For aviators and aviation professionals, AIRMETs and SIGMETs are two of the most essential weather advisories to understand. But what are they, and what’s the difference between them? This article will discuss the similarities and differences between AIRMETs and SIGMETs and how they are used in aviation.
What is the difference between an AIRMET and a SIGMET?
AIRMET and SIGMET weather advisories are issued to warn pilots of potentially hazardous weather conditions they might encounter while flying. AIRMETs consist of turbulence, visibility, and icing-related warnings that are less severe than those in a SIGMET. A SIGMET includes thunderstorms, volcanic ash, dust storms, as well as other weather (described below), and is more severe overall.
While AIRMETs are generally issued for six-hour periods and amended as necessary due to changing weather conditions, SIGMETs are issued as needed. They may be valid for a specific area and time.
What is an AIRMET?
An AIRMET is a weather advisory for pilots that alerts them to potential hazards in the air. The acronym “AIRMET” stands for “Airmen’s Meteorological Information.”
There are three types of AIRMET, referred to by their phonetic alphabet letter: Sierra S, Tango T, and Zulu Z:
- AIRMET SIERRA (Instrument Flight Rules or Mountain Obscuration): mountain obscuration or ceilings less than 1000 feet or visibility less than 3 miles affecting more than 50% of the stated area at one time
- AIRMET TANGO (Turbulence): moderate turbulence, or continuous surface winds of 30 knots or more, or non-convective low−level wind shear.
- AIRMET ZULU (Icing): moderate icing (AIRMET ZULU also provides freezing-level heights.)
An AIRMET is only issued if the weather conditions are widespread, which means that it covers an area of at least 3000 square miles. However, because the weather condition can move across this extended area during its validity time, only a small part of the entire region may be experiencing these effects at any given moment.
AIRMETs are issued by the National Weather Service (NWS) and are valid for a specific area and time. AIRMETs are generally issued for six-hour periods and amended as necessary due to changing weather conditions or the issuance or cancellation of a SIGMET.
What is a SIGMET?
A SIGMET is a weather advisory for pilots that alerts them to potentially hazardous weather conditions. The acronym “SIGMET” stands for “Significant Meteorological Information.” Compared to