Chapter 9 Air Masses and Fronts

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Chapter 9Air Masses and Fronts
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Source RegionsFigure from apollo.lsc.vsc.edu/clas
ses/met130
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Cold Front

• Cold air advances, replaces warm air at the
  surface
• Change in wind direction/speed
• Minimum in atmospheric pressure

Fig. 9-14, p. 266
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Cold Front Cross Section

• A front is a 3-D boundary
• Front slopes back over the cold air mass
• Warm, less dense air is lifted
• Clouds/precipitation associated with a front
  depend on stability and moisture
• Sharp vertical motion at cold front can force
  thunderstorm activity

Fig. 9-15, p. 266
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Fig. 9-16, p. 267
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Warm Front

• Warm air advances
• Replaces the cold air at the surface
• Change in wind direction/speed

Fig. 9-17, p. 268
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Warm Front Cross Section
Fig. 9-18, p. 269

• Front slopes back over the cold air mass
• Slope is more gentle than with a cold front (less
  thunderstorm activity)
• Warm, less dense air lifted over the cold air
  (called overrunning)
• Clouds/precipitation depend on moisture and
  stability, usually follow a set progression with
  an increase in altitude
• Responsible for a lot of hazardous winter weather

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Fig. 9-19, p. 270
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Stationary Front

• Air masses at surface do not move, so the front
  is stationary
• Overrunning still occurring, so we often still
  see cloudiness
• Figure from ww2010.atmos.uiuc.edu

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Occluded Front

• Separates cool air from relatively colder air at
  the surface
• Sometimes thought of as the cold front catching
  up to warm front
• The warm air mass is found above the ground
• Two types
• Cold-type occluded front
• Warm-type occluded front
• Figure from ww2010.atmos.uiuc.edu

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Development of Occluded FrontFigures from
ww2010.atmos.uiuc.edu
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Cross Section of Occluded Front
Fig. 9-20, p. 271
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Occluded Front
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Dryline

• Dry air (lower dewpoint temperatures) found to
  west, moist air (higher dewpoint temperatures)
  found to east
• Temperature change is rather limited across the
  boundary
• Common in the southern plains during the spring
• It is a convergence line for wind at the surface,
  and is therefore responsible for initiating many
  of our tornadic thunderstorms in the south Plains
• Motion is tied strongly to insolation, and
  typically exhibits a diurnal sloshing motion
  (moving eastward during the day, westward at
  night)

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Fig. 9-21, p. 272
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Air Masses with the Drylinewww.geog.umn.edu/facul
ty/klink/geog1425/images/front/dryline_airmass.jpg
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Surface Dew Points
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5/10/10 Tornado Outbreak
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http//www.youtube.com/watch?vtdDRBaDRf40
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