Transcript Cold Fronts

Chapter 8: Air Masses,
Fronts, and Middle-Latitude
Cyclones
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Air masses
Fronts
Middle-latitude cyclones
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 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
Classification:
• Temperature and humidity
• Naming conventions
• continental: ‘dry’,
maritime:
‘moist’
Q1: 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.
Q2: which mP is warmer when reaching the U.S.?
a) from the Pacific, b) from the Atlantic, c) the same
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
Q3: 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.
Q4: Can we have lake-effect snow if the lake is frozen?
a) yes,
b) no,
.
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
Q5: What are the small white
clouds over the Pacific?
a) cirrocumulus, b) altocumulus,
c) cumulus
Q6: What are the symbols in
California?
A: see Appendix C
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.
The ‘pineapple express’
on January 1, 1997
High T on April 17, 1976
cT (Continental Tropical Air Masses)
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Northern Mexico
and southwestern U.S.
Summer
Max T in
July 2005
Front
• A transition zone between two air masses of different temperature
and/or humidity
• This terminology was developed by Norwegian meteorologists
shortly after World War I.
• cold front
• warm front
• stationary front
• occluded front
• dryline
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
Cold Fronts
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cold front: temperature, humidity,
wind direction differences
clouds and precipitation
vertical cross section:
slope of 1:50
Q7: before the cold front approaches you, what do you see first?
a) high clouds, b) rain shower,
c) thunderstorm
Q8: Where is the surface minimum pressure located?
a) before cold front, b) at cold front, c) behind cold front
Q9: Where do you expect snowfall?
a) before cold front, b) at cold front,
c) behind cold front
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
Q10: which front slope is more steep?
a) cold front, b) warm front
Q11: where is the warm area with small cumulus clouds most
probably located?
a) right behind a warm front, b) right behind a cold front
Q12: 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
Q13: 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
Dryline: primary difference in dew-point T
Occluded Fronts
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cold occlusion
warm occlusion
• Occluded fronts have
characteristics of both
warm and cold fronts.
Polar Front Theory
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Stationary front
frontal wave
open wave
mature cyclone
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
Q14: what is the reason for the Gulf
of Alaska Low?
a) permanent Aleutian low;
b) water vapor from Pacific
Q15: what do Alberta Clipper and
Colorado Low have in common?
a) both are cold
b) both are on the leeside of Rockies
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)
Q16: 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
Figure 4, p. 226
Developing Mid-Latitude Cyclones
and Anticyclones
• convergence and divergence patterns aloft are extremely
important to the development of mid-latitude cyclones
Q17: 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
Q18: 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
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
• During World War II, the jet stream was used by
the Japanese to carry balloon bombs across the
Pacific Ocean to North America.
Fig. 8-30, p. 231