Transcript 3/27/2012 Chap. 8

Chapter 8: Air Masses,
Fronts, and Middle-Latitude
Cyclones
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Air masses
Fronts
Middle-latitude cyclones
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Air Mass Source Regions
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air mass: an extremely large body of air whose properties of
temperature and humidity are fairly similar in any horizontal
direction at any given altitude
source regions: regions dominated by surface high pressure
over flat surface
• Because air sinks in
high pressure systems,
air stays in contact with
the surface and acquires
its temperature and
moisture characteristics
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Classification:
• Temperature and humidity
• Naming conventions
• continental: ‘dry’,
maritime:
‘moist’
Q: Since Arctic is a sea, why can we use the term ‘continental
Arctic air mass’? A: because Arctic is covered by sea ice in
winter.
Q: Why are midlatitudes not good source regions?
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Q: which mP is warmer when reaching the U.S.?
a) from the Pacific, b) from the Atlantic, c) the same
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cP (Continental Polar) and cA
(Continental Arctic) Air Masses
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Continental polar
continental Arctic
In the continental US, the coldest
winter air is associated with cA
air masses
(unfrozen) lake effect snows
Tmin in Dec 1990
At the downwind side of
.
the lake, additional
lifting is provided by low
hills and the convergence
of air as it slows down
over the rough terrain 5
Q: What is the lake-effect snow and what is the mechanism?
A: Lake-effect snows are snowstorms that form on the
downwind side of a large lake. Cold, dry air crossing a
lake gains moisture and warmth from the water. As the
more buoyant air rises, clouds forms that deposit snow
on the lake's lee shore.
Q: Can we have lake-effect snow if the lake is frozen?
a) yes,
b) no,
.
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mP (Maritime polar)
Air Masses
mP air often brings rain to
the west coast of the US.
Modification of mP air mass
by mountain ranges
Q: What are the small white
clouds over the Pacific?
a) cirrocumulus, b) altocumulus,
c) cumulus
Q: What are the symbols in
California?
A: see Appendix C
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Surface station
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mT (Maritime Tropical Air Masses)
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subtropical air
Bermuda high
• mT air brings
hot, muggy air
to the eastern
US in summer.
Max and Min T
on April 17,
1976
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mT (Maritime Tropical Air Masses)
The ‘pineapple express’ on January 1, 1997
Atmospheric River: caused flooding and extensive damage
(~$1.5 billion)
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cT (Continental Tropical Air Masses)
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Northern Mexico
and southwestern U.S.
Summer
Max T in
22 July 2005
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What types of air mass would be responsible for the weather
conditions listed below? choose the right answer:
a) cP, b) cT, c) mP, d) mT
Q1: hot, muggy summer in the Midwest and East
Q2: cool breeze after a summer hot spell on the Central Plains
Q3: persistent cold, damp weather along the east coast
Q4: drought with high temperature over the Great Plains
Q5: record-breaking low temperature over U.S.
Q6: cool weather with showers over the Pacific Northwest
Q7: daily afternoon thunderstorm along the Gulf Coast
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Front
• A transition zone between two air masses of different temperature
and/or humidity. It represents low pressure area.
• This terminology was developed by Norwegian meteorologists
shortly after World War I.
• cold front
• warm front
• stationary front
• occluded front
• dryline
Air masses:
mP, cP, mT
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Stationary Fronts
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Has essentially no
movement, and wind is
usually parallel with
the stationary front
Large T and Td
differences still exist
Light precipitation may
or may not appear on
the cold air side
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Cold Fronts
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cold front: temperature, humidity,
wind direction differences
clouds and precipitation
vertical cross section:
slope of 1:50
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Q: before the cold front approaches you, what do you see first?
a) high clouds, b) rain shower,
c) thunderstorm
Q: Where is the surface minimum pressure located?
a) before cold front, b) at cold front, c) behind cold front
Q: Where do you expect snowfall?
a) before cold front, b) at cold front,
c) behind cold front
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Warm Fronts
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overrunning: slope of 1:300
Temperature, humidity, and wind direction changes
T and Td differences not as large as
those for cold fronts
Cloud and precipitation changes
vertical cross section
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Q: which front slope is more steep?
a) cold front, b) warm front
Q: where is the warm area with small cumulus clouds most
probably located?
a) right behind a warm front, b) right behind a cold front
Q: If the wind is northerly behind a cold front, the wind
direction ahead of the cold front can not be
a) westerly, b) southwesterly, c) easterly
Q: if the wind is southerly behind a warm front, the wind
direction ahead of the warm front can not be
a) westerly,
b) southeasterly, c) easterly
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Dryline: primary difference in dew-point T
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Occluded Fronts
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cold occlusion
warm occlusion
• Occluded fronts have
characteristics of both
warm and cold fronts.
• After occlusion, the
front would die out
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Polar Front Theory
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Stationary front
frontal wave
open wave
mature cyclone
Q: for the left figure, what is the
corresponding stage based on the
above figure?
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Where Do Mid-Latitude Cyclones
(i.e., initial Low’s) Tend to Form?
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Both Lows and Highs move
from west to east
Highs also from north to south
Lows also from south to north
Q: what is the reason for the Gulf of
Alaska Low?
a) permanent Aleutian low;
b) water vapor from Pacific
Q: what do Alberta Clipper and
Colorado Low have in common?
a) both are cold
b) both are on the leeside of Rockies
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Northeasters (or nor’easters):
develops or intensified off the
eastern seaboard of North
America then move
northeastward along the coast
(not unlike a tropical cyclone)
Q: what do the Gulf Low and
Hatteras Low have in
common?
a) both have strong ocean
currents;
b) both are located between
warm ocean and cold land
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Developing Mid-Latitude Cyclones
and Anticyclones
• convergence and divergence patterns aloft are extremely
important to the development of mid-latitude cyclones
Q: Why would the surface low be weakened if the low aloft
is right above it?
A: because convergence will bring air
molecules to the column and increase surface pressure
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•
In the figure, there is a divergence aloft above surface
Low pressure (and convergence); a convergence aloft
above surface High; trough aloft is behind surface Low
but ahead of surface High
Q: under what conditions
would surface Low be further
strengthened?
a) surface convergence is
stronger than divergence aloft
b) divergence aloft is stronger
than surface convergence
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Jet Streams and Developing MidLatitude Cyclones
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jet stream (usually near tropopause of ~10 km)
jet streak: jet stream core with maximum wind
upper-air support
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Q: list four regions in North America where midlatitude
cyclones tend to develop.
Q: For surface low to intensify, the upper level trough should
be located: a) west of surface low, b) above surface low,
c) east of surface low
Q: A mid-latitude cyclonic storm over eastern U.S. usually
moves: a) northeastward, b) southeastward, c) southward
Q: in winter, which front produce more violent weather:
a) stationary front, b) warm front, c) cold front
Q: describe the cloud change before passing, while passing,
and after passing of a cold front.
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