Pilots count on accurate and timely weather data to make informed decisions both before and during their flights. Good pilots never go anywhere without checking and rechecking the weather. It may be clear and calm on the ground, but we know that things could look much different en-route, especially during a long cross-country flight. AIRMETs and SIGMETs are important tools during the flight planning process and while underway. Think of them as the aviation version of the local weather report and the Emergency Broadcast System (EBS) used on TV and radio to alert citizens of severe weather conditions on the ground.
Both AIRMETs and SIGMETs provide pilots abbreviated in-flight severe weather advisories for both existing and predicted adverse conditions. Conditions must be widespread, affecting an area of at least 3,000 square miles, to trigger an AIRMET or SIGMET. AIRMETs and SIGMETs are forecasts issued when the Aviation Weather Center (AWC) determines that the conditions pose a degree of risk to aircraft safety.
But what is the difference between the two and should you be more concerned about an AIRMET vs SIGMET or a SIGMET vs AIRMET? What types of AIRMET can be issued and what does each mean? How do you read a graphical AIRMET? Is one type of SIGMET more dangerous than the other? Read on to get answers and learn everything you need to know about AIRMET vs SIGMET.
An AIRMET, or Airman’s Meteorological Information, is an in-flight weather bulletin that is issued when there are weather conditions present which may affect the safety of lighter aircraft, though the information they contain is relevant for all aircraft.
What is an AIRMET?
The types of conditions that may trigger the issuance of an AIRMET are divided into three categories, each with a unique designation. Pilots whose flight plans take them into an area with an active AIRMET may wish to consider modifying their flight plans to avoid the inclement weather. Since it is unlikely that the weather condition in question will impact the entire 3,000 square mile area at once, it may be possible to navigate around the hazards.
The three types of AIRMETs, otherwise known as WAs, are:
- AIRMET Sierra
An AIRMET Sierra, or AIRMET S, is issued for mountain obscuration and/or IFR conditions with ceilings that are less than 1,000’ and/or visibility of under 3 miles over an range of at least 50% of the area covered by the AIRMET
- AIRMET Tango
An AIRMET Tango, or AIRMET T, is issued for light to moderate turbulence, sustained surface winds at 30 knots or higher, or low-level windshear.
- AIRMET ZULU
An AIRMET Zulu, or AIRMET Z, is issued when weather conditions are at freezing levels making light to moderate icing a possibility.
Graphical AIRMETs, or G-AIRMETs are used to plot AIRMET conditions on a map for visual reference. Six unique colors and types of boundary lines depict the various conditions. Review the Aviation Weather Center’s What is an AIRMET? document to learn more about the graphical AIRMET symbols and their usage.
How often are AIRMETs issued?
AIRMETs are issued every six hours by the Aviation Weather Center (AWC) starting at 0245 UTC (Zulu time). An AIRMET is valid for the period of six hours until the next regularly scheduled AIRMET is released, although the AWC can terminate the AIRMET earlier if the condition causing the AIRMET resolves. The AIRMET can also be extended if inclement conditions persist past the standard expiration time.
A SIGMET, or Significant Meteorological Information, is an in-flight weather bulletin that is issued when there are severe weather conditions present which may affect the safety of all aircraft in the vicinity.
What is an SIGMET?
The weather conditions that precipitate a SIGMET are substantially more severe than those that trigger an AIRMET. Pilots whose flight plans take them into an area with an active SIGMET should highly consider canceling the flight since conditions pose a significant threat. Weather in an area under a SIGMET advisory is highly volatile and the rapidly evolving nature of the threat means that modifying the flight plan to simply skirt the adverse conditions is often not as feasible or prudent as it is with an AIRMET.
There are two categories of SIGMETs, one for weather patterns with more stable overall atmospheric conditions and the other for weather patterns characterized by rapid air movement, otherwise known as convection.
The two categories of SIGMETs are:
- Non-Convective SIGMET (WS)
Non-Convective SIGMETs are issued for conditions similar to but more severe than those of the three types of AIRMETs. A non-convective SIGMET can be issued for severe icing, severe to extreme turbulence, dust storms or sandstorms that lower visibility to less than three miles, and for volcanic ash. The dust, sand, and ash are significant causes for concern because they can incapacitate engines.
- Convective SIGMET (WST)
Convective SIGMETs are issued for the most dangerous of conditions with atmospheric instability. They cover severe thunderstorms with surface winds of greater than 50 knots, surface hail ¾ inch or larger in diameter, tornadoes, and embedded thunderstorms, lines of thunderstorms, or thunderstorms with heavy precipitation affecting at least 40% of a region
How often are SIGMETs issued?
Non-convective SIGMETs are an unscheduled forecast, and as such they are simply issued as needed when significant meteorological conditions that meet non-convective SIGMET criteria exist. Once a non-convective SIGMET is issued, it is valid for 4 hours, except for non-convective SIGMETs relating to hurricanes. Hurricane-related non-convective SIGMETS are valid for 6 hours.
Convective SIGMETs are routinely issued at 55 minutes past the hour and remain valid for two hours, though additional unscheduled convective SIGMETs may be issued in the interim as conditions evolve. If no severe weather is forecast or existing when a scheduled convective SIGMET is due to be released, the SIGMET will simply read: “CONVECTIVE SIGMET…NONE.” For purposes of convective SIGMET issuance, the United States is divided into three zones: Eastern, Central, and Western.
The main thing to remember is that AIRMETs are issued when there is weather that may impact aircraft safety, especially that of smaller, lighter aircraft. Weather conditions that prompt the issuance of a SIGMET are more severe and affect the safety of all aircraft regardless of size. While you may be able to fly during an AIRMET by slight modifications to your flight plan, a SIGMET should be cause for seriously considering canceling your flight.
Looking for some guidance and fresh perspective on making go/no-go flight decisions in inclement weather? Consult your copy of the ASA’s Severe Weather Flying.