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

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 Weather
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The Impact of Storms on the
Landscape
• Storm conditions can result in widespread damage
through flooding and wind damage
• Can provide diversity in vegetative cover and
increase lake and pond size
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Figure 7-B

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Air Masses
• Properties of an air mass
– Large (diameter > 1600 km)
– Uniform horizontal
properties
– Recognizable entity; travel
as one
• Origins of air masses
– Remains over a uniform
land or sea surface long
enough to acquire its
uniform characteristics
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Figure 7-2

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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 region
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Air Masses
• Source regions
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Figure 7-1

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Air Masses
• Properties
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Fronts
• Definition of a front
• History of the name “front”
• Clash over midlatitudes between polar and
tropical 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 air
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Fronts
• Cold Front
– Protruding “nose” of
cold air
– Faster than warm
fronts
– Lift warm air ahead of
cold fronts
– Identified by blue line
with triangles pointing
in direction of frontal
motion
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Figure 7-3

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Fronts
• Warm Front
– Gentle slope of warm
air rising above cool
air
– Slow cloud formation
and precipitation
– Indicated by red line
with semicircles
pointing in the
direction of warm air
motion
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Figure 7-4

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Atmospheric Disturbances
• Midlatitude disturbances
—i.e., midlatitude
cyclones
• Tropical disturbances—
easterly waves and
hurricanes
• Localized severe weather
—thunderstorms and
tornadoes
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Figure 7-7

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Midlatitude Cyclones
• Exist between 35–70°
latitude
• Roughly 1600 km in size
• Central pressure near 990
to 1000 mb
• Converging
counterclockwise circulation
in Northern Hemisphere
• Circulation creates fronts
• Westward tilt with increasing
elevation in Northern
Hemisphere 12
Figure 7-6

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Midlatitude Cyclones
• Weather changes
behind front
– Temperature
– Winds
– Pressure
• Cyclone movement
– Steered by jet stream
– System has a cyclonic
wind circulation
– Cold front advances faster
than center of the storm
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Figure 7-8

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Midlatitude Cyclones
• Life cycle of a cyclone—cyclogenesis to occlusion
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Figure 7-9

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

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Midlatitude Cyclones
• Occlusions—occluded front
– cold front catches warm front, removing the energy of
the storm (which is the warm air)
– occlusions mark the end of the cyclone’s life
– Marked as a purple line with alternating triangles and
half circles in direction of advancing cold air
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Figure 7-11

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Midlatitude Cyclones
• Occurrence and distribution
– Typically 6–15 cyclones exist worldwide
– More numerous and better developed in winter than in
summer
– Move more equatorward during summer
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Figure 7-13

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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 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

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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

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Major Tropical Disturbances:
Hurricanes
• Hurricane characteristics
– Prominent 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

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Major Tropical Disturbances:
Hurricanes
• Hurricane origin
– Over warm water
– A few degrees N or S of
equator
– No significant wind shear
– Hurricane season
• Hurricane movement
– Irregular tracks within the
flow of the trade winds
– Typically begin moving east–
west, some curve poleward
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Figure 7-19
Figure 7-21

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Major Tropical Disturbances:
Hurricanes
• Damage and destruction
– High winds, torrential rain,
and isolated tornadoes
– Primary destruction—
storm surge flooding
• Saffir-Simpson scale
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Figure 7-24

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Localized Severe Weather
• Thunderstorms
– Violent convective storms
– Accompanied by 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 thunderstorms
per latitude: Figure 7-26

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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
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Figure 7-29

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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
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Figure 7-30

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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
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Figure 7-31

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Localized Severe Weather
• 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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31. © 2011 Pearson Education, Inc.
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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