1. Chapter 7: Atmospheric
Disturbances
McKnight’s Physical Geography:
A Landscape Appreciation,
Tenth Edition, Hess

2. Class 27
Midlatitude Cyclones,
Anticyclones
Tropical Cyclones or Hurricanes

3. Atmospheric Disturbances
• Midlatitudes cyclones and
anticyclones
• Tropical disturbances—
easterly waves and
hurricanes
• Localized severe weather
—thunderstorms and
tornadoes
3
Figure 7-7

4. Midlatitude Cyclones
• 35–70° latitude
• ~ 1600 km across
• Central pressure - 990 to 1000 mb
• Converging counterclockwise in Northern Hemisphere
• Circulation creates fronts
4
Figure 7-6

5. Stages
• 1. Clashing air
masses
• 2. “Wave” disturbance
begins
• 3. Cold and warm
fronts
• 4. Occluded front
• 5. Dissipation
5
Physical Geography: A Landscape
Appreciation 10e

6. Physical Geography: A Landscape
Appreciation 10e
6

7. Physical Geography: A Landscape
Appreciation 10e
7

8. • Weather changes
as front moves by
– Temperature
– Winds
– Pressure
– Dew Point
• Cyclone movement
– Steered by jet stream and
westerlies
– Cyclonic winds
– Cold front advances faster
than center of the storm
8
Figure 7-8

9. • Cyclogenesis to
Occlusion
9
Figure 7-9

10. Midlatitude Cyclones
• Upper level divergence and convergence related to
cyclogenesis
10
Figure 7-10

11. ~ 6–15 worldwide
11
Figure 7-13

12. Midlatitude Anticyclones
• Anticyclones—high pressure systems
– Subsiding, diverging winds at the surface
– Flow is clockwise
• Relationship to cyclones
– Independent, but related
– Anticyclones typically follow behind cyclone’s cold front
12

13. Minor Tropical Disturbances:
Easterly Waves
• Easterly wave characteristics
– Oriented N–S
– Little cyclonic circulation
– Convergence behind wave,
divergence ahead of wave
– Can intensify to tropical
cyclones
13
Figure 7-15

14. Major Tropical Disturbances:
Hurricanes
• Tropical cyclone definition
• Tropical depression—winds
< 38 mph
• Tropical storm—winds 38–74
mph
• Hurricane—winds > 74 mph
– Typhoons
– Baguios
– Cyclones
14
Figure 7-16

15. – Low pressure center, winds spiral inward
– Steep pressure gradient and strong winds
– Warm moist air enters storm to form rain and release latent
heat
– Eye wall and eye
– Anticyclonic winds aloft, divergence aloft 15
Figure 7-18
Hurricane
Characteristics

16. Development and Movement
– Needs warm water and Coriolis
– Irregular paths within trade winds
– Typically begin moving east–west, some
curve poleward
16

17. When would hurricanes be most
frequent in north hemisphere?
Physical Geography: A Landscape
Appreciation 10e
17
0%
0%
0%
0%
0% 1. January – March
2. March –May
3. May – July
4. August – October
5. November - December

18. When?
18© 2011 Pearson Education, Inc.

19. Where do most hurricanes
develop?
Physical Geography: A Landscape
Appreciation 10e
19
0%
10%
40%
50% 1. Between 0 and 5 degrees latitude
2. Between 5 and 15 degrees latitude
3. Between 35 and 45 degrees latitude
4. Between 45 and 60 degrees latitude

20. Damage
– High winds, torrential rain, and isolated tornadoes
– Flooding: Storm surge and inland flooding
20
Figure 7-24

21. Camille Storm Surge
21© 2011 Pearson Education, Inc.

22. Physical Geography: A Landscape
Appreciation 10e
22

23. Winds and wind-driven waves
Physical Geography: A Landscape
Appreciation 10e
23

24. Inland Flooding
Physical Geography: A Landscape
Appreciation 10e
24

25. Saffir-Simpson scale
Physical Geography: A Landscape
Appreciation 10e
25

26. Localized Severe Weather
• Thunderstorms
– Violent storms with
thunder and lightning
– Formation stages
• Cumulus stage
• Mature stage
• Dissipating stage
– Atmospheric conditions
prone to thunderstorm
formation
26
Figure 7-25
Number of thunderstorms
per latitude: Figure 7-26

27. Localized Severe Weather
• Lightning
– Electric discharge in
thunderstorms
– Separation of charges due to
ice particles in a cloud
– Positive charges on Earth’s
surface
– Lightning types
• Cloud to ground
• Cloud to cloud
• Within cloud
– Thunder
27
Figure 7-29

28. Localized Severe Weather
• Tornadoes
– Deep low pressure vortex,
typically less than 400
meters in diameter
– Fast winds, sometimes in
excess of 300 mph
– Originate above ground,
water vapor condenses into
funnel cloud
– Contains vapor and debris
28
Figure 7-30

29. Localized Severe Weather
• Tornado formation
– Vertical wind shear creates rotation with horizontal axis
– Horizontal rotation tilted into vertical by thunderstorm updraft
– Mesocyclone and tornado development
29
Figure 7-31

30. Localized Severe Weather
• Tornado classification
30

31. Summary
• Storms can impact the landscape through damaging
winds and flooding rains
• Air masses form in regions of stagnant air and are
important for the weather in the midlatitudes
• Fronts are the boundaries between different air masses
• There are four primary types of fronts
• Midlatitude cyclones are low pressure systems that are
responsible for a majority of the weather in the
midlatitudes.
31

32. Summary
• Midlatitude anticyclones are related to midlatitude
cyclones
• Easterly waves are minor tropical disturbances
responsible for thunderstorms in the tropics
• Hurricanes are strong tropical cyclones which cause
catastrophic wind and storm surge flooding damage
• Thunderstorms are localized strong weather
phenomenon that include thunder, lightning, and heavy
rainfall.
32

33. Summary
• Lightning results from charge separation within a cloud.
• Thunder is caused by superheating of the atmosphere by
lightning and the resulting sound waves
• Tornadoes are violent vortices associated with strong,
rotating thunderstorms
33