How to decode the image Legends

 How can I read the legends lines to figure out what meteorological fields or parameters are being displayed on the graphics? 
      
This question falls into the, "how do I know what I'm looking at when viewing graphics," category.   This tutorial won't teach you how to interpret meteorological charts, rather it's one of the foundation stones.  You've gotta know what you're looking at before you can start interpreting it.

There are two sources we use for graphics, our AWIPS system and FXC.  The output from both is quite similar but the FXC graphic legends are smaller and a little harder to read than the AWIPS graphic legends.

The same information is included in the legends from both AWIPS and FXC, but since the AWIPS legends are larger and thus a little easier to read, we'll start there.

The first image below is an example of an AWIPS graphic.  The second image is zoomed in on the legends block in the lower right hand corner.

                                             Example AWIPS Graphic


                                          Example AWIPS legends block 

This legends block will tell you:

        1.  What parameter you are looking at and which model it came from.
        2.  The model valid time, i.e., when the model was run.
        3.  The forecast hour of the individual graphic, i.e., the 42 hour forecast.
        4.  The valid date/time of that forecast hour.
        5.  When there is more than parameter being displayed on the graphic
               the color of that particular legend line will be the same color as the parameter.
        6.  If the parameter is being displayed as the colored image, then it's legends line
               will be white and also contain the word "Img".

The examples below decode the 1st, 2nd, and 4th lines of the Legend.  So you may ask what about the 3rd, 5th, and 6th lines? Notice that these lines are all a pale gray.  This indicates that these parameters were loaded in the original image on AWIPS but were "turned-off" in the image that is displayed.   The AWIPS system is an operational forecasting tool that allows forecasters to look at all sorts of meteorological parameters.  It would rapidly become impossible to make sense of an image if every parameter that was loaded was turned on simultaneously.  So we turn off stuff we don't want to look at during that particular moment, then turn it back on when we are ready to look at it.

Example 1:   The 2nd line of the legend.

CODED     GFS90 MSL Pressure (mb)   06.18  12HR  Wed 06:00Z  07-May-08

De-coded:  Model is GFS90,  Parameter is  MSL Pressure (mb),  or Mean Sea Level Pressure in millibars.
                   06.18  is the date and time the model was created, or on the 6th at 18Z.  So this is the 18Z run
                   of the model.  The forecast hour of the image being displayed is hour 12.    12HR 
                   The valid time of forecast hour 12 is    Wed 06:00Z  07-May-08    or 06Z on May 7th, 2008.
  Note that the color of the Legend line is cyan, which matches the color of the MSL Pressure contours.

Example 2:  The 1st line of the legend.

CODED     GFS90 Surface Wind (Kts)    06.18  12HR  Wed 06:00Z  07-May-08

 De-coded: This one is pretty much the same as example 1 above.  The only difference is the parameter
                     being viewed.
                    Model is GFS90,  Parameter is  Surface Wind (Kts),  or the Surface Wind in knots.
                   06.18  Is the date and time the model was created, or on the 6th at 18Z. 
                   So this is the 18Z run of the model.
                   The forecast hour of the image being displayed is hour 12.   12HR
                   The valid time of forecast hour 12 is    Wed 06:00Z  07-May-08    or 06Z on May 7th, 2008.
 Note that the color of the Legend line is pale yellow, which matches the color of the Surface Wind barbs.

Example 3:   The 4th line of the legend.

CODED     GFS90 Layer Relative Humidity Img  06.18  12HR  Wed 06:00Z  07-May-08

De-coded:  Model is GFS90,  Parameter is  Layer Relative Humidity,
                                                     *Average Relative humidity in the 1000 mb to 500 mb layer, which
                                                         runs from about the surface up to around 18,000 feet.
                 Since Img is included, this parameter is the image, or is displayed as the colors on the graphic.
                 06.18  Is the date and time the model was created, or on the 6th at 18Z.
                 The forecast hour of the image being displayed is hour 12.   12HR
                 The valid time of forecast hour 12 is    Wed 06:00Z  07-May-08    or 06Z on May 7th, 2008.

 

The next set of examples are from the FXC system.  You will notice right away that the legends are very similar to the first set of examples.

The first image below is an example of an FXC graphic.  The second image below is the legends block taken  from the lower right hand corner.


                                        Example FXC Graphic

  
                              Example FXC legends block 

This legends block will tell you:

        1.  What parameter you are looking at and which model it came from.
        2.  The model valid time, i.e. when the model was run.
        3.  The forecast hour of the individual graphic, i.e., the 42 hour forecast.
        4.  The valid date/time of that forecast hour.
        5.  When there is more than one parameter being displayed on the graphic
               the color of that particular legend line will be the same color as the parameter.
        6.  If the parameter is being displayed as the colored image, then it's legends line
               will be white and also contain the word "Img"

The first 3 examples below decode the 1st, 5th, and 6th lines of the Legend.  So you may ask what about the 2nd through 4th lines?  Notice that once again these lines are all gray.  
This means that these parameters were loaded in the original image on FXC but were "turned-off" in the image that is displayed.  Just to show you a bit more information on
decoding legends, example 4 below decodes the 2nd line of the legend, which is also the first greyed out line. 

From the legends block above:

Example 1:   The 6th or bottom line of the legend.

CODED     GFS40 1000MB/500MB Rel Humidity Img 15.06 42HR Thu 00:00Z 17/Apr/08

De-coded:  Model is GFS40,  Parameter is  1000MB/500 MB Rel Humidity,
                                                     *Average Relative humidity in the 1000 mb to 500 mb layer, which
                                                         runs from about the surface up to around 18,000 feet.
                 Since Img is included, this parameter is the image, or is displayed as the colors on the graphic.
                 15.06  Is the date and time the model was created, or on the 15th at 06Z.
                 The forecast hour being displayed is hour 42.   42HR
                 The valid time of forecast hour 42 is    00:00Z 17/Apr/08    or 00Z on April 17th, 2008.

Example 2:   The 5th line, or second line from the bottom in the legend.

CODED   Reg States/Counties   

De-coded:  This is the map background being used on the image, regional scale
                                                   (as opposed to North America), and state and county maps.

Example 3:    The 1st or top line of the legend.

 CODED     GFS40 550 MB Height  ??????

 De-coded:   This is only a partial legend because if you look closely in the example legend
                     this line is black.  These are the black lines on the graphic.  Black shows up really
                     well overlaid on colored images, but since the legend text is the same black color,
                     the rest of the legend line is lost in the black background.  Otherwise, you know
                     the model is the GFS40 and the parameter is 550 MB Height.

Example 4:  The 2nd line of the legend.

CODED     GFS40 1000MB/500MB Thickness 15.06 42HR Thu 00:00Z 17/Apr/08

 De-coded: This one is pretty much the same as example 1 above.  The only differences are
                    (1)  this is the 1000 mb to 500 mb thickness, or distance or height between the
                           two pressure surfaces.  
                    (2)  This is NOT part of the image. 
                    (3) Because it is greyed out this parameter isn't actually being displayed on the
                          graphic.  The forecaster did have it available when he put the graphic together,
                          but at the end opted to "turn off" or not display this parameter.