Plain English Meaning
A weather alert sent out during the day to warn pilots about widespread weather that could be dangerous, especially for smaller aircraft, but isn't extreme enough to be a top-tier warning.
Full Definition
An in-flight weather advisory issued by the National Weather Service warning of weather phenomena that may affect aircraft safety but are less severe than those described in a SIGMET. AIRMETs cover moderate icing, moderate turbulence, sustained surface winds of 30 knots or more, widespread areas of ceilings less than 1,000 feet and/or less than 3 miles, and . They are of particular concern to pilots of and to all pilots operating without qualifications or equipment.