1. Fundamentals of Physical Geography 1e
Chapter 6: Air Masses and Weather Systems
Cover Image.
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2. Air Masses and Weather Systems
cover image for Chapter 6 (p. 123).

3. Air Masses
Air Mass: large body of air whose temperature and moisture characteristics are similar
Source Region:
m = Maritime (sea)
c = Continental (land)
E = Equatorial (very warm)
T = Tropical (warm)
P = Polar (cold)
A = Arctic (very cold)
Table 6.1

4. Air Masses Air Masses Maritime Equatorial (mE) Maritime Tropical (mT)
Continental Tropical (cT)
Continental Polar (cP)
Maritime Polar (mP)
Continental Arctic (cA)
Table 6.1

5. Air Masses
Table 6.1

6. Air Masses Air Mass Modification and Stability
cP or cA moves over Great Lakes and picks up moisture, and may cause Lake effect snow.
What two main factors contribute to increased precipitation caused by the lake effect?
Figure 6.1

7. Air Masses North American Air Masses Continental Arctic (cA)
Continental Polar (cP)
Maritime Polar (mP)
Maritime Tropical (mT)
Continental Tropical (cT)
Figure 6.2

8. Air Masses Continental Arctic (cA): Dry and very cold
Occasionally impacts U.S. in the winter
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 eastern U.S. as Nor’easters

9. 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

10. Air Masses
Which air masses affect your location? Are there seasonal variations?
Figure 6.2

11. Fronts Fronts Clash between air masses Sloping boundary
Generally move with westerlies
3-dimensional
Frontal uplift
U.S. and Canada in a zone between source regions

12. 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
Figure 6.3

13. Cirrus anvil top Warm air mass Cumulonimbus Cold air mass Cumulus
Cold front surface
Cumulonimbus
Cold air
mass
Cumulus
Stepped Art
Fig. 6-3, p. 127

14. 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
Figure 6.4

15. Warm air mass Cirrus Cirrostratus Warm front surface Altostratus
Cool air mass
Nimbostratus
Figure 7.3
Cross section of a warm front. Warm fronts advance more slowly than cold fronts and replace rather than displace cold air by sliding upward over it. The gentle rise of the warm air produces stratus clouds and gentle rain.
Compare Figures 7.2 and 7.3. How are they different? How are they similar?
Stratus
Stepped Art
Fig. 6-4, p. 128

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

17. Four major frontal symbols used on weather maps
Figure 6.5

18. Atmospheric Disturbances
Anticyclones and Cyclones
Atmospheric disturbance
Anticyclone
Cyclone
Wind and pressure gradient
Where would be the strongest winds in this figure? Where would be the weakest winds?
Figure 6.6

19. Atmospheric Disturbances
Anticyclone (H)
Move with path of westerlies
Divergence, sinking air
Sources:
Northern Canada and Arctic  Polar outbreak
Subtropical High  Dry and warmer weather

20. Atmospheric Disturbances
Cyclones
Low pressure
Convergence and rising air
Clouds and precipitation

21. Atmospheric Disturbances
Mapping pressure systems
Horizontal structure
Vertical Structure
Figure 6.6

22. Atmospheric Disturbances
General Movement
Track of storms (mid-latitude cyclones)
What storm track influences your location?
Figure 6.7

23. Atmospheric Disturbances
Middle-Latitude Cyclone
Also known as extratropical cyclones
Migrating storms
Clash between air masses
Ex: Cp vs. mT
Vary in intensity, longevity, speed of travel, wind strength, amount and type of cloud cover, the quantity and type of precipitation, and the area they affect.
Associated with polar front
Movement with the seasons

24. Atmospheric Disturbances
Middle-Latitude Cyclone
Stages in the development of a mid-latitude cyclone
In (c), where would you expect rain to develop. Why?
Figure 6.8

25. Atmospheric Disturbances
Cyclones and Local Weather
Describe the front(s), temp, air mass type, wind direction, and precip. in Pittsburg and Detroit.
Figure 6.9

26. Atmospheric Disturbances
Cyclones and the Upper Air Flow
Alternating pressure ridges (highs/divergent winds) and troughs (lows/convergent winds)
Where would you expect storms to develop?
Figure 6.10

27. Atmospheric Disturbances
Cyclones and the Upper Air Flow
Polar Jet Stream Analysis
Which country does most of this pattern occupy? Where is one trough?
Figure 6.11

28. Atmospheric Disturbances
Hurricanes
Circular, cyclonic storm with wind speeds greater than 74 mph
Smaller than mid-latitude cyclone
Same air mass type
More destructive
Calm winds at center
Also called tropical cyclones
Require/fueled by warm water (80oF or more)
Figure 6.12

29. Atmospheric Disturbances
Hurricane cross-section
Figure 6.12

30. Atmospheric Disturbances
Major “Hurricane Alleys”
Which coastlines seem unaffected by these tracks?
Figure 6.13

31. Atmospheric Disturbances
Hurricane development
Warm water (>27oC or 80oF)
Most air
Coriolis force (does not form or survive near equator)
Stages of development
Tropical disturbance (easterly waves)
Tropical depression
Tropical storm (becomes named; mph)
Hurricane (categorized by Saffir-Simpson Scale)
Dissipation (dies) over land or cool water

32. Atmospheric Disturbances
Hurricane Intensities and Impacts
Storm surges
Saffir-Simpson Scale
What can people who live in such regions do to protect themselves when a serious storm surge is threatening?
Figure 6.14

33. Atmospheric Disturbances
Saffir-Simpson Hurricane Scale
Table 6.2

34. Atmospheric Disturbances
Hurricane Intensities and Impacts
2004 was a record- breaking year
3 in Florida
Hurricane Katrina in 2005
New Orleans
Levee failure
Hurricane Ike in 2008
Figure 6.15

35. Atmospheric Disturbances
Snow storms and blizzards
Mid and high latitudes
Blizzard
Severe weather event
Heavy snow and strong winds (35 mph)
Visibility reduced
Figure 6.16

36. 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
Figure 6.17

37. Atmospheric Disturbances
Types of Thunderstorms
Convective (thermal)
Orographic
Frontal
Figure 6.18

38. 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
Figure 6.19

39. Destruction caused by an F5 tornado in Greensburg, Kansas on May 16, 2007
Figure 6.20

40. Atmospheric Disturbances
Tornado
Typically small and short lived
80% associated with thunderstorms
Temporal Variability
March to July
Late afternoon or early evening
Figure 6.21

41. Atmospheric Disturbances
Doppler radar:
Improves tornado detection and forecasting
Able to determine wind speed and direction
Hook Echo is a signature of a tornado
Figure 6.22

42. Atmospheric Disturbances
Fujita Scale (F-0 to F-5)
Enhanced Fujita Scale (EF-0 to EF-5)
Table 6.3

43. Weather Forecasting Weather Forecasting Doppler radar
Weather satellites (e.g. GOES East)
High speed computers
Improving!
Is there cloud cover over your state on this day?
Figure 6.23

44. Fundamentals of Physical Geography 1e
End of Chapter 6: Air Masses and Weather Systems
Petersen
Sack
Gabler