AIRMETs and SIGMETs

AIRMET chart
Example AIRMET chart from aviationweather.gov

All pilots should be very familiar with the meaning of AIRMET and SIGMET – they both represent warnings of conditions which could threaten the safety of flight. Find out what they mean and why it is important to understand them in this quick introductory guide.

What is an AIRMET?

AIRMET is an abbreviation of Airman’s Meteorological Information. It is a warning issued to all aircraft that conditions may exist that pose a safety to flight. Although the warning is issued to all aircraft, AIRMETs are particularly important to pilots of smaller aircraft. AIRMETs are only issued when the conditions are expected to affect a widespread area, over the valid period – 3,000 square miles or more. They are available as part of an official weather brief, and are commonly broadcast over ATC and ATIS. AIRMETs are typically issued for 6 hour periods. Although it may be the case that only a small region within the warning area is affected at any one time, conditions may move and may affect the whole area over the valid time.

There are three types of AIRMET:

AIRMET Sierra – Mountain Obscuration or Instrument Flight Rules

A Sierra AIRMET pertains to restricted visibility. Either mountain obscuration is expected, or the cloud ceilings are at or below 1,000ft and the visibility is at or below 3 miles. This type of AIRMET means that fields could be IFR only (meaning that operations are only permitted according to Instrument Flight Rules, and in accordance with a filed IFR plan), or that elevated terrain may be difficult to spot.

AIRMET Tango – Turbulence

Light or moderate turbulence is expected over a wide area, and/or surface winds are expected to be greater than 30 knots sustained.

AIRMET Zulu – Freezing potential

Light to moderate freezing is possible.

AIRMET Memory Aid

Do you have trouble remembering which AIRMET is which? Here is a simple system I made up to help me – feel free to steal it:

Sierra Sierra Mountains (also, sierra means mountain range in Spanish)
Tango Turbulence
Zulu FreeZing

What is a SIGMET?

A SIGMET is a Significant Meteorological warning, issued to all aircraft. These conditions pose a very serious threat to all aircraft – even the large jets. As the pilot of a small airplane you must avoid these conditions at all costs. SIGMETs are issued as needed, and are typically valid for 2-6 hours, depending on type. Again, SIGMETs are issued when conditions are expected to be widespread, that is to say covering an area of at least 3,000 miles over the valid period. SIGMETs are classified as follows:

Convective SIGMET

A convective SIGMET indicates significant convective activity – thunderstorms. It is recommended that pilots stay at least 25 miles away from thunderstorms at all times, as they can contain threats easily capable of destroying an airplane in flight, causing rapid an uncontrollable changes in altitude, or upsetting the attitude of the airplane to the point where control is lost.

Convective SIGMETs may also be issued for tornados, surface winds in excess of 50 knots, and large hail at the surface. They are valid for 2 hours.

Non-convective SIGMET

The non-convective SIGMET can be issued for the threat of severe or greater turbulence, severe or greater icing conditions, or Instrument Meteorological Conditions (IMC) due to particulates such as dust, volcanic ash, or sand. They can be valid for up to 6 hours.

Summary

As the pilot of a small airplane, every AIRMET and SIGMET is a potential hazard that you must assess and proactively decide how to deal with. Always get that official weather briefing, and remember that actual conditions may worsen or be more severe than forecast. NEVER mess with a SIGMET – avoid at all costs!

Thanks to:

 

… for suggesting this topic.

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