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  1. 1. Chapter 7: AtmosphericDisturbancesMcKnight’s Physical Geography:A Landscape Appreciation,Tenth Edition, Hess
  2. 2. © 2011 Pearson Education, Inc.Atmospheric Disturbances• The Impact of Storms on the Landscape• Air Masses• Fronts• Atmospheric Disturbances• Midlatitude Cyclones• Midlatitude Anticyclones• Minor Tropical Disturbances: Easterly Waves• Major Tropical Disturbances: Hurricanes• Localized Severe Weather2
  3. 3. © 2011 Pearson Education, Inc.The Impact of Storms on theLandscape• Storm conditions can result in widespread damagethrough flooding and wind damage• Can provide diversity in vegetative cover andincrease lake and pond size3Figure 7-B
  4. 4. © 2011 Pearson Education, Inc.Air Masses• Properties of an air mass– Large (diameter > 1600 km)– Uniform horizontalproperties– Recognizable entity; travelas one• Origins of air masses– Remains over a uniformland or sea surface longenough to acquire itsuniform characteristics4Figure 7-2
  5. 5. © 2011 Pearson Education, Inc.Air Masses• Air mass classification– Two letter classification system– Lowercase letter indicates moisture content• c—continental, dry• m—maritime, humid– Uppercase letter indicates source region• P—polar source region• T—tropical source region• A—arctic source region• E—equatorial source region5
  6. 6. © 2011 Pearson Education, Inc.Air Masses• Source regions6Figure 7-1
  7. 7. © 2011 Pearson Education, Inc.Air Masses• Properties7
  8. 8. © 2011 Pearson Education, Inc.Fronts• Definition of a front• History of the name “front”• Clash over midlatitudes between polar andtropical air masses• Four primary frontal types:– Cold front: cold air advancing– Warm front: warm air advancing– Stationary front: no advance of air masses– Occluded front: cold air overtakes warm air8
  9. 9. © 2011 Pearson Education, Inc.Fronts• Cold Front– Protruding “nose” ofcold air– Faster than warmfronts– Lift warm air ahead ofcold fronts– Identified by blue linewith triangles pointingin direction of frontalmotion9Figure 7-3
  10. 10. © 2011 Pearson Education, Inc.Fronts• Warm Front– Gentle slope of warmair rising above coolair– Slow cloud formationand precipitation– Indicated by red linewith semicirclespointing in thedirection of warm airmotion10Figure 7-4
  11. 11. © 2011 Pearson Education, Inc.Atmospheric Disturbances• Midlatitude disturbances—i.e., midlatitudecyclones• Tropical disturbances—easterly waves andhurricanes• Localized severe weather—thunderstorms andtornadoes11Figure 7-7
  12. 12. © 2011 Pearson Education, Inc.Midlatitude Cyclones• Exist between 35–70°latitude• Roughly 1600 km in size• Central pressure near 990to 1000 mb• Convergingcounterclockwise circulationin Northern Hemisphere• Circulation creates fronts• Westward tilt with increasingelevation in NorthernHemisphere 12Figure 7-6
  13. 13. © 2011 Pearson Education, Inc.Midlatitude Cyclones• Weather changesbehind front– Temperature– Winds– Pressure• Cyclone movement– Steered by jet stream– System has a cyclonicwind circulation– Cold front advances fasterthan center of the storm13Figure 7-8
  14. 14. © 2011 Pearson Education, Inc.Midlatitude Cyclones• Life cycle of a cyclone—cyclogenesis to occlusion14Figure 7-9
  15. 15. © 2011 Pearson Education, Inc.Midlatitude Cyclones• Upper level divergence and convergence related tocyclogenesis15Figure 7-10
  16. 16. © 2011 Pearson Education, Inc.Midlatitude Cyclones• Occlusions—occluded front– cold front catches warm front, removing the energy ofthe storm (which is the warm air)– occlusions mark the end of the cyclone’s life– Marked as a purple line with alternating triangles andhalf circles in direction of advancing cold air16Figure 7-11
  17. 17. © 2011 Pearson Education, Inc.Midlatitude Cyclones• Occurrence and distribution– Typically 6–15 cyclones exist worldwide– More numerous and better developed in winter than insummer– Move more equatorward during summer17Figure 7-13
