How do the blizzards from?
Review of last lecture
Tropical climate:
 Mean state: The two basic regions of SST? Which region has
stronger rainfall? What is the Walker circulation?
 Mean state: Two types of ocean upwelling, ocean-atmosphere
feedback
 El Nino and La Nina: Which region has warm SST anomaly
during El Nino? 4-year period.
 Land-sea contrasts: seasonal monsoon
Extratropical climate:
 Mean state: westerly winds, polar vortex
 What is the primary way El Nino affect extratropics? (PNA)
 The oscillations associated with strengthening/weakening of polar
vortex: AO, AAO
Video: Weather fronts
http://www.youtube.com/watch?v=tkK4
_F0VKhM
Air masses
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An airmass is a large (usually thousands of km across) volume of air that
has horizontally uniform properties of temperature and moisture.
Airmasses acquire their properties from spending days to weeks over the
same part of the Earth.
“Polar” airmasses are colder than “tropical” airmasses
“Maritime” airmasses are wetter than "continental" airmass
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Other specific airmass types include "arctic", "equatorial", and “monsoon”
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Bergeron classification of air masses
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3 letters: e.g. mTk, cPw
1st letter for moisture properties: c - continental, m - maritime
2nd letter for thermal characteristics: T - tropical, P -polar, A Artitic/Antarctic, M - monsoon, E - equatorial, S -superior air(dry air
formed by significant downward motion in the atmosphere)
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3rd letter for stability: k/w - air colder/warmer than ground
Source regions
The areas where air masses form
are called source regions.
Fronts
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A weather front is a boundary separating two air masses
Types: cold front, warm front, stationary front, occluded
front, dry line, squall line
Cold Fronts
• A cold front is a mass of
cold air advancing towards
warm air.
• Typically associated with
heavy precipitation, rain or
snow, combined with rapid
temperature drops.
• Since friction decreases with
height, winds move faster at
higher altitude. Then the
surface of cold front
becomes more steeper
through time, leading to a
narrow belt of precipitation.
• Moving speed up to 30mph
Satellite and radar images of cold fronts
(narrow belt of clouds/precipitation)
Warm Fronts
• Warm fronts are warm air
moving towards cold air.
• Friction decreases with height,
so winds move faster at higher
altitude. This causes the
surface of the front to become
less steep through time. Then
clouds will be spread to a wider
region.
• Shallow stratus clouds
dominate and bring light
precipitation. Frontal fogs
may occur as rain
evaporates in the colder air
near the surface.
• Moving speed about 12 mph
Satellite and radar images of warm fronts
(wide region of clouds/precipitation)
Stationary Fronts
• Stationary fronts do not move. They do not advance.
They are two unlike air masses side by side.
• They may slowly migrate and warmer air is displaced
above colder.
From Environment
Canada
Video: Science of a Blizzard

http://www.history.com/videos/science-of-ablizzard#science-of-a-blizzard
What is a mid-latitude cyclone?
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The mid-latitude cyclone
is a synoptic scale low
pressure system that has
cyclonic (counterclockwise in northern
hemisphere) flow that is
found in the middle
latitudes (30N-55N, 30S55S).
It has a larger size than a
tropical cyclone
Midlatitude cyclones often form near the
fronts and jet streams
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Jet streams are caused by steep
temperature gradients between
cold and warm air masses
Polar front - marks area of
contact, steep pressure gradient
 polar jet stream
Low latitudes  subtropical jet
stream
Stronger in winter, affect daily
weather patterns
How does a mid-latitude cyclone form?
In mid-latitude there is a boundary
between northern cold air and
southern warm air
In the boundary a initial cyclone
can advect warm air northward
and cold air southward
Mature stage. Cold air begins to
catch up with warm air (occluded).
If the upper level low is to the west
of surface low, the cyclone will
amplify and precipitation will form.
Cold air cools down the cyclone.
Dissipation.
Why do some frontal waves develop
into huge cyclonic storms, but others
don’t?
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Complex challenge to forecasting;
Atmospheric conditions at the surface and aloft affect
cyclogenesis.
Key is to look at the upper level winds (longwaves,
shortwaves).
Longwave disturbances (Rossby waves)
Earth's poles are
encircled by 3 to 6
longwaves, or Rosby
waves, directing
upper level winds
around lows at the
500 mb surface.
Small disturbances in
these waves can
trigger storms.
Shortwave Disturbances
Shortwave ripples
within the Rossby
waves move faster,
and propagate
downwind into the
Rossby troughs and
cause them to deepen.
Barotropic conditions,
where isobars and
isotherms are parallel,
then degenerate into a
baroclinic state where
the lines cross and cold
or warm air is advected
downwind.
Regions of cyclogenesis and typical tracks
• Gulf of Mexico, east coast
• Alberta Clipper from eastern side of Canadian Rockies
• Colorado Low from eastern slope of American Rockies
 Lee-side lows, lee cyclogenesis
Summary
1. Definition of airmasses. Bergeron classification of air
masses (3 letters).
2. Fronts: 6 types (cold, warm, stationary, occluded, dry line,
squall line)
3. Cold front (narrow, fast, heavy precipitation), Warm front
(wide, slow, light precipitation)
4. The developmental stages and vertical structure of middle
latitude cyclones (boundary between northern cold air
and southern warm air, upper level low to the west of
surface low)
5. How upper level longwaves and shortwaves may
enhance cyclonic development at the surface (upper level
low to the west of surface low)
6. The three regions of cyclogenesis and typical tracks