Airmasses & Fronts Explained by Dr. Sam Miller - Weather & Climate

1. AIRMASSES & FRONTS Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University 1

2. Ahrens, Ch. 8

3. Airmasses

4. Airmasses • Definition • 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

5. 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)

6. 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

7. 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

8. 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

10. Fronts

11. 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

12. cP airmass FRONT mT airmass

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

16. 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

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

23. Types of Fronts • Cold Fronts • Warm Fronts • Stationary Fronts • Occluded Fronts • Dry Lines

24. 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

25. Cold Front cP mT

26. Cold Front

27. 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

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

29. Types of Fronts • Cold Fronts • Warm Fronts • Stationary Fronts • Occluded Fronts • Dry Lines

30. 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

31. Warm Front mT cP

32. Warm Front mT cP

33. Warm Front mT cP

34. Warm Front OVERRUNNING mT cP

35. Warm Front mT cP RAIN OR DRIZZLE

36. Warm Front mT cP FREEZING PRECIP

37. Warm Front mT cP ICE PELLETS

38. Warm Front mT cP SNOW

39. Warm Front

40. Red line with semi-circular pips • 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

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

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

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

44. Lows rotate in a CCW direction in the Northern Hemisphere L

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

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

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

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

49. Eventually… L

50. Real world example: