1. Intro. to Atmospheric Sciences Plymouth State University
AIRMASSES & FRONTS
Dr. Sam Miller
Intro. to Atmospheric Sciences
Plymouth State University 1

2. Airmasses

3. Airmasses Definition Dimensions:
Large body of air with similar temperature and moisture characteristics
Dimensions:
Area: 100,000’s km2 or more
Depth: Up to three km (surface – 700 mb)
Form when a large mass of air spends a lot of time over a source region
Source regions are regions of the Earth’s surface with homogeneous characteristics

4. Airmass Classification
Based on temperature and moisture content
Dry air comes from the middle of continents (c), and moist air comes from the oceans (m)
Warm air comes from the tropics (T), and cold air comes from the polar (P) or the Arctic/Antarctic regions (A)
Moisture
Temperature
Dry  Continental (c)
Moist  Maritime (m)
Warm  Tropical (T)
Cold  Polar (P)
Very Cold  Arctic (A)

5. Airmass Classification
Based on temperature and moisture content
Temperature and moisture are two most important characteristics defining the density of the airmass
Cold = Dense
Warm = Light
Dry = Dense
Moist = Light

6. Airmass Classification
mT – maritime Tropical
moist and warm
mP – maritime Polar
moist and cold
cT – continental Tropical
dry and warm
cP – continental Polar
dry and cold
cA – continental Arctic
dry and very cold

7. Source regions affecting the U.S.
Rocky Mountain states
Desert Southwest and Mexican Plateau
Northern and interior Canadian provinces
Gulf of Mexico
Atlantic Ocean
Pacific Ocean

9. Fronts

10. Fronts
Front - boundary between two airmasses of differing temperature and/or moisture (i.e. density) characteristics
When two airmasses of differing densities collide at a front, the lighter airmass is forced upward

11. cP airmass
FRONT
mT airmass

14. Identifying Fronts From Weather Observations
Sharp changes in temperature
Changes in moisture content (dew point)
Shift in wind direction
Organized regions of cloud cover and precipitation
Trough (area) of low pressure
- Sometimes can find a distinct cyclonic (CCW) bend in the isobars or height contours

15. Identifying Fronts From Weather Observations
Sharp changes in temperature
Changes in moisture content (dew point)
Shift in wind direction
Organized regions of cloud cover and precipitation
Trough (area) of low pressure
- Sometimes can find a distinct cyclonic (CCW) bend in the isobars or height contours
PRESSURE FALLS AS FRONT APPROACHES YOUR STATION, AND RISES AFTER FRONT HAS PASSED

21. Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts
Dry Lines

22. Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts
Dry Lines

23. Cold Front
A transition zone where a cold (polar) airmass advances and replaces a warm (tropical) airmass
Colder airmass is denser, so warm airmass is lifted aloft

24. Cold Front cP mT

25. Cold Front

26. Shown as blue line with triangular pips
Point to direction of cold air flow
Generally moves toward the E or SE
Usually fast moving, esp. in summer
May have showers and thunderstorms (cumuliform clouds)
On eastern side of front
May be severe
Band of showers is usually narrow
Roughly linear area up to several thousand miles long

27. Colder, drier conditions after front passes
Wind shifts from southerly to northwesterly

28. Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts
Dry Lines

29. Warm Front
A transition zone where advancing warm air (tropical) replaces retreating cold air (polar)
Colder airmass is denser, so invading warmer airmass is forced aloft
Overrunning

30. Warm Front mT cP

31. Warm Front mT cP

32. Warm Front mT cP

33. Warm Front
OVERRUNNING mT cP

34. Warm Front mT cP
RAIN OR DRIZZLE

35. Warm Front mT cP
FREEZING PRECIP

36. Warm Front mT cP
ICE PELLETS

37. Warm Front mT cP
SNOW

38. Warm Front

39. Red line with semi-circular pips Generally moves toward the N or NE
Point to direction of warm air flow
Generally moves toward the N or NE
May have precipitation
On northern side of front
Due to overrunning (warm air sliding over cold air)
Steady NS-type (stratiform) precipitation more common than showery CB type (cumuliform)
Roughly circular area the size of a state
Winter precipitation may include all phases
snow, ice pellets (“sleet”), freezing rain

