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 SURFACE VERSUS UPPER AIR STATION PLOTS

METEOROLOGIST JEFF HABY

Although they look identical at first inspection, there are huge fundamental differences between surface station plots and upper level station plots. Station plots give information such as temperature, dewpoint, cloud cover, weather, pressure (or height), dewpoint depression, visibility, wind direction, wind speed and height change. The four biggest distinctions between surface and upper air observations are the temperature units, the representation of dewpoint, the meaning of a filled in station plot circle, and the height/pressure representation.

Surface observations in the U.S. have the dewpoint and temperature plotted in degrees Fahrenheit. Observations plotted on an upper level chart have the temperature and dewpoint depression plotted in units of Celsius. On a surface chart, dewpoint is plotted while on an upper level chart the dewpoint depression is plotted. The dewpoint depression is the temperature difference between the temperature and the dewpoint. While the dewpoint is plotted directly on a surface plot it must be inferred on an upper air chart. The dewpoint is located below the temperature on a surface plot. The dewpoint depression is plotted below the temperature on an upper air plot. The meaning of a filled in circle is cloudy on a surface chart but means the dewpoint depression is less than 6 on an upper air chart. On an upper air chart the circle will either be blank or shaded it. On a surface plot, the circle could be shaded none, 1/4,1/2,3/4 or completely shaded depending on how much cloud cover is present. A station plot shows pressure without the leading 9 or 10. Example: 945 is 994.5 millibars while 034 is 1003.4 millibars. Upper air plots have a separate coding for each analysis chart level.

A surface chart is available at:

http://www.atmos.albany.edu/weather/difax.html