Weather map drawing of slp isobars, Other Subject

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Your charge is to analyze the surface map on the last page of this document.  Temperature, dew point, SLP (sea-level-pressure), cloud cover, wind direction and speed are plotted following the surface station model. You are to draw isobars of SLP, locate fronts (type and position), indicate the center of a cyclone and identify air masses. You may want to briefly review material in the lectures that discussion extratropical cyclones as they contain the most relevant material.  

1. Perform a SLP analysis by drawing isobars to define the pressure pattern. Isobars are drawn at 4 mb intervals. Permitted values for isobars include: ...992 mb, 996 mb, 1000 mb, 1004 mb, 1008 mb, 1012 mb, 1016 mb, ... or any value evenly divisible by 4. Depending on the actual pressure values plotted on your map, you may need to use only some of these values or you may need to use values that are larger or smaller than the values listed.  

What are the largest and the smallest pressure values on your map? Which of the allowed isobar values in the list above fall between the largest and smallest pressure values on the map? Answer these questions before you begin the analysis. You should find that 1002.4 1008.0 mb and1019.41020.0 mb are the extreme observations on your map.  So your map will have isobars for values of 1004 mb, 1008 mb, 1012 mb, 1016 mb, 1020 mb.

 Sketch lightly in ordinary pencil on a practice map the position of the isobars. You will be making frequent changes in the position of the isobars, so keep a big handy. Contours can be darkened and labeled, and the analysis can be "prettied up" just before scanning it.

Only analyze the map east of Denver CO (125° W)  

2. Label the low-pressure center with a large "L".  Use a red pencil.  

3a. Properly position the cold front on the map. The northern end of the cold front should originate from the center of cyclone, where the "L" is centered. Cold fronts are found at the leading edge of a cold, dry air mass that is advancing equatorward and replacing a moister, warmer air mass. Try to identify large air masses on the map with distinct temperature and moisture (dew point) differences. The boundary between these air masses is a good place to start to try locating the front. From your readings you know also that winds will often blow from different directions on the two sides of a front. Clouds, rain or snow showers, and thunderstorms are often found along a cold front.   

3b. Properly position the warm front on the map. The western end of the warm front should originate from the center of low pressure. Warm fronts are found where warm, humid air is advancing poleward and replacing a colder air mass. Again, you should try to identify large air masses with distinct differences in temperature and moisture content. The boundary between these air masses is a good to start to try locating the front. You should then use the wind direction, clouds, and precipitation data to confirm or to adjust the location of the front.   

4.  Label a continental polar (cP) and maritime tropical (mT) air mass.

5. Outline the area where cloud cover is 6/8's or greater. Use a scalloped line.

6. Shade in green continuous areas receiving steady precipitation.

This assignment is given with the hope that you find analyzing a weather map fun, but a quality analysis takes time and effort. For that reason, I urge that you do not wait until the last moment to start the assignment. I want all of you to score well on this assignment, so if you have any questions, please email them ASAP over the class list serve.        

Additional Guidance for Analyzing Your First Weather Map Drawing of SLP Isobars and Identification of Fronts

Ackerman and Whitaker (1997) nicely summarize the basic rules of contouring. I quote verbatim:   

"1. isopleths should never cross,

  2. isopleths should not branch or fork,

  3. only the area on the map that has data should be contoured,

  4. you should label your isopleths..."  

Want a good grade on the exercise? Read the link. Twice if necessary. Then follow its advice. Closely.  

Drawing SLP contours can be challenging in that two weather observations, SLP and wind (speed and direction), must be jointly considered at each station to define the placement of, orientation of and spacing between isobars.  

1. In order to help facilitate for your first surface analysis, write out all the sea-level-pressures, which are in the upper right of each station. 

a. Put a decimal point between the last digit and next to the last digit so there is no confusion what the value of the tens, units and tenths digits are. 

b. Put a 9 or 10 to the front of the three plotted digits so that the value of the SLP report is between 950 mb and 1050 mb.   

2. Look for the highest and lowest pressures.   

3. Permitted values for SLP contours are ...996 mb, 1000 mb, 1008 mb, 1012 mb, 1016 mb, 1020 mb, 1020 mb... etc.  Pick the next highest permitted contour for the lowest SLP observation and the next permitted lowest for the highest SLP observation. These are the contour values you will use. For example, the highest SLP report on your map is 1020.0 mb, which means the biggest isobar value that you should draw is 1020 mb.  

4. Mark all the pressures between the low value and the first contour. Note stations with SLP reports that are the exactly divisible by 4 (e.g. 996, 1000, 1004, 1008, 1012, 1016 etc.) as the appropriate contour would be placed at that station; your weather map has observations like this. You will find a cluster of low SLP observations concentrated in one area of the map, and higher SLP values elsewhere.   

5. Estimate where the contour should be placed by interpolation between stations.  For example, if adjacent stations report a SLP of 1000 mb at site #1 and 1012 mb at site #2, the 1004 mb contour should be twice as far from site #2 than site #1.


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