Plain English Meaning
Weather warnings sent out specifically to pilots while they are flying, telling them about dangerous weather happening now or expected soon. Different types of advisories cover different kinds of hazards, from moderate turbulence and icing up to severe thunderstorms.
Full Definition
Unscheduled forecasts issued by the National Weather Service to alert pilots in flight of the actual or potential development of hazardous weather. They include AIRMETs, SIGMETs, and Convective SIGMETs, each addressing different categories and severities of weather hazards along a route or within a forecast area.