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Colorado Snow-‐-‐ Fronts
Nelson pages 22-26, 211,
229-230
Air Masses• air mass– an extremely large body of air whose properties of
temperature and humidity are fairly similar in any horizontal direction at any given latitude
• Source regions– regions where air masses originate– Over water?– Over dry land?– In a cold region?– In a warm region?
• Air mass classification– Air mass classification uses a two letter
classification scheme– First letter:
• c – continental source region• m – maritime source region
– Second letter:• P – polar source region• T – tropical source region
– There are four basic air mass types:• cP – continental polar air mass• mP – maritime polar air mass• cT – continental tropical air mass• mT – maritime tropical air mass
– Extreme air mass:• cA – continental Arctic air mass
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Figure11-3 p296
Figure11.3 A shallow but large dome of extremely cold air — a conCnental arcCc air mass — moves slowly southeastward across the upper plains. The leading edge of the air mass is marked by a cold front. (Numbers represent air temperature, °F.)
• Cold Front– Cold, dry air replaces warm, moist air– A cold front is drawn on weather maps as a line with triangle
symbols, with the triangles pointing in the direction that the front is moving towards.
– How do meteorologists locate fronts on a weather map?• Large change in temperature over a short distance• Change in moisture content of air• Shift in wind direction• Pressure and pressure changes• Cloud and precipitation patterns
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Fig. 8-‐11, p. 215
Fig. 8-‐15, p. 217
Figure: A verCcal view of the weather across the cold front in Fig. 8.13 along the line X–X’.
Fig. 8-‐13, p. 216
Figure 8.13: A closer look at the surface weather associated with the cold front situated in the southeastern United States in Fig. 8.12. (Gray lines are isobars. Dark green-‐shaded area represents rain; white-‐shaded area represents snow.)
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Fig. 8-‐14, p. 216
Figure 8.14: A Doppler radar image showing precipitaCon paUerns along a cold front similar to the cold front in Fig. 8.13. Green represents light-‐to-‐moderate precipitaCon; yellow represents heavier precipitaCon; and red the most likely areas for thunderstorms.
Figure: C) Clouds and precipitation D) Wind direction
Figure: Oklahoma City cold frontal passage.
Data from David M. Schultz, University of Helsinki/ASM/NOAA
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• Stationary Front– A nearly stationary (not moving) boundary between two air
masses– A stationary front is drawn on weather maps as alternating
triangle and semi-circle symbols on opposite sides of the front
– What types of weather are associated with stationary fronts?
• Clear or cloudy skies• Light rain possible
• Warm Fronts– Warm, moist air replaces cold,
dry air– A warm front is drawn on
weather maps as a line with semi-circle symbols, with the semi-circles pointing in the direction that the front is moving towards.
Figure 8.17: Surface weather associated with a typical warm front. (Green-‐shaded area represents rain, pink-‐shaded area represents freezing rain and sleet; white-‐shaded area represents snow.) Watch this AcCve Figure on ThomsonNow website at www.thomsonedu.com/login.
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Figure: Surface weather associated with a warm front
Warm Front
Figure: VerCcal view of clouds, precipitaCon, and winds across the warm front in Fig. 8.17 along the line P–P’.
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• Occluded Fronts – Form when a cold front catches up to a warm front
– An occluded front is drawn on weather maps as a line with triangle and semi-‐circle symbols on the same side of the line, with the symbols poinCng in the direcCon that the front is moving towards.
– Cold-‐type occluded front (cold occlusion)
• Air behind the cold front is colder than the air ahead of the warm front
– Warm-‐type occluded front (warm occlusion)
• Air behind the cold front is not as cold as the air ahead of the warm front
– O]en it is difficult to idenCfy occluded fronts from a surface weather map only.
Fronts
• Front – a transition zone between two air masses of different densities• There are four basic types of fronts:
– Cold– Warm– Stationary– Occluded
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Fig. 8-‐22, p. 223
Figure: A visible satellite image showing a mid-‐laCtude cyclonic storm with its weather fronts over the AtlanCc Ocean during March, 2005. Superimposed on the image is the posiCon of the surface cold front, warm front, and occluded front. PrecipitaCon symbols indicate where precipitaCon is reaching the surface.
Typical Frontal locations in a cyclone
Colorado FROPAs (Frontal Passages)
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Cme
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The November 2014 cold snap that killed many trees/bushes on the Front Range
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The Great Freeze of 1951
• January 31 Denver temps were -‐24 • February 1, -‐60 at Taylor Park Dam and -‐40 in some Front Range areas, killing many cherry orchards and wine growers, Denver was -‐25 and Fort Collins -‐41
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Cold Christmas of 1983
• Temperatures fell below zero for 115 straight hours
• Colorado Springs set six consecuCve record lows from December 19-‐25 with a Christmas morning low of -‐15!