This chapter discusses air masses and weather systems. It describes the major air masses that affect weather in different regions, including maritime tropical (mT), continental polar (cP), and continental arctic (cA). Fronts are boundaries between differing air masses, such as cold fronts and warm fronts. Mid-latitude cyclones form in zones of clash between polar and subtropical air masses, bringing variable weather. Other atmospheric disturbances discussed include hurricanes, thunderstorms, tornadoes, blizzards, and tropical waves. Modern weather forecasting utilizes Doppler radar, satellites, and high-speed computing.

1. Chapter 7: Air Masses and Weather Systems Physical Geography Ninth Edition Robert E. Gabler James. F. Petersen L. Michael Trapasso Dorothy Sack

2. Ch. 7: Air Masses and Weather Systems

3. 7.1 Air Masses Air Mass: large body of air whose temperature and moisture characteristics are similar. Due to General Atmospheric Circulation, air masses move and cause changing weather. Source Region: m = maritime (sea) c = continental (land) E = Equatorial (very warm) T = Tropical (warm) P = Polar (cold) A = Arctic (very cold)

4. 7.1 Air Masses

5. 7.1 Air Masses Q: Which air mass affect your location? Are there seasonal variations?

6. 7.1 Air Masses Air Mass Modification cP or CA moves over Great Lakes and picks up moisture, and may cause Lake effect snow.

7. 7.1 Air Masses Continental Arctic (cA): Dry and very cold occasionally impacts U.S. in the winter it often produces record breaking cold. Continental Polar (cP): Dry and cold. May reach Gulf of Mexico in winter. Rarely affects west coast. Maritime Polar (mP): moist and cool Westerlies bring air mass to west coast, especially in winter. Occasionally affects east U.S.

8. 7.1 Air Masses Maritime Tropical (mT): moist and warm May originates in Gulf of Mexico Major impact on central and eastern U.S T-storms in the summer Clashes with cP Continental Tropical (cT): dry and hot Small source region (SW deserts and N. Mexico Dry line Smallest player in U.S. weather

9. 7.2 Fronts Fronts clash between air masses Generally move with westerlies 3-dimensional Frontal uplift

10. 7.2 Fronts Cold Front: Cold air moves in on warm air Warm air (less dense) rises above cold air Steep slope Cumulonimbus may form a Squall line Sharp changes in temperature, pressure, and wind

11. 7.2 Fronts Warm Front: Warm air moves in on cooler air Warm air (less dense) rises above cold air Slope is not as steep Light precipitation which may last longer Usher in warmer conditions

12. 7.2 Fronts Stationary Front Boundary between air masses that is not moving Extended period of light precipitation and occasionally strong T-storms Occluded Front Cold air is overtaking warm air Dying storm

13. 7.2 Fronts Four Major Frontal Symbols used on weather maps.

14. 7.3 Atmospheric Disturbances Middle Latitude Cyclones Common location for a clash between cold, polar air and warm, subtropical air Convergence Guided by polar jet stream Variable weather

15. 7.3 Atmospheric Disturbances Cyclones (L) Move with jet stream/westerlies Converging, rising air Fronts Precipitation Anticyclone (H) Move with path of westerlies Divergence, sinking air Dry weather

16. 7.3 Atmospheric Disturbances Horizontal and Vertical Structure of pressure systems Where would the strongest and weakest winds be?

17. 7.3 Atmospheric Disturbances Common track of storms (mid-latitude cyclones) What storm track (jet stream) influences your location?

18. 7.3 Atmospheric Disturbances Stages in the development of a mid-latitude cyclone Q: Where would you expect precipitation. Why?

19. 7.3 Atmospheric Disturbances Cyclones and Local Weather Q: Describe the front(s), temp, air mass type, wind direction, and precip. at Des Moines, St Louis, and Pittsburg.

20. 7.3 Atmospheric Disturbances Cyclones and Upper Air Flow Polar jet stream Major impact on temperature and precip. Q: Where would you expect storms to develop?

21. 7.3 Atmospheric Disturbances Polar Jet Stream Analysis Anticyclones Q: Which country does most of this pattern occupy? Where is an anticyclone?

22. 7.3 Atmospheric Disturbances Hurricanes Circular, cyclonic storm with wind speeds greater than 74 mph (118 km/hr) Smaller than mid-latitude cyclone Same air mass type More destructive Also called tropical cyclones

23. 7.3 Atmospheric Disturbances Cross section of a hurricane

24. 7.3 Atmospheric Disturbances Storm Surge Q: Why is the timing of landfall so critical to coastal areas?

25. 7.3 Atmospheric Disturbances Major “Hurricane Alleys” Which coastlines seem unaffected by these tracks?

26. 7.3 Atmospheric Disturbances Hurricane development Warm water (>25 o C or 77 o F) Most air Coriolis force (do not form or survive near equator) Stages of Development Tropical disturbance Tropical depression Tropical Storm (become named) Hurricane (categorized by Saffir-Simpson Scale)

27. 7.3 Atmospheric Disturbances Saffir-Simpson Scale

28. 7.3 Atmospheric Disturbances Three hurricane tracks Subtropical High (Bermuda High)

29. 7.3 Atmospheric Disturbances Hurricane Strikes Q: What areas of our coasts seem to have escaped landfall?

30. 7.3 Atmospheric Disturbances Snow Storms and blizzards Mid and high latitudes Blizzard Severe weather event Heavy snow and strong winds (55 km/hr or 35 mph) Visibility reduced

31. 7.3 Atmospheric Disturbances Thunderstorms Low and mid latitudes Lightning: intense discharge of electricity Thunder: sonic boom created by the expansion of air around the lightning bolt Usually cover a small area

32. 7.3 Atmospheric Disturbances Types of Thunderstorms Convective (thermal) Orographic Frontal

33. 7.3 Atmospheric Disturbances Tornadoes Occur almost anywhere but are most common in North America (Tornado Alley) Small intense, cyclonic storm of low pressure, violent winds, and converging air

34. 7.3 Atmospheric Disturbances Destruction caused by an F5 tornado in Greensburg, Kansas on May 16, 2007

35. 7.3 Atmospheric Disturbances Average annual # of tornadoes and seasonal variation

36. 7.3 Atmospheric Disturbances Doppler radar: improves tornado detection and forecasting Able to determine wind speed and direction Q: How many squall lines can you see on this image?

37. 7.3 Atmospheric Disturbances Fujita Scale (F-0 to F-5) Enhanced Fujita Scale (EF-0 to EF-5)

38. 7.3 Atmospheric Disturbances Weak tropical Disturbances Easterly Wave Polar Outbreak

39. 7.3 Atmospheric Disturbances Weather Forecasting Doppler radar Weather satellites (e.g. GOES East) High speed computers Improving!

40. Physical Geography End of Chapter 7: Air Masses and Weather Systems