The following is a description from the documentation of the WXMAP program used to generate the surface analysis images. Used with permission of the author.

Interpreting surface analysis weather maps

The cloud symbol is always centered on the station location itself and appears as a circle. The degree to which the circle is filled in is an indication of the amount of cloud cover.
  • a clear sky

  • a partly cloudy sky

  • a mostly cloudy sky

  • an overcast sky

  • a sky obscured by fog or other visibility limitation

  • indicates that the cloud cover could not be parsed out of the original report

  • If clouds are present, a number below and slightly to the right of the circle will indicate, in hundreds of feet, the height of the cloud base. If a broken (mostly cloudy) or overcast layer lies above a scattered (partly cloudy) layer, the scattered layer is not considered a cloud base. Thus a scattered layer at 3000 feet and another at 9000 feet will be reported as "30", whilst a scattered layer at 3000 feet and an overcast layer at 9000 feet will be reported as "90."

    The temperature in Fahrenheit will appear above and to the left of the cloud symbol, and the dew point will appear below and to the left. Other indicies which can be computed from these, such as relative humidity and THI, don't appear on the map but can be gotten from the output of the REPORT command.

    The wind direction is indicated by a barb sticking out of the cloud circle. It points INTO the wind, so one pointing off to the upper right indicates wind out of the northeast. The wind speed is indicated by flags on the end of the barb. Each full flag is ten knots, and a half flag is five knots. Thus two full flags and a half flag would be read as a 25 knots wind. A knot is 1.15 miles per hour, for you landlubbers.

    If any weather is occuring at the station, a symbol for the dominant weather will appear directly to the left of the cloud circle. If this weather restricts visibility to six miles or less, the visibility in miles will appear directly to the left of the weather symbol.


    A good reference for reading weather maps is:

         Aviation Weather Services
         AC 00-45C
         US Department of Transportation
         Federal Aviation Administration