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For an icing AIRMET to be issued, moderate icing must be forecast over an area of at least 3,000 square miles. What probability of moderate icing is required before that AIRMET will be issued?

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For any AIRMET, the event must have a probability of 50% at least (that is more than playing playing heads or tails).

Related US NOAA definition for AIRMET Z:

AIRMETs are issued every 6 hours, where there is a 50% probability that any of the following en-route weather phenomena will cover an area of 3000 square miles or greater.

Phenomena for AIRMET Z are:

Icing: Areas of moderate airframe icing, other than convectively induced, including the areal extent. Freezing Level information is included after AIRMETs for moderate icing or statements indicating that no significant icing is expected.

Freezing level is defined as:

The lowest freezing level above the ground or at the SFC as appropriate. Freezing levels are delineated using high altitude VOR locations describing the location of the lowest freezing level above the ground or SFC as appropriate.

Freezing levels above the ground are delineated at 4000 feet intervals above mean sea level (AMSL). Multiple freezing levels above the ground are delineated using "BOUNDED BY" followed by high altitude VOR locations. The range of freezing levels across the forecast area is also included.

G-AIRMETs, the graphical version, are issued every 3 hours (but still describe the next 6 hours).

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  • $\begingroup$ Thanks for the quick answer. This one stumped a number of people, including in follow-up to my asking this during a webinar on the subject. I'll take a brief wet noodle lashing for inadequate searching before asking the question here: I didn't phrase my searches broadly enough. $\endgroup$
    – ammPilot
    Nov 2, 2016 at 15:55
  • $\begingroup$ @mins is a much respected Hanshi master in the ancient art of Google-Fu. Do not despair. Few can match his skills. $\endgroup$
    – TomMcW
    Nov 2, 2016 at 19:38
  • $\begingroup$ Btw, your GPS knows where you are. The NSA knows what you're doing there. Google knows what you're thinking while you're doing it $\endgroup$
    – TomMcW
    Nov 2, 2016 at 19:45

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