What Is the Difference Between AIRMETs and SIGMETs?

In this article we’re going to explore the difference between AIRMETs and SIGMETs.

A wise pilot once said it’s better to be on the ground wishing you were in the air than to be in the air wishing you were on the ground.

Getting a good pre-flight briefing and understanding the basics of weather are the keys to making sound go/no-go decisions.  

But looking at the weather can be confusing, especially when starting out. Pilots use specialized weather reports to understand what’s happening aloft.

Some of the most important weather advisories are called AIRMETs and SIGMETs. Here’s a look at the difference between them. 

What Are AIRMETs and SIGMETs?

Both AIRMETs and SIGMETs are aviation weather advisories. Knowing if there might be any adverse weather is a critical part of every pre-flight weather briefing, and AIRMETs and SIGMETs are your keys to knowing.

You can think of AIRMETs and SIGMETs like the “Severe Thunderstorm Warnings” the NWS issues over broadcast TV and radio. Many kinds of watches and warnings are issued, and when conditions are expected or observed, a watch box alert is issued.

AIRMETs and SIGMETs are the aviation equivalent, and they occur for things that are hazardous to flight: icing, turbulence, low visibility, or thunderstorms. 

In simple terms, SIGMETs are for severe weather that affects all flights, and AIRMETs are important updates for all operators.

All pilots will want to note the locations and conditions, but an AIRMET might not be a ‘no-go’ for everyone. While a small Cessna cannot operate in moderate icing, many transport category aircraft can (with the right systems and pilot training). 

SIGMETs are for more severe weather that all operators must carefully consider. Here’s a look at the criteria for when an AIRMET or SIGMET will be issued.

AIRMETs

AIRMETs are AIRmen’s METerological advisories. There are three types of AIRMET issued: AIRMETs Sierra, Tango, and Zulu.

To warrant an AIRMET, the conditions must affect a wide area outside convective activity. 

AIRMET Sierra is for low visibility. There are two circumstances: extensive mountain obscurment or ceilings less than 1,000 feet and/or visibilities less than 3 miles over more than half the area.

AIRMET Tango is for moderate turbulence or sustained winds of more than 30 knots at the surface.

AIRMET Zulu is for moderate icing. It also reports the freezing level. 

SIGMETs

SIGMETs are SIGnificant METological information. In other words, SIGMETs are issued for extreme and severe weather. Here are the conditions that would require one.

• Severe icing
• Severe turbulence
• Sand or dust storms reducing visibility to less than three mile
• Volcanic ash

Convective SIGMETs

A Convective SIGMET is a special advisory for especially severe thunderstorms. 

Normal summertime thunderstorms do not usually warrant a Convective SIGMET. In most instances, these airmass thunderstorms can be easily avoided by remaining well clear of the convection.

But remember, anytime you see a thunderstorm, its presence indicates hazards to flight, including turbulence, icing, and wind shear. 

Convective SIGMETs, on the other hand, advise pilots of conditions so severe that they may be unable to avoid them. Here are the conditions under which a Convective SIGMET is issued.

• A line of thunderstorms
• Wide-spread thunderstorms (affecting more than 40 percent of a 3,000 square-mile area)
• Embedded thunderstorms
• Tornadoes
• Hail greater than 3/4-inch
• Surface winds over 50 knots 

Where to Find AIRMETs and SIGMETs

Both of these severe weather products can be found in several ways. If you call for a briefing, the briefer will always check for them when looking for any adverse conditions.

If you’re self-briefing on the web, you should always check for them. 

Most weather apps and portals like the Aviation Weather Center or Foreflight have special layers that allow you to see active advisories graphically. 

Both AIRMETs and SIGMETs are text weather products that have been used for decades, made for the days of the fax machine. You can still download and read the text version.

A new version is the G-AIRMET (Graphical-AIRMET). These are shown on a map, with the specific areas and times affected by the weather indicated. Check out this article from FLYING Magazine on how to use these new products. 

AIRMETs, SIGMETs, and Convective SIGMETs are covered in Chapter 26: Advisories of the FAA’s Aviation Weather Handbook.