• Has little/no movement
• Denoted by alternating cold/warm
frontal symbols
– Semicircles face toward cold air
– Triangles point toward warmer air
• Associated weather:
– clear, partly cloudy, cloudy, light precip
– usually nothing severe
• If the stationary front starts moving
north in the example to the right it
will become a warm fronts
• If the stationary front starts moving
south in the example to the right it
will become a cold fronts
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Cold Fronts
• Zone/boundary between warmer, more moist, unstable air
(usually mT) being replaced by colder, drier, more stable air
(usually cP).
• Location of cold front:
– leading edge of sharp
temperature change
– moisture content (dew point)
changes dramatically
– wind shift (direction and speed)
– Pressure and pressure change
(pressure tendency is useful!)
Since the cold front is a trough
of low pressure, sharp changes
in pressure can be significant in
locating the front’s position.
Figure 11.13
• Location of cold front:
– often cloudy/showers/thunderstorms/sometimes severe
A squall line
A Doppler radar image showing precipitation patterns along a
cold front similar to the cold front in Fig. 11.13. Green represents
light-to-moderate precipitation; yellow represents heavier
precipitation; and red the most likely areas for thunderstorms.
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A vertical view of the weather across the cold frontin
Fig. 11.13 along the line X–X’.
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2
5
4
1
6
7
1.
2.
3.
4.
5.
6.
7.
8.
Warm air ahead of front is lifted up and over
Can get intense showers/thunderstorms at frontal boundary
Cs and Ci clouds are blown ahead of the front by upper level winds
Cloud base is generally lower behind the front
Further behind the front, the air is quite dry, few clouds
Steep frontal boundary (1:50), slopes backward into the cold air
Frontal speed averages 15-25 knots
Temperature and wind profiles on either side of cold front
Frontolysis: a condition cause a front weaken and dissipate –
the temperature contrast across a front lessens.
Frontogenesis: a condition cause a front to strengthen and
regenerate into a more vigorous frontal system – the temperature
contrast across a front increases.
The infrared satellite image (a) shows a weakening cold front over land on Tuesday
morning, November 21, intensifying into (b) a vigorous front over warm Gulf Stream
water on Wednesday morning, November 22.
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A “back door” cold front moving into New England during the
spring. Notice that, behind the front, the weather is cold and
damp with drizzle, while to the south, ahead of the front, the
weather is partly cloudy and warm.
In fact, no two fronts are exactly alike.
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Warm
Fronts
• Zone/boundary
between
advancing
warmer, more
moist air (usually
mT) and cooler,
drier air (usually
mP)
• Average speed
is about 10 knots
•
•
•
•
Often associated with "overrunning"
Frontal passage:
Clouds associated with warm fronts
frontal surface has a much smaller slope (1:300) than for cold
fronts
• Often produces wide-spread nimbostratus precip near front
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• Temperature and wind profiles on either side of warm front?
– Frontal inversion
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Drylines represents a narrow boundary where is a steep
horizontal change in moisture.
A dryline moves
across Texas and
Oklahoma during the
late afternoon in May.
Radar fine lines
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Dryline and convective initialization !
Occluded Fronts (Occlusion)
• Why do they form
– Cold front moves
fast -- catches up
the warm front
• There are two
types of occluded
fronts:
– cold occlusion
– warm occlusion
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Cold Occlusion
• Cold front "lifts" the warm
front up and over the very
cold air
• Associated weather is
similar to a warm front as
the occluded front
approaches
• Once the front has passed,
the associated weather is
similar to a cold front
• Most common type of
occluded front
• Vertical structure is often
difficult to observe
Warm Occlusion
• cold air behind cold
front is not dense
enough to lift cold air
ahead of warm front
• cold front rides up
and over the warm
front
• upper-level cold front
reached station
before surface warm
occlusion
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Visible satellite image
showing a mid-latitude
cyclonic storm with its
weather fronts over the
Atlantic Ocean during
March, 2005.
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An idealized vertical view of an upper-air front showing
tropopause (heavy red line), isotherms in °C (dashed gray lines),
and vertical air motions. The polar jet stream core (maximum
winds) is flowing into the page (from west to east).
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