This document summarizes atmospheric disturbances including storms, air masses, fronts, midlatitude cyclones, hurricanes, thunderstorms, lightning, and tornadoes. It explains that storms can damage landscapes through flooding and winds. Air masses form over land or sea and drive weather patterns. Fronts are boundaries between different air masses like cold and warm fronts. Midlatitude cyclones bring weather changes as they move with associated fronts. Hurricanes are powerful tropical cyclones that cause high winds and flooding from storm surge. Thunderstorms, lightning, and tornadoes are localized severe weather phenomena.

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
2

3. © 2011 Pearson Education, Inc.
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
3
Figure 7-B

4. © 2011 Pearson Education, Inc.
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
4
Figure 7-2

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 region
5

6. © 2011 Pearson Education, Inc.
Air Masses
• Source regions
6
Figure 7-1

7. © 2011 Pearson Education, Inc.
Air Masses
• Properties
7

8. © 2011 Pearson Education, Inc.
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
8

9. © 2011 Pearson Education, Inc.
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
9
Figure 7-3

10. © 2011 Pearson Education, Inc.
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
10
Figure 7-4

11. © 2011 Pearson Education, Inc.
Atmospheric Disturbances
• Midlatitude disturbances
—i.e., midlatitude
cyclones
• Tropical disturbances—
easterly waves and
hurricanes
• Localized severe weather
—thunderstorms and
tornadoes
11
Figure 7-7

12. © 2011 Pearson Education, Inc.
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

13. © 2011 Pearson Education, Inc.
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
13
Figure 7-8

14. © 2011 Pearson Education, Inc.
Midlatitude Cyclones
• Life cycle of a cyclone—cyclogenesis to occlusion
14
Figure 7-9

15. © 2011 Pearson Education, Inc.
Midlatitude Cyclones
• Upper level divergence and convergence related to
cyclogenesis
15
Figure 7-10

16. © 2011 Pearson Education, Inc.
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
16
Figure 7-11

17. © 2011 Pearson Education, Inc.
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
17
Figure 7-13

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 front
18

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 tropical
cyclones
19
Figure 7-15

20. © 2011 Pearson Education, Inc.
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
20
Figure 7-16

21. © 2011 Pearson Education, Inc.
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
21
Figure 7-18

22. © 2011 Pearson Education, Inc.
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
22
Figure 7-19
Figure 7-21

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 scale
23
Figure 7-24

24. © 2011 Pearson Education, Inc.
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
24
Figure 7-25
Number of thunderstorms
per latitude: Figure 7-26

25. © 2011 Pearson Education, Inc.
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
25
Figure 7-29

26. © 2011 Pearson Education, Inc.
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
26
Figure 7-30

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 development
27
Figure 7-31

28. © 2011 Pearson Education, Inc.
Localized Severe Weather
• Tornado classification
28

29. © 2011 Pearson Education, Inc.
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.
29

30. © 2011 Pearson Education, Inc.
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.
30

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
31