Air Masses, Fronts,
Cyclogenesis
Air masses
Large bodies of air with uniform temperature
and moisture conditions at given altitude.
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~1000 miles diameter
several miles deep
Move separately from surrounding air
Originate over source regions
Take on temperature and moisture
characteristics of source region
Source regions
• Extensive (10s of 1000s of km2)
• High or low latitudes (not mid)
• Temperature characteristics (second letter of air mass)
– By latitude: Polar, Arctic, Tropical, Equatorial, Antarctic
• P, A, T, E, AA
• Moisture char (first letter of air mass)
– By underlying surface: Continental, Maritime
• m, c
• Resulting:
– mE, mP, mT
– cP, cA, cAA, cT,
fronts
Boundary between two unlike air masses
(they don’t readily mix)
unlike in temperature or moisture
in both cases, unlike in density
– drier air masses are more dense than
wetter air masses at same temp
– Water vapor is light gas
Warm or cold front?
• Depends on movement of denser air mass
– If cold (dense) side is advancing : COLD front
– If cold side is retreating and being replaced by
warm air: WARM front
– Look at winds in cold sector: are they blowing
into the front or away from (or parallel) to the
front?
Dew point front
• Air masses have same temps but different
dew points
• Called “dry line” in tornado alley in US
– Where warm dry cT air masses from SW meet
mT air masses from Gulf
Dew points
Cold front
Cold front
• Steep leading edge due to friction with
surface
• Warm air rises rapidly up steep slope of
leading edge
• Very unstable: thunderstorms, heavy
precipitation, sometimes tornadoes
Warm front
Warm front
• Cold air retreats
• More gradual slope because dense air is
not the aggressor
• Warm air gently climbs over cold air; up
the ramp
• Atmosphere is not as unstable: air rises
gradually
• Clouds lower and thicken in advance of
front
Stationary front
Stationary front
• Surface winds in cold sector blow parallel
to front
• No movement
• Varied weather, but there are clouds and
precipitation because there is still a
difference in density which causes rising
air
Occluded front
Midlatitude / Wave Cyclones
• Large cyclonic circulation systems,
associated with cold and warm fronts in
the mid latitudes.
– Most common type of storm in mid-latitudes
– Cyclogenesis: process of development and
dissolution of a wave cyclone
Favorable
conditions:
• “Trough”
– between two Highs
along polar front
• Converging air
• Unstable
1. Early stage
surface convergence; lifting around Low
2. Open Stage
wave develops; warm air in touch with ground;
counterclockwise around Low strengthens (N. hem)
Warm front
Cold front
3. Occluded Stage
occluded front; warm air mass eventually leaves ground
Comma-shaped storm
4. Dissolving Stage
warm air mass completely cut off from ground; no more
uplift
Upper level control
Not all wave cyclones develop into intense
storms; need upper level help to intensify
into big storms.
Convergence and Divergence aloft
Caused by:
1. Crowding or dispersing
Geostrophic flow
2. Speed
500 mb map
Meridional flow in Rossby waves:
deep ridges and troughs
500 mb map
Convergence aloft
Divergence aloft
If: 1) divergence aloft is above surface Low and
2) divergence aloft exceeds surface convergence,
storm intensifies.
Wave cyclone will intensify if the surface Low is
just East of an upper level trough.
• Wave cyclone animation 1, animation 2
• video
Favored when jet max is in trough
April 3, 2014, UMD was closed!
April
2
Next
day
Later …
Another
look
Thursday
Friday
October 27, 2010
Oct. 27
Oct 27
Oct 27; 500 mb
Oct 27; 300 mb