What defines an AIRMET?

An AIRMET, or Airmen's Meteorological Information, is specifically defined as an advisory that provides information about significant weather phenomena that may affect aircraft operations. These advisories are used to inform pilots of weather conditions that could affect their safety, such as moderate turbulence, icing, and widespread areas of instrument flight rules (IFR) conditions, among other factors.

The key aspect of an AIRMET is its focus on weather phenomena that are significant but do not meet the criteria for a brief severe weather advisory, such as a SIGMET. This advisory is particularly important for pilots operating under visual flight rules (VFR) and is vital for planning their flights safely through areas where they might encounter adverse weather conditions.

In contrast, the other options pertain to different types of notices. For example, advisories for light turbulence are covered under different weather advisories, and notices for alternate airports are not relevant to AIRMETs. Warnings about flight restrictions may relate to specific airspaces but do not describe the broad weather-related focus of an AIRMET. Thus, the correct definition encompasses the comprehensive scope of significant weather conditions that affect overall flight safety.