Comparing the differences between AIRMETs and SIGMETs can seem tedious. After all, both describe non-ideal weather conditions for flying. But there are important differences.
AIRMETs focus on
Understanding the differences between AIRMETs and SIGMETs can help pilots protect their own safety, as well as the safety of their passengers.
AIRMET
Imagine that you’ve been planning a day-trip to the mountains. The big day comes and, being “the responsible adult” you are, you check the weather. The news isn’t good. The weatherman is saying that there’s a storm moving in—complete with thunderbolts and lightning.
It’s all very, very frightening.
The weatherman goes on to say that the storm should break by the late-afternoon, but this will be after you’ve started
Do you:
- Cancel the whole hike?
- Wait to see if the storm passes?
- Brave the storm and go anyway?
What is an AIRMET?
Think of AIRMETs as your local weather report but for aviation. The weatherman tells you the weather regardless of the conditions. AIRMETs are weather advisories that alert pilots to conditions that may put them in danger.
AIRMET is shorthand for “Airmen’s Meteorological Information.” They are:
- An abbreviated forecast of actual and possible adverse weather conditions along a given flight route.
- Issued when conditions reach a point where the safety of all nearby aircraft is believed to be at risk.
AIRMETs used to only be applied to light aircraft but this has since changed. Now AIRMETs apply to all aircraft in a given area.
AIRMETs serve to protect the safety of pilots and their passengers. Like checking the weather before a hike, they give pilots important weather information to help plan the safest route possible.
AIRMET Conditions
It’s up to the pilot to decide how to proceed once an AIRMET has been issued. This decision should not be made lightly.
The decisions you make will be influenced by:
- The type of AIRMET that has been issued.
- Your level of preparedness.
- Your experience level.
AIRMETs have three different signifiers:
- S (Sierra)
- T (Tango)
- Z (Zulu)
Each letter alerts a pilot to specific, unfavorable flying conditions. The Aviation Weather Center defines them as:
- S (Sierra): mountain obscuration or IFR
Cloud ceilings are at less than 1,000 feet. There is also the possibility of having less than three miles of visibility over half of the affected area, which can result in mountain obscuration.
- T (Tango): turbulence
Possible light-moderate turbulence as well as surface winds of 30 or more knots.
- Z (Zulu): icing
Weather conditions are at freezing levels, creating the possibility for light-moderate icing. Dieicing your plane isn’t the hardest thing in the world. However, pilots should take the possibility of icing very seriously.
Conditions must have the potential to affect 3,000 square miles of the surrounding area for an AIRMET to be issued. Once issued, an AIRMET will last for 6 hours.
The three AIRMET signifiers alert pilots to actual and possible conditions in their area. Once issued, AIRMETs are considered to extend over a large area and last for a considerable amount of time. It’s a pilot’s responsibility to be prepared for these conditions and to keep track of them as they develop.
SIGMET
Imagine again that you’re planning on going hiking. Only this time, the night before your hike, your regularly scheduled programming is interrupted by the Emergency Broadcast System (EBS).
A Severe Weather Alert has been put in effect for the next 24 hours. This would drastically affect your plans to go hiking.
What is a SIGMET?
SIGMETs are a more extreme version of AIRMETs. Think of them like the EBS. You can check the weather whenever the local news is on, but sometimes conditions are so bad that your regularly scheduled programming is interrupted by an alert from the EBS.
When this happens, it means that conditions have worsened and immediate action must be taken.
SIGMET is shorthand for “Significant Meteorological Information.” They’re like AIRMETs in that they:
- Are an abbreviated forecast of actual and possible adverse weather conditions.
- Cover an area of 3,000 square miles.
- Are issued when the safety of all nearby aircraft is considered to be at risk.
They differ from AIRMETs in that:
- They alert pilots to weather that is significantly more severe than AIRMETs.
- Their alphabetic designators are N-Y. SIGMETs don’t use S, T, or Z to avoid confusion with AIRMET designators.
- They last up to six hours if they involve hurricanes or volcanic ash outside of the continental U.S.
- They come in two types: non-convective and convective.
SIGMETs are like AIRMETs in a few ways. However, the severity of SIGMETs puts aircraft at significantly greater danger than AIRMETs.
Non-Convective SIGMETs
The first type of SIGMET is non-convective. Basically, take the conditions for AIRMETs and replace “light-moderate” with “sever-extreme.”
According to the Aviation Weather Center, these conditions include:
- Severe icing
- Sever-extreme turbulence
- Visibility-impairing conditions like dust storms, sandstorms, or a combination of the two
- Volcanic ash
Compare to AIRMETs where low cloud ceilings are the main form of impaired visibility. Non-convective SIGMETs are valid for four hours, just like AIRMETs.
Convective SIGMETs
To understand convective SIGMETs, you may need to remember what you’ve forgotten since 6th-grade science class. A refresher on what convection couldn’t hurt.
Convection adds a new dimension to SIGMETs by creating storms. The Aviation Weather Center describes these conditions as:
- A line of thunderstorms that is 60 miles long, with thunderstorms affecting 40% of its length.
- An area of thunderstorms that comprises at least 40% of the 3,000 square mile area.
- Storms that reflect strongly on radar or have a prominent satellite or lightning signature.
- 30+ minutes of severe or embedded thunderstorms.
Authorities can also broadcast a special issuance for convective SIGMETs. To qualify for a special issuance, the weather must show signs of:
- At least one tornado
- Hail at least 3/4 inches in diameter
- Gusts of wind that are at least 50 knots
Do not take SIGMETs lightly. Both types pose real dangers to pilots and their passengers.
Dealing with AIRMETs and SIGMETs
AIRMETs and SIGMETs can be a minor inconvenience depending on where you live. Some pilots may decide to just wait it out. However, not everyone has that luxury. Some folks may live in areas where they’d rarely get to fly if they waited for perfect conditions.
With that in mind, remember that:
- An AIRMET or SIGMET being issued does not necessarily mean that the entire area will experience those conditions at the same time, if at all. 3,000 square miles is a lot of area to cover.
- The time period for both lasts a while. Stay tuned to ATIS and listen for any PIREPS (pilots flying and reporting to ATC what they are seeing in real-time) that may come in.
If you fly or plan on flying in areas where AIRMETs are common:
- Consider getting your instrument rating.
- Develop a pilot’s checklist to be as prepared as possible.
- Ask a CFI to go up with you and show you how to handle different conditions.
- Submit PIREPS to help other pilots know what to expect.
- Familiarize yourself with the Aviation Weather Overview.
While surmountable, AIRMETs and SIGMETs should never be taken lightly. AIRMETs and SIGMETs can be a major challenge for pilots. That doesn’t mean that you’ll be grounded every time one is issued. Exercise extreme caution and good
At best, you may have to delay your flight until conditions have improved. At worst, you’ll cancel your flight and go on to fly another day. Both options are better than flying in conditions that could lead to an accident, or worse.