  18. 18. © 2011 Pearson Education, Inc.Midlatitude Anticyclones• Anticyclones—high pressure systems– Subsiding, diverging winds at the surface– Flow is clockwise around an anticyclone– Move slightly slower than cyclones• Relationship to cyclones– Occur independently, but have a functional relationship– Anticyclone follows a cyclone– Anticyclones typically reside behind cyclone’s cold front18
  19. 19. © 2011 Pearson Education, Inc.Minor Tropical Disturbances:Easterly Waves• Easterly wave characteristics– Oriented N–S– Little cyclonic circulation– Convergence behind wave,divergence ahead of wave– Can intensify to tropicalcyclones19Figure 7-15
  20. 20. © 2011 Pearson Education, Inc.Major Tropical Disturbances:Hurricanes• Tropical cyclone definition• Tropical depression—winds< 38 mph• Tropical storm—winds 38–74mph• Hurricane—winds > 74 mph– Typhoons– Baguios– Cyclones20Figure 7-16
  21. 21. © 2011 Pearson Education, Inc.Major Tropical Disturbances:Hurricanes• Hurricane characteristics– Prominent low pressurecenter, winds spiral inward– Steep pressure gradientand strong winds– Warm moist air entersstorm to form rain andrelease latent heat– Eye wall and eye– Anticyclonic winds aloft,divergence aloft21Figure 7-18
  22. 22. © 2011 Pearson Education, Inc.Major Tropical Disturbances:Hurricanes• Hurricane origin– Over warm water– A few degrees N or S ofequator– No significant wind shear– Hurricane season• Hurricane movement– Irregular tracks within theflow of the trade winds– Typically begin moving east–west, some curve poleward22Figure 7-19Figure 7-21
  23. 23. © 2011 Pearson Education, Inc.Major Tropical Disturbances:Hurricanes• Damage and destruction– High winds, torrential rain,and isolated tornadoes– Primary destruction—storm surge flooding• Saffir-Simpson scale23Figure 7-24
  24. 24. © 2011 Pearson Education, Inc.Localized Severe Weather• Thunderstorms– Violent convective storms– Accompanied by thunderand lightning– Formation stages• Cumulus stage• Mature stage• Dissipating stage– Atmospheric conditionsprone to thunderstormformation24Figure 7-25Number of thunderstormsper latitude: Figure 7-26
  25. 25. © 2011 Pearson Education, Inc.Localized Severe Weather• Lightning– Electric discharge inthunderstorms– Separation of charges due toice particles in a cloud– Positive charges on Earth’ssurface– Lightning types• Cloud to ground• Cloud to cloud• Within cloud– Thunder25Figure 7-29
  26. 26. © 2011 Pearson Education, Inc.Localized Severe Weather• Tornadoes– Deep low pressure vortex,typically less than 400meters in diameter– Fast winds, sometimes inexcess of 300 mph– Originate above ground,water vapor condenses intofunnel cloud– Contains vapor and debris26Figure 7-30
  27. 27. © 2011 Pearson Education, Inc.Localized Severe Weather• Tornado formation– Vertical wind shear creates rotation with horizontal axis– Horizontal rotation tilted into vertical by thunderstorm updraft– Mesocyclone and tornado development27Figure 7-31
  28. 28. © 2011 Pearson Education, Inc.Localized Severe Weather• Tornado classification28
  29. 29. © 2011 Pearson Education, Inc.Summary• Storms can impact the landscape through damagingwinds and flooding rains• Air masses form in regions of stagnant air and areimportant 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 areresponsible for a majority of the weather in themidlatitudes.29
  30. 30. © 2011 Pearson Education, Inc.Summary• Midlatitude anticyclones are related to midlatitudecyclones• Easterly waves are minor tropical disturbancesresponsible for thunderstorms in the tropics• Hurricanes are strong tropical cyclones which causecatastrophic wind and storm surge flooding damage• Thunderstorms are localized strong weatherphenomenon that include thunder, lightning, and heavyrainfall.30
  31. 31. © 2011 Pearson Education, Inc.Summary• Lightning results from charge separation within a cloud.• Thunder is caused by superheating of the atmosphere bylightning and the resulting sound waves• Tornadoes are violent vortices associated with strong,rotating thunderstorms31