At this point in time all pilots should have long moved away from the legacy AIRMET and should now be using the "new" Graphical AIRMET or G-AIRMET. The legacy AIRMET is primarily a textual product that can be depicted graphically. However, beginning October 1, 2008, meteorologists at the Aviation Weather Center (AWC) began to issue a product called G-AIRMETs in concert with issuing legacy AIRMETs.
On March 16, 2010, the G-AIRMET became an operational product for pilots and essentially replaced the existing textual AIRMET. 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 a 6 hour period. So the G-AIRMET depicts coverage of that hazard valid at a particular time within an area defined by a polygon. 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 limits of the icing threat like those advisories shown below.
In most cases you will notice that particular advisory area (polygon) will be tagged with meta data containing two numbers like the label in the black circle in the image above. These are altitudes represented in hundreds of feet above mean sea level (not above ground level). For example, in the image above over Idaho, the bottom number is 030 which means the base of the icing forecast for this area is 3,000 ft MSL. For this polygon, the top is shown to be 130 which identifies the top of the icing threat as 13,000 feet MSL. So essentially, widespread moderate icing should be expected at 0300Z from 3,000 ft MSL to 13,000 feet MSL within the polygon defined shown on the map.
In some cases the base is represented by SFC like you see in the G-AIRMET label highlighted by the red circle. This tells you that the base of the threat of icing can be expected down to the surface. Or in the case of the polygon with meta data highlighted within the violet circle, the base is variable based on the expected freezing level. In this case the freezing level varies from the surface (SFC) to 090 or 9,000 feet MSL.
You can find the G-AIRMET snapshots on aviationweather.gov.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS meteorologist