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Intro. to Atmospheric Sciences Plymouth State University

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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

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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

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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

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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 front
mP 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


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