SIGMET, or Significant Meteorological Information, is a weather

Weather

Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...

 advisory that contains meteorological information concerning the safety of all aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

. There are two types of SIGMETs, convective and non-convective. The criteria for a non-convective SIGMET to be issued are severe or greater turbulence

Turbulence

In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...

 over a 3000 square miles (7,770 km²) area, or severe or greater icing

Atmospheric icing

Atmospheric icing occurs when water droplets in the atmosphere freeze on objects they contact. This can be extremely dangerous to aircraft, as the built-up ice changes the aerodynamics of the flight surfaces, which can increase the risk of a subsequent stalling of the airfoil...

 over a 3000 square miles (7,770 km²) area or IMC

Instrument meteorological conditions

Instrument meteorological conditions is an aviation flight category that describes weather conditions that require pilots to fly primarily by reference to instruments, and therefore under Instrument Flight Rules , rather than by outside visual references under Visual Flight Rules . Typically, this...

 conditions over a 3000 square miles (7,770 km²) area due to dust

Dust

Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

, sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

, or volcanic ash

Volcanic ash

Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...

 http://www.aviationweather.gov/exp/product_overlay/help/p-sigmets.html.

This information is usually broadcast on the ATIS

Automatic Terminal Information Service

Automatic Terminal Information Service, or ATIS, is a continuous broadcast of recorded noncontrol information in busier terminal areas. ATIS broadcasts contain essential information, such as weather information, which runways are active, available approaches, and any other information required by...

 at ATC

Air traffic control

Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to separate aircraft to prevent collisions, to organize and expedite the flow of traffic, and to provide information and other...

 facilities. A SIGMET is a forecast valid for up to four hours. They are assigned an alphabetic

NATO phonetic alphabet

The NATO phonetic alphabet, more accurately known as the NATO spelling alphabet and also called the ICAO phonetic or spelling alphabet, the ITU phonetic alphabet, and the international radiotelephony spelling alphabet, is the most widely used spelling alphabet...

designator from N (November) through Y (Yankee), excluding S and T. http://www.faa.gov/ATpubs/AIM/Chap7/aim0701.html#7-1-6

A Convective SIGMET is issued for convection over the Continental U.S. Convective SIGMETs are issued for an area of thunderstorms affecting an area of 3000 square miles (7,770 km²) or greater, a line of thunderstorms at least 60 nm long, and/or severe or embedded thunderstorms affecting any area that are expected to last 30 minutes or longer. A Convective SIGMET is valid for 2 hours and they are issued every hour + 55 min. http://aviationweather.gov/exp/product_overlay/help/p-csigmets.html