Pg class27-midlat cyclones and anti, hurricanes

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

• Localized severe weather—thunderstorms and tornadoes

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Figure 7-7

Midlatitude Cyclones• 35–70° latitude

• ~ 1600 km across

• Central pressure - 990 to 1000 mb

• Converging counterclockwise in Northern Hemisphere

• Circulation creates fronts

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Figure 7-6

Stages• 1. Clashing air

masses• 2. “Wave” disturbance

begins• 3. Cold and warm

fronts• 4. Occluded front• 5. Dissipation

5Physical Geography: A Landscape

Appreciation 10e

Physical Geography: A Landscape Appreciation 10e

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• 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

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Figure 7-8

• Cyclogenesis to Occlusion

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Figure 7-9

Midlatitude Cyclones

• Upper level divergence and convergence related to cyclogenesis

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Figure 7-10

~ 6–15 worldwide

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Figure 7-13

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

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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

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Figure 7-15

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

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Figure 7-16

– 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

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Figure 7-18

HurricaneCharacteristics

Development and Movement

– Needs warm water and Coriolis– Irregular paths within trade winds– Typically begin moving east–west, some

curve poleward16

When would hurricanes be most frequent in north hemisphere?


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0%

0%

0%

0%

0% 1. January – March

2. March –May

3. May – July

4. August – October

5. November - December

When?

18© 2011 Pearson Education, Inc.

Where do most hurricanes develop?


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0%

10%

40%

50% 1. Between 0 and 5 degrees latitude

2. Between 5 and 15 degrees latitude



Damage– High winds, torrential rain, and isolated tornadoes

– Flooding: Storm surge and inland flooding

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Figure 7-24

Camille Storm Surge

21© 2011 Pearson Education, Inc.


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Winds and wind-driven waves


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Inland Flooding


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Saffir-Simpson scale


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Localized Severe Weather

• Thunderstorms– Violent storms with

thunder and lightning– Formation stages

• Cumulus stage• Mature stage• Dissipating stage

– Atmospheric conditions prone to thunderstorm formation

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Figure 7-25

Number of thunderstormsper latitude: Figure 7-26


• 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

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Figure 7-29


• 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

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Figure 7-30


• Tornado formation– Vertical wind shear creates rotation with horizontal axis– Horizontal rotation tilted into vertical by thunderstorm updraft– Mesocyclone and tornado development

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Figure 7-31


• Tornado classification

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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.

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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.

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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

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