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A Surface Map tries to depict as much about weather conditions at
the surface as possible. This is accomplished by taking the hourly weather reports issued from the major airports, the smaller airports and any automated observation stations and plotting the information on a map. The image to the left is an example of a surface map.The elements which make up our surface maps are:Station Plot
Front Locations
Pressure Contours
Station Plots
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The data from each station is plotted in a standard format called a Station Plot. All station plots resemble the following
All temperature data is in Fahrenheit, except in Canada where it is in Celsius
For an explanation of the symbols which indicate the weather at a particular station, see the Weather Symbols section. For an explanation of the wind barb, see the Wind Barb section.
Fronts indicate the boundary between two different air masses and are also plotted on the surface maps. This information is updated once every 3 hours, and the valid time for the frontal portion of the analysis is displayed in the lower right corner of the image.
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Pressure contours (isobars), or lines of equal surface pressure, are contoured every 4 millibars. They are a good indicater of frontal positions as well as high and low pressure centers.
Weather Depiction
The Weather Depiction chart is a chart aimed at the aviation industry. This chart is similar to the surface map in that the hourly weather reports are plotted on the map as are the frontal locations.· Center circle represents cloud cover
· Number below the circle is the ceiling (100's of feet)
· The symbol (if any) to the left of the circle represents the current weather
The contours represent Flight Rules. Any region shaded in Red is experiencing Instrument Flight Rules (IFR), while any area hatched in Blue is experiencing Marginal Visual Flight Rules (MVFR).
Radar Summary/Severe Watch Boxes
The Radar Summary indicates where precipitation is falling with the intensity according to the color bar to the left. Any severe weather watches are also plotted. Tornado Watches are labeled with a 'T' while Severe Thunderstorm Watches are labeled with a 'S'. Following the letter designation is the watch number (i.e. the 307th watch of the year in this example) along with the time the watch is valid until (Zulu, or GMT time).
Weather Symbols
Here is a list of the symbols indicating the weather which has occurred during the past hour or was occurring at the time of the observation.
Cloud Coverage/Wind Barbs
Nexrad Radar Legends
Individual Nexrad site radars run in one of two modes: either Clear Air or Precipitation mode. The image on the left is the appropriate legend for Clear Air mode, while the image on the right is for Precip mode. As you can see, the same color on one legend does not match the same color on the other legend from a dBZ standpoint. Clear Air mode is more sensitive then Precip mode, which is why the color scale begins at a much lower dBZ value. As a general rule, precipitation first begins to reach the ground at a dBZ value of 15-20.