0
Chapter 7: Atmospheric
Disturbances
McKnight’s Physical Geography:
A Landscape Appreciation,
Tenth Edition, Hess
Class 27
Midlatitude Cyclones,
Anticyclones
Tropical Cyclones or Hurricanes
Atmospheric Disturbances
• Midlatitudes cyclones and
anticyclones
• Tropical disturbances—
easterly waves and
hurricanes
•...
Midlatitude Cyclones
• 35–70° latitude
• ~ 1600 km across
• Central pressure - 990 to 1000 mb
• Converging counterclockwis...
Stages
• 1. Clashing air
masses
• 2. “Wave” disturbance
begins
• 3. Cold and warm
fronts
• 4. Occluded front
• 5. Dissipat...
Physical Geography: A Landscape
Appreciation 10e
6
Physical Geography: A Landscape
Appreciation 10e
7
• Weather changes
as front moves by
– Temperature
– Winds
– Pressure
– Dew Point
• Cyclone movement
– Steered by jet strea...
• Cyclogenesis to
Occlusion
9
Figure 7-9
Midlatitude Cyclones
• Upper level divergence and convergence related to
cyclogenesis
10
Figure 7-10
~ 6–15 worldwide
11
Figure 7-13
Midlatitude Anticyclones
• Anticyclones—high pressure systems
– Subsiding, diverging winds at the surface
– Flow is clockw...
Minor Tropical Disturbances:
Easterly Waves
• Easterly wave characteristics
– Oriented N–S
– Little cyclonic circulation
–...
Major Tropical Disturbances:
Hurricanes
• Tropical cyclone definition
• Tropical depression—winds
< 38 mph
• Tropical stor...
– Low pressure center, winds spiral inward
– Steep pressure gradient and strong winds
– Warm moist air enters storm to for...
Development and Movement
– Needs warm water and Coriolis
– Irregular paths within trade winds
– Typically begin moving eas...
When would hurricanes be most
frequent in north hemisphere?
Physical Geography: A Landscape
Appreciation 10e
17
0%
0%
0%
0...
When?
18© 2011 Pearson Education, Inc.
Where do most hurricanes
develop?
Physical Geography: A Landscape
Appreciation 10e
19
0%
10%
40%
50% 1. Between 0 and 5 de...
Damage
– High winds, torrential rain, and isolated tornadoes
– Flooding: Storm surge and inland flooding
20
Figure 7-24
Camille Storm Surge
21© 2011 Pearson Education, Inc.
Physical Geography: A Landscape
Appreciation 10e
22
Winds and wind-driven waves
Physical Geography: A Landscape
Appreciation 10e
23
Inland Flooding
Physical Geography: A Landscape
Appreciation 10e
24
Saffir-Simpson scale
Physical Geography: A Landscape
Appreciation 10e
25
Localized Severe Weather
• Thunderstorms
– Violent storms with
thunder and lightning
– Formation stages
• Cumulus stage
• ...
Localized Severe Weather
• Lightning
– Electric discharge in
thunderstorms
– Separation of charges due to
ice particles in...
Localized Severe Weather
• Tornadoes
– Deep low pressure vortex,
typically less than 400
meters in diameter
– Fast winds, ...
Localized Severe Weather
• Tornado formation
– Vertical wind shear creates rotation with horizontal axis
– Horizontal rota...
Localized Severe Weather
• Tornado classification
30
Summary
• Storms can impact the landscape through damaging
winds and flooding rains
• Air masses form in regions of stagna...
Summary
• Midlatitude anticyclones are related to midlatitude
cyclones
• Easterly waves are minor tropical disturbances
re...
Summary
• Lightning results from charge separation within a cloud.
• Thunder is caused by superheating of the atmosphere b...
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  1. 1. Chapter 7: Atmospheric Disturbances McKnight’s Physical Geography: A Landscape Appreciation, Tenth Edition, Hess
  2. 2. Class 27 Midlatitude Cyclones, Anticyclones Tropical Cyclones or Hurricanes
  3. 3. Atmospheric Disturbances • Midlatitudes cyclones and anticyclones • Tropical disturbances— easterly waves and hurricanes • Localized severe weather —thunderstorms and tornadoes 3 Figure 7-7
  4. 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. 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. 6. Physical Geography: A Landscape Appreciation 10e 6
  7. 7. Physical Geography: A Landscape Appreciation 10e 7
  8. 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. 9. • Cyclogenesis to Occlusion 9 Figure 7-9
  10. 10. Midlatitude Cyclones • Upper level divergence and convergence related to cyclogenesis 10 Figure 7-10
  11. 11. ~ 6–15 worldwide 11 Figure 7-13
  12. 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. 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. 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. 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. 16. Development and Movement – Needs warm water and Coriolis – Irregular paths within trade winds – Typically begin moving east–west, some curve poleward 16
  17. 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. 18. When? 18© 2011 Pearson Education, Inc.
  19. 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. 20. Damage – High winds, torrential rain, and isolated tornadoes – Flooding: Storm surge and inland flooding 20 Figure 7-24
  21. 21. Camille Storm Surge 21© 2011 Pearson Education, Inc.
  22. 22. Physical Geography: A Landscape Appreciation 10e 22
  23. 23. Winds and wind-driven waves Physical Geography: A Landscape Appreciation 10e 23
  24. 24. Inland Flooding Physical Geography: A Landscape Appreciation 10e 24
  25. 25. Saffir-Simpson scale Physical Geography: A Landscape Appreciation 10e 25
  26. 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. 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. 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. 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. 30. Localized Severe Weather • Tornado classification 30
  31. 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. 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. 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
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