AIRMET
An AIRMET, or Airmen's Meteorological Information, is a concise description of weather phenomena that are occurring or may occur (forecast) along an air route that may affect aircraft safety. Compared to SIGMETs, AIRMETs cover less severe weather: moderate turbulence and icing, sustained surface winds of 30 knots or more, or widespread restricted visibility.

AIRMETs are broadcast on the ATIS at ATC facilities, and are referred to as Weather Advisories.  AIRMETs are valid for six hours.  NOTE: The definition has changed and no longer says "light aircraft";  AIRMETs are intended for all aircraft.

There are three types of AIRMET, all identified by a phonetic letter: S (Sierra), T (Tango), and Z (Zulu).  

 AIRMET SIERRA (Mountain obscuration or IFR) ceilings less than 1000 feet and/or visibility less than 3 miles affecting over 50% of the area at one time; extensive mountain obscuration
AIRMET TANGO (Turbulence) moderate turbulence, sustained surface winds of 30 knots or greater, and/or non convective low−level wind shear.
AIRMET ZULU (Icing) Moderate icing and provides freezing level heights.

For an authority to issue an AIRMET, applicable conditions must be widespread.  "Widespread" means that the applicable area covers at least 3000 square miles.  Because conditions across the forecast period can move across the area, it is possible that only a small portion of the area is affected at any time.

AIRMETs are routinely issued for six-hour periods beginning at 0145Z during Daylight Saving Time and at 0245Z during Standard Time. AIRMETS are also amended as necessary due to changing weather conditions or issuance/cancellation of a SIGMET.

Graphical representation of AIRMETs can be found at: https://www.aviationweather.gov/.  Select the G-AIRMET checkbox for "Graphical AIRMET".