Updated: Sep 17
It has been over 10 years since the Graphical AIRMET or G-AIRMET has replaced the legacy AIRMET. However, it seems that on various podcasts, webinars, magazine articles and in the aviation discussion forums, pilots are still clinging to the term "AIRMET" for some reason. Perhaps it's because they don't really understand that on March 16, 2010, the G-AIRMET became the operational product for pilots and REPLACED the existing textual AIRMET. The legacy AIRMET is a byproduct of the new G-AIRMET and has stuck around for this long for a variety of reasons.
At this point in time all pilots should have long moved away from the legacy AIRMET and should now be using the G-AIRMET. In fact, the Aviation Weather Center website (https://aviationweather.gov) you will not find a graphical depiction of the legacy AIRMET. There hasn't been one for more than five years. Well, that's not entirely true. It's still there if you know the "secret" URL, but it's not part of the menu structure.
Let's do a little history lesson. The legacy AIRMET has been always a textual product that can be depicted graphically. That is, before sophisticated computer systems were in place, an aviation meteorologist issued an AIRMET solely with a keyboard typing in the forecast one character at a time. And pilots would get a briefing and pull out a map and plot the AIRMET as a polygon. When the Internet became alive with weather guidance, websites such as from aviationweather.gov started plotting these for us as polygons.
The problem with an AIRMET is that it is a forecast valid over a six hour period. Its temporal resolution long became a joke for many pilots who said that most AIRMETs were useless. It's not that they were useless, but the pilot needed to understand that the AIRMET had to cover a six hour period...what if an area of weather was moving quickly through the Midwest? Well, it had to account for that movement and the AIRMET ended up covering a lot more area. This means that some regions would likely not contain adverse weather during the six hour window, hence why pilots suggested it was useless.
Even though the legacy AIRMET still gets issued today, the primary difference is that a G-AIRMET is a "snapshot" of a particular hazard valid at a specific time (e.g., 0300Z) whereas the legacy AIRMET is valid over that six hour period. So the G-AIRMET depicts coverage of that hazard valid at a particular time within an area defined by a polygon, typically a smaller area. Consequently, the G-AIRMET provides a much better temporal resolution of the weather hazards in time and space than the legacy AIRMET.
In the case of G-AIRMETs, you will notice there's no textual component like the legacy AIRMET. Instead G-AIRMETs are strictly graphical and include some meta data. For G-AIRMETs depicting widespread moderate ice, the meta data simply consists of upper and lower altitude limits of the icing threat. Each forecast cycle, five snapshots are issued to include an initial snapshot and snapshots with a lead time of 3, 6, 9 and 12 hours. Then, once the forecaster has completed the snapshots, the software automatically generates the legacy AIRMET text by taking the union of the first three (initial, 3 and 6) snapshots. Then the AIRMET outlook is a union of the last three snapshots (6, 9 and 12).
What you end up with at the end with G-AIRMETs is guidance with better spatiotemporal resolution. But there's an even better advantage. The legacy AIRMET was split into three groups, Sierra, Tango and Zulu for mountain obscuration and IFR conditions, turbulence and icing, respectively. The problem is that each one of these had embedded subcategories. That is, AIRMET Tango was issued for moderate non-convective turbulence, non-convective low level wind shear (LLWS) and strong surface winds. Now, those three are split out into their own G-AIRMET.
For example, the G-AIRMET below was issued for non-convective LLWS. It is valid at 2100Z. When you look at the AIRMET, it'll be buried in AIRMET Tango with potentially an AIRMET for moderate turbulence and one for strong surface winds. So it's easy to get lost in the shuffle as you can see below.
WAUS46 KKCI 262045
SFOT WA 262045
AIRMET TANGO UPDT 3 FOR TURB STG WNDS AND LLWS VALID UNTIL 270300
AIRMET TURB...WA OR CA ID MT WY NV UT CO AZ NM AND CSTL WTRS
FROM YDC TO 70SE YQL TO 30S BOY TO 20W DBL TO 50S DVC TO BZA TO
50SSE OAL TO 110WSW PYE TO 140WSW FOT TO 80W ONP TO 140W TOU TO
MOD TURB BTN FL180 AND FL380. CONDS CONTG BYD 03Z THRU 09Z.
OTLK VALID 0300-0900Z
AREA 1...TURB WA OR CA ID MT WY NV UT CO AZ NM AND CSTL WTRS
BOUNDED BY YDC-30WNW GGW-CYS-20ESE TBE-INK-ELP-60SE SSO-30WNW
INW-110SSW SNS-140WSW FOT-120WNW ONP-140W TOU-YDC
MOD TURB BTN FL180 AND FL400. CONDS CONTG THRU 09Z.
AREA 2...TURB WA OR CA ID MT WY NV UT AND CSTL WTRS
BOUNDED BY 20NW HUH-90WSW YXC-60SW BIL-30N BCE-20N LAX-150SW SNS-
140WSW ENI-80NNW FOT-60W ONP-120W TOU-20NW HUH
MOD TURB BLW FL180. CONDS CONTG THRU 09Z.
In the end, start using the term G-AIRMET. Just like the termination of the area forecast (FA), the legacy AIRMET is on the chopping block and will someday "just go away, and magically, just disappear."
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist