SHW Ch. 3 - Weather Maps

• Refer to your ATM210 Weather Workshop Booklets to refresh yourself on the "Station Model Plotting Convention" for surface plots

• Surface Charts: all barometric pressure readings are "reduced" to sea level (SLP) in order to produce a mean sea level surface analysis

• Constant Pressure Charts: standard pressure levels include 850 mb, 700 mb, 500 mb, 300 mb, 250 mb and 200 mb ; note that constant altitude charts would convey similar information (e.g., 5700 m chart would look similar to a 500 mb chart); below are some numbers one must commit to memory:

  
  	  Pressure    Approx.         Approx.
  	  Level       Altitude	      Altitude
  	  850 mb      5,000 feet      1,500 m
  	  700 mb     10,000 feet      3,000 m
  	  500 mb     18,000 feet      5,500 m
  	  300 mb     30,000 feet      9,000 m
  	  250 mb     35,000 feet     10,500 m
  	  200 mb     39,000 feet     12,000 m
  
  

• 1 meter = 3.281 feet ; 1 degree latitude = 60 nautical miles (69 statute miles)

• 850 mb Chart: location of the low-level jet (which transports heat and moisture northward); useful in identifying frontal boundaries; the 0° C isotherm provides a crude rain/snow line; this becomes a "fictitious" level for those places with elevations above 1500 m (e.g., Denver, CO)

• 700 mb Chart: this level intersects many clouds, thus moisture distribution is important; dry intrusions at this level are a precursor to severe weather; weak surface low pressure systems will be "driven" by the flow at this level

• 500 mb Chart: this level is used to determine the location of short waves and long waves (associated with ridges and troughs in the air flow pattern); absolute vorticity is plotted at this level; moderate to strong low pressure systems will be "driven" by the flow at this level

• 300 mb, 250 mb & 200 mb Charts: these levels are located near the top of the troposphere and in the lower stratosphere; the jetstream winds and jetstreaks are found near these levels; important levels for confluence/difluence