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 more.
AIRMET ZULU (Icing) Moderate icing, freezing levels
In order for an airmet to be issued, the applicable conditions must be widespread. By definition, "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 given time.
AIRMET’s are routinely issued for 6 hour periods beginning at 0145Z during Central Daylight Time and at 0245Z during Central Standard Time. AIRMETS are also amended as necessary due to changing weather conditions or issuance/cancellation of a SIGMET.