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An AIRMET (AIRman's METeorological Information) advises of weather that may be hazardous, other than convective activity, to single engine, other light aircraft, and Visual Flight Rule (VFR) pilots. However, operators of large aircraft may also be concerned with these phenomena. The items covered are:

AIRMET Sierra (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 at the surface.

AIRMET Zulu (Icing): Moderate icing. Freezing levels.

These AIRMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time. However, if the total area to be affected during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.

AIRMETs are routinely issued for 6 hour periods beginning at 0145 UTC during Central Daylight Time and at 0245 UTC during Central Standard Time. AIRMETS are also amended as necessary due to changing weather conditions or issuance/cancellation of a SIGMET.

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A SIGMET (SIGnificant METeorlogical Information) advises of weather that is potentially hazardous to all aircraft other than convective activity. In the conterminous U.S., items covered are: severe icing, severe or extreme turbulence, dust storms and sandstorms lowering visibilities to less than three (3) statute miles, and volcanic ash.

These SIGMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time. However, if the total area to be affect during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.

SIGMETs are issued for six (6) hour periods for conditions associated with hurricanes and four (4) hours for all other events. If conditions persist beyond the forecast period, the SIGMET is updated and reissued.

Convective SIGMETs are text consisting of an observation including a two-hour trend forecast and remarks. It is a description of phenomena compiled from radar reports, satellite data, pilot reports (PIREPs), or other reports which satisfy the criteria below. Convective SIGMETs are issued in the conterminous U.S. for any of the following: Thunderstorms greater than or equal to VIP level 4 affecting 40% or more of an area at least 3000 square miles, line of thunderstorms, isolated severe thunderstorm due to surface winds greater than or equal to 50 knots, hail at the surface greater than or equal to 3/4 inches in diameter, tornadoes, or embedded thunderstorms.

Any Convective SIGMET implies severe or greater turbulence, severe icing, and low level wind shear. A Convective SIGMET may be issued for any convective situation which the forecaster feels is hazardous to all categories of aircraft.

Convective SIGMET bulletins are issued for the Eastern (E), Central (C), and Western (W) United States. The areas separate at 87 and 107 degrees west longitude with sufficient overlap to cover most cases when the phenomenon crosses the boundaries. Bulletins are issued hourly at Hour+55. The text of the bulletin consists of either an observation and a forecast or just a forecast. The forecast is valid for up to two (2) hours.

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Valid Times (UTC)