What is an AIRMET?
Source: www.pilotworkshop.com/tips/aviation_weather_airmets.htm, Featuring Scott Dennstaedt
AIRMETs are advisories to pilots for widespread adverse weather that is expected to affect an area of at least 3,000 square miles. Now that sounds kind of big. However, 3,000 square miles is only a little more than half the size of the state of Connecticut. Very few AIRMETs are issued by the Aviation Weather Center that are that small. In fact, many of them can be rather large, covering a portion of 8 western states as it is in this AIRMET for moderate icing.
Does this imply that icing is possible throughout this entire advisory area? Not necessarily. An AIRMET is a time-smeared forecast valid over a six-hour period. If the total area to be affected during the forecast period is very large as it is in this AIRMET, it could be that only a small portion of the total area would be affected at any one time.