By Jordan Bullock CFI, Boeing 737 Pilot
Weather by nature is unpredictable and chaotic. As a pilot, its important to understand weather, as well as your personal limitations in regards to both the aircraft, and your personal skill set. Unfortunately throughout my aviation career I’ve witnessed multiple pilots underestimate weather as a force and put themselves in extremely compromising positions, as if we can just pull over on the side of the road and wait out a storm. Being a professional pilot requires you to make tough decisions that impact both your lives and the lives of others, and often times this decision boils down to a go or no-go based on weather.
Thankfully there are plenty of resources out there for pilots to use to help make weather based decisions. Most people are competent enough to use a radar based app, or read the current meter translation on ForeFlight. But what else can you use to help you make your decision, or predict what the weather can be like enroute? Thankfully the National Weather Service (NWS) helps us. Let’s take a look at Airmets and Sigmets, and the difference between the two.
Airmet
Airmet stands for “Airman’s Meteorological Information”, and it is exactly that. An Airmet advises pilots of hazardous weather that may effect specifically single engine, light aircraft, and VFR pilots. It does not include convective activity. Airmets are issued for 6 hour periods and are amended as needed.
Airmets are important because even though weather might be VFR at the current time of departure, if an Airmet is active, then the conditions for deteriorating weather exists. An Airmet is issued if the considered area for potential or active weather is widespread. The NWS requires the effected area to be at least 3,000 square miles. So what kind of weather is an Airmet issued for? There are 3 different types of Airmets, lets take a look at all three:
Airmet Sierra (IFR): I refer to this Airmet as “Sierra MIST”. This is due to the IFR conditions required for this to be issued. An Airmet Sierra is issued for Ceilings less than 1,000 ft and/or visibility less than 3 miles affecting 50% of the area as well as extensive mountain obscuration. In short, anything that affects visibility will issued as an Airmet Sierra.
Airmet Tango (Turbulence): “T” for turbulence, this Airmet is pretty straightforward. Excessive surface wind of 30 knots or more as well as moderate turbulence in the air will require this Airmet to be issued. In Florida, Airmet Tango’s are issued on a regular basis.
Airmet Zulu (Icing): I don’t have any cool trick or name for this Airmet. Zulu for Icing? I have no idea, take it up with the FAA. Anyway, this Airmet is issued if the potential for moderate icing or freezing flight levels exists. Obviously this Airmet carries extreme importance as icing can effect the integrity of the aerodynamics of your aircraft. Steer clear of this one.
Sigmet
So now you understand Airmets and the 3 different types. So, what is a Sigmet? Well, its the Airmets big bad brother. Sigmets are defined as significant meteorological conditions with hazardous weather. Airmets are specific to small aircraft (though everyone should heed their warning), while Sigmets effect ALL aircraft.
Within the Sigmet issuance, there are two different types: Sigmet and Convective Sigmet. As the name suggests, the Convective Sigmet is issued specifically for convective activity. Let’s take a look at the main differences of each:
Sigmet (Non-Convective): Sigmets are issued for widespread weather covering at least 3,000 square miles, though the specific activity can cover a small portion of this area. Sigmets are issued for multiple reasons, including severe icing, severe turbulence, volcanic ash or other nature inspired events that moderately effect visibility. Think sandstorm or dust storms. This is similar to Airmets, but with much more severe impact. Steer clear of any of these.
Convective Sigmet: It’s all in the name: Convective. This Sigmet is issued when there’s a high chance of convective activity. Specifically convective activity leading to embedded thunderstorms, a line of thunderstorms, surface winds greater than 50 knots due to convective activity, hail greater than ¾ of an inch or tornadoes.
Convective Sigmets are indicative of terrible flying weather. With that being said, they’re extremely common in tropical areas during the summer, specifically Florida due to our afternoon thunderstorms. If a Convective Sigmet is issued for your area, I’d avoid flying for large amounts of time or covering great distance.
Airmets and Sigmets are issued by the NWS to help pilots make flying decisions by giving us a great tool to add to our knowledge bank. Knowing what an Airmet or Sigmet is not only important because it will undoubtedly be on every checkride you take, it gives you a invaluable tool to use in real world flying.
By Jordan Bullock CFI, Boeing 737 Pilot
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