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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!