Geography 1 (41100). Prof. Dean . Wint., Jan 17, 2012 . Lect and Chp 7
Chapter 7
Transient Atmospheric Flows and Disturbances
I. The Impact of Storms on the Landscape
a. Storm conditions can result in widespread damage through flooding
and wind damage.
b. Can provide diversity in vegetative cover and increase lake and pond
size.
II. Air Masses
a. Properties of an air mass
i. Large (diameter > 1600 km)
ii. Uniform horizontal properties.
iii. Recognizable entity; travel as one.
iv. When this air travels it will then change the temperature and
weather of the air it passes through.
b. Origins of air masses
i. Remains over a uniform land or sea surface long enough to
acquire its uniform characteristics.
ii. Is created in areas of sinking air.
c. Air mass classification
i. Two letter classification system.
ii. Lowercase letter indicates moisture content.
1. c=continental, dry
2. m=maritime, humid
iii. Uppercase letter indicates source region (and temperature)
1. P=polar source region
2. T=tropical source region
3. A=arctic source region
4. E=equatorial source region
iv. Source regions
a. The warmest water will be above land and the
coldest will be above, if you are excluding Arctic
and Equatorial.
2. Arctic=A
Very cold, dry and stable
3. Continental Polar=cP
Cold, dry, very stable
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Geography 1 (41100). Prof. Dean . Wint., Jan 17, 2012 . Lect and Chp 7
4. Maritime polar=mP
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Cold, moist, relatively
untable
5. Continental tropical=cT
Hot, very dry, unstable
6. Maritime Tropical=mT
Warm, moist, variable
7. Equatorial=E
Warm, moist, unstable
III. Fronts
a. Meeting of two air masses where a distinct and narrow gradient will
develop. Initially it was compared to two armies meeting in battle.
b. Clash over midlatitudes between polar and tropical air masses.
c. Four primary frontal types:
i. Cold front: cold air advancing.
ii. Warm front: warm air advancing
iii. Stationary front: no advance of air masses
iv. Occluded front: cold air overtakes warm air
d. Cold Front
i. Protruding “nose” of cold air.
ii. Faster than warm fronts
iii. Life warm air ahead of cold fronts
iv. Identified by blue line with triangles pointing in direction of
frontal motion.
e. Warm Front
i. Gentle slope of warm air rising above cool air
ii. Slow cloud formation and precipitation
iii. Indicated by red line with semicircles pointing in the direction
of warm air motion.
IV. Atmospheric Disturbances
i. Midlatitude disturbances—i.e., midlatitude cyclones
ii. Tropical disturbances—easterly waves and hurricanes
iii. Localized severe weather—thunderstorms and tornadoes
V. Midlatitude Cyclones
a. Exist between 35-70 Degrees latitude
b. Roughly 16—km in size
c. Central pressure near 990 to 1000 mb
d. Converging counterclockwise circulation in Northern Hemisphere.
e. Circulation creates fronts
Geography 1 (41100). Prof. Dean . Wint., Jan 17, 2012 . Lect and Chp 7
f. Westward tilt with increasing elevation in Northern Hemisphere
VI. Midlatitude Cyclones
a. Weather changes behind front
i. Temperature
ii. Winds
iii. Pressure
b. Cyclone movement
i. Steered by jet stream
ii. System has a cyclonic wind circulation
iii. Cold front advances faster than center of the storm.
c. Life cycle of cyclone is called cyclogensis to occlusion
d. Upper level divergence and convergence related to cyclogenesis
e. Occlusions—occluded front
i. Cold front catches warm front, removing the energy of the
storm (which is the warm air)
ii. Occlusions mark the end of the cyclone’s life
iii. Marked as a purple line with alternating triangles and half
circles in direction of advancing cold air.
f. Occurrence and distribution
i. Typically 6-15 cyclones exist worldwide
ii. More numerous and better developed in winter than in
summer
iii. Move more equatorward during summer.
VII. Midlatitude Anticyclones
a. Anticyclones- high pressure systems
i. Subsiding, diverging winds at the surface
ii. Flow is clockwise around and anticyclone
iii. Move slightly slower than cyclones
b. Relationship to cyclones
i. Occur independently, but have a functional relationship
ii. Anticyclone follows a cyclone
iii. Anticyclones typically reside behind cyclone’s cold front
VIII. Minor Tropical Disturbances: Easterly Waves
a. Easterly wave characteristics
i. Oriented N-S
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Geography 1 (41100). Prof. Dean . Wint., Jan 17, 2012 . Lect and Chp 7
ii. Little cyclonic circulation
iii. Convergence behind wave, divergence ahead of wave
iv. Can intensify to tropical cyclones
IX. Major Tropical Disturbances: Hurricanes
a. Tropical cyclone definition
b. Tropical depression—winds < 38 mph
c. Tropical storm—winds 38-74 mph
d. Hurricane—winds > 74 mph
i. Typhoons
ii. Baguios
iii. Cyclones
e. Hurricane characteristics
i. Prominent low pressure center, winds spiral inward
ii. Steep pressure gradient and strong winds
iii. Warm moist air enters storm to form rain and release latent
heat.
iv. Eye wall and eye
v. Anticyclonic winds, aloft divergence aloft.
f. Hurricane origin
i. Over warm water
ii. A few degrees N or S of equator
iii. No significant wind shear
iv. Hurricane season
g. Hurricane movement
i. Irregular tracks within the flow of the trade winds
ii. Typically begin moving east—west, some curve poleward
h. Damage and destruction
i. High winds, torrential rain, and isolated tornadoes
ii. Primary destruction—storm surge flooding
X. Localized Severe Weather
a. Thunderstorms
i. Violent connective storms
ii. Accompanied by thunder and lightning
iii. Formation stages
1. Cumulus stage
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Geography 1 (41100). Prof. Dean . Wint., Jan 17, 2012 . Lect and Chp 7
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2. Mature stage
3. Dissipating stage
iv. Atmospheric conditions prone to thunderstorm formation
b. Lightning
i. Electric discharge in thunderstorms
ii. Separation of charges due to ice particles in a cloud
iii. Positive charges on Earth’s surface
iv. Lightning types
1. Cloud to ground
2. Cloud to cloud
3. Within cloud
v. Thunder
c. Tornadoes
i. Deep low pressure vortex, typically less than 400 meters in
diameter
ii. Fast winds, sometimes in excess of 300 mph
iii. Originate above ground, water vapor condenses into funnel
cloud
iv. Contains vapor and debris
v. Tornado formation
1. Vertical wind shear creates rotation with horizontal axis
2. Horizontal rotation titled into vertical by thunderstorm
updraft
3. Mesocyclone and tornado development.