40. Warmer, more humid conditions after front passes
Wind shift from easterly to southerly

41. There is really only one front – The Polar Front
POLAR AIRMASS
TROPICAL AIRMASS

42. L The Polar Front is usually associated with areas of low pressure
POLAR AIRMASS L
TROPICAL AIRMASS

43. Lows rotate in a CCW direction in the Northern Hemisphere

44. L Lows rotate in a CCW direction in the Northern Hemisphere
WEST OF LOW CENTER
FRONT IS PUSHED TO
THE SOUTH

45. L Lows rotate in a CCW direction in the Northern Hemisphere
EAST OF LOW CENTER
FRONT IS PUSHED TO
THE NORTH

46. L Lows rotate in a CCW direction in the Northern Hemisphere
WHEN THE POLAR FRONT
MOVES SOUTH, WE CALL IT
A COLD FRONT

47. L Lows rotate in a CCW direction in the Northern Hemisphere
WHEN THE POLAR FRONT
MOVES NORTH, WE CALL IT
A WARM FRONT

48. Eventually… L

49. Real world example:

50. Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts
Dry Lines

51. Stationary Front
Boundary between two airmasses that are not moving with respect to each other.
One airmass is usually colder than the other

52. Characteristics of a stationary frontmP cP mT

53. Alternating red and blue pips on opposite sides of line
Blue triangles away from cold air
Red semi-circles away from warm air
Little or no movement
Stationary boundary between cold and warm air
Surface air flow parallel or weak warm air overrunning
May have clouds and precipitation
If rain happens can cause floods

54. Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts
Dry Lines

55. Occluded Front Occurs when a cold front overtakes a warm front
There are three airmasses involved

56. Evolution of an Occluded Front

57. Before occlusion

58. Before occlusion cP mP mT

59. Before occlusion cP mP mT

60. Before occlusion
CROSS-SECTION

61. Before occlusion

62. Before occlusion cP mT mP

63. Occlusion occurs cP mT mP

64. Occlusion continues cP mT mP

65. Occlusion continues
TRIPLE POINT

66. WARM SECTOR IS ALOFT NORTH OF TRIPLE POINT
Occlusion continues
WARM SECTOR IS ALOFT NORTH OF TRIPLE POINT

67. Occlusion continues
OCCLUDED FRONT IS THE EXTENSION OF THE COLD FRONT ON THE SURFACE NORTH OF THE TRIPLE POINT

68. Evolution of an Occluded Front
Over time, occlusion closes up like a zipper

69. Alternating purple triangles and semi-circles
On the same side of the line
Pointing in the direction of motion
Often has overcast skies
Precipitation and clouds have characteristics of both a warm and cold front
Stratiform clouds (continuous precip) with embedded cumuliform clouds (showery precip)
Colder temperatures after front passes
Wind shift from southeasterly to northwesterly

70. Types of Fronts Cold Fronts Warm Fronts Stationary Fronts
Occluded Fronts
Dry Lines

71. Dry Line Boundary between warm, dry air and warm, moist air
Both airmasses are tropical
The dry airmass is denser than the moist airmass

72. Dry Line cP mT cT

73. Solid line with open semi-circles Found in Southern Plains
All brown
Found in Southern Plains
TX, OK, KS
Winds shift from SE to SW
Important in tornadic thunderstorm development
Found south of low pressure systems
Often located between cold and warm fronts

74. Review

75. Definition of an airmass Horizontal & vertical scales
- Normal range at sea level
Horizontal & vertical scales
Source regions & naming conventions
- cA
- cP
- mP
- cT mT

76. Definition and characteristics of a front
How to identify front from plotted observations
How to identify front from satellite and radar data
When airmasses meet, the lighter one is forced aloft
Types of fronts
- Cold
- Warm
- Stationary
- Occluded – THREE AIRMASSES; TIME EVOLUTION
Dry Line – TWO TROPICAL AIRMASSES