In 2008 the FAA’s Aviation Weather Center began phasing out AIRMETs (Airman’s Meteorological Information) in favor of G-AIRMETs (Graphical AIRMETs). In anticipation of the switch to exclusively using G-AIRMETs, ForeFlight recently made the change to displaying G-AIRMETs in the AIR/SIGMETs/CWAs layer in the Maps view.
AIRMETs and G-AIRMETs have their differences, but since they are made from the same data, they are consistent in their messages. Why are G-AIRMETs used and how does that affect pilots using ForeFlight?
Most pilots will notice that G-AIRMETs in ForeFlight don’t contain the same text that is usually associated with AIRMETs. That text usually indicated geographical information for the AIRMET and since G-AIRMETs are graphically depicted AIRMETs, the additional text is not needed. In the past, ForeFlight would parse this geographical description to display it on the map the same way we display G-AIRMETs, resulting in no noticeable change to pilots.
Additionally, AIRMETs are limited by a character count, making it difficult for forecasters to fully convey weather conditions. Some forecasters have been forced to combine geographical areas of the forecast or cut out sections of the text to get their message through the FAA system.
AIRMETs provide geographic data using VORs to describe the area of weather conditions, while G-AIRMETs use lat/long coordinates and can use many more points to describe an area (due to the lack of character limit), giving pilots a more accurate geographical depiction of weather.
The FAA issues G-AIRMETs in 3 hour blocks and AIRMETs in 6 hour blocks that blend the data into one image. This means that multiple G-AIRMETs issued for different times could be turned into a single AIRMET lasting for much longer than required, possibly preventing pilots from flying in an area that would be otherwise cleared with G-AIRMETs.
Since G-AIRMETs use smaller time increments, ForeFlight provides a time slider at the bottom of the Maps view when you enable the AIR/SIGMETs/CWA layer. Scrub the time slider to view G-AIRMETs that will be active at different times in the future. If you have another time-based weather layer turned on, ForeFlight will replace the time slider increments with the other layer, letting you view G-AIRMETs that are active at the time of the other layer’s frame. If you combine the AIR/SIGMETs/CWA layer with a longer term forecast layer like Surface Analysis or Icing, you’ll find that the G-AIRMETs will disappear if you move the slider past the expiration time of the final active G-AIRMET.