"Will AIRMET Zulu tell me if there are known icing conditions along my route of flight?" - John G.
“According to AOPA’s Air Safety Institute, known icing conditions exist if any of the following (or combination thereof) exists along your route of flight:
• Pilot reports of icing
• Icing AIRMETs and/or SIGMETs
• Any other forecast for icing
All of these items are important to consider prior to and during your flight; however, AIRMET Zulu in of itself, does not imply known icing conditions as suggested by the Air Safety Institute. Here’s why…
AIRMET Zulu issued by the Aviation Weather Center is an advisory to pilots for the potential of widespread moderate structural icing. So it would seem plausible that AIRMET Zulu would be a forecast for known icing conditions. However, AIRMETs are what forecasters at the Aviation Weather Center (AWC) refer to as time-smeared forecasts. That is, they are forecasts valid over a six hour period. In fact, the Aviation Weather Center provides a good explanation on their website. They explain that:
These AIRMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3,000 square miles at any one time. However, if the total area to be affected during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.
Consequently, icing conditions that are expected to develop, move or dissipate within the six hour forecast period must be included within the overall boundary of the AIRMET. AIRMETs can cover large regions of airspace at times because of their time-smeared nature. Therefore, AIRMET Zulu, in of itself, does not always imply known icing conditions.
In the end, it is prudent to integrate other weather data and forecasts such as the area forecast (FA), terminal forecasts (TAFs), pilot reports, surface observations, ground-based radar, satellite images as well as the gridded icing analysis and forecasts found on the Aviation Weather Center website.”
Online source: http://aviationweather.gov/