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Chapter 11

1 Concordia University Geog/Sci-381 Chapter 11

Air Masses

�  Introductory Anecdote: Illinois, 1836. � Extremely large body of air whose

temperature and humidity are similar horizontally and vertically.

� Source Regions: area where air mass originates, usually flat and uniform composition with light surface winds



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Air Masses

� Classification � Classification based upon temperature and

humidity ○  P = polar ○  T = tropical ○  A = Arctic ○  m = maritime ○  c = continental



Air Masses

� North America cP and cA �  Source region: N. Canada, Alaska � Dry, cold, stable (A more extreme)

� Topic: Lake Effect Snow �  cP air passes over unfrozen lake, absorbs

moisture and drops snow on leeward side of lake


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Fig. 11-3, p. 290

Fig. 1, p. 291

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Fig. 2, p. 291



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Fig. 3, p. 294

Air Masses

� North American mP �  Source region: North Pacific, North Atlantic � Cool, moist, unstable

� North American mT �  Source region: Gulf of Mexico, Caribbean,

SE Pacific � Wet, warm, unstable �  Pineapple Express, Bermuda High


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Fig. 11-8, p. 295

Fig. 11-10, p. 296


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Air Masses

� North American cT �  Source Region: SW US, Mexican Plateau � Hot, dry, stable


Fig. 11-12, p. 297

Fronts

�  Transition zone between two air masses of different densities

�  Identification on Charts 1.  Sharp temperature change 2.  Sharp change in dew point 3.  Shift in wind direction 4.  Sharp pressure change 5.  Clouds and precipitation Frontolysis (a front weakening and dissipating)

and frontogenesis (a strengthening front)


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Fig. 11-14, p. 299

Fronts

� Stationary �  Front with no movement �  Alternating red and blue line with blue

triangles and red semi-circles � Winds parallel but opposite direction �  Variable weather



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Fronts

� Cold � Cold, dry stable air replaces warm, moist

unstable air �  Blue line with blue triangles � Clouds of vertical development �  Thunderstorms, squall lines




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Table 11-2, p. 303

Fronts

� Warm � Warm, moist unstable air overrides cold, dry

stable air � Red line with red semi-circles � Horizontal cloud development with steady

rain � Topic: Dry Line

� Not a cold or warm front but a narrow boundary of steep change in dew point


Fig. 11-20, p. 304

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Fig. 11-21, p. 304

Fig. 4, p. 305

Table 11-3, p. 306

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Fronts

� Occluded Front � Cold front catches up to and over takes a

warm front �  Purple line with purple triangles and semi-

circles � Cold occlusion, warm occlusion




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Table 11-4, p. 308

Current Surface Map

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Homework for Chapter 11 �  Chapter 11 Questions for Review

�  #2-5, 8

�  Chapter 11 Questions for Thought �  #4, 6

�  Chapter 11 Problems and Exercises �  None


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