1. Air Masses, Fronts, Cyclogenesis

2. Air masses
Large bodies of air with uniform temperature and moisture conditions at given altitude.
~1000 miles diameter
several miles deep
Move separately from surrounding air
Originate over source regions
Take on temperature and moisture characteristics of source region

3. 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,

5. fronts Boundary between two unlike air masses (they don’t readily mix)
unlike in temperature or moisture (density)
drier air masses are more dense than wetter air masses at same temp
Water vapor is light gas

7. 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?

9. 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
weather map
Water vapor

12. Dew points

13. Cold front

14. 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

15. Weather map
Water vapor

16. Warm front

17. 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

18. weather map IR

19. Stationary front

20. 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
weather map IR

23. Occluded front
weather

24. 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

26. Favorable conditions:
“Trough”
between two Highs along polar front
Converging air
Unstable

27. Early stage surface convergence; lifting around Low

29. 2. Open Stage wave develops; warm air in touch with ground; counterclockwise around Low strengthens (N. hem)
Warm front
Cold front

31. 3. Occluded Stage occluded front; warm air mass eventually leaves ground

35. Comma-shaped storm

40. 4. Dissolving Stage warm air mass completely cut off from ground; no more uplift

41. Upper level control
Not all wave cyclones develop into intense storms; need upper level help to intensify into big storms.

42. Convergence and Divergence aloft
Caused by:
1. Crowding or dispersing
2. Speed
500 mb map
Geostrophic flow

43. Meridional flow in Rossby waves: deep ridges and troughs
500 mb map
Divergence aloft
Convergence aloft

44. If: ) divergence aloft is above surface Low and 2) divergence aloft exceeds surface convergence, storm intensifies.
Divergence!
LOW

45. Wave cyclone will intensify if the surface Low is just East of an upper level trough.

48. Wave cyclone animation 1, animation 2

49. video

50. Favored when jet max is in trough

51. April 3, 2014, UMD was closed!
April 2

52. Next
day

53. Later …

55. Another look

56. April 3

57. Apr 4

58. October 27, 2010

59. Oct. 27

60. Oct 27

61. Oct 27; 500 mb

62. Oct 27; 300 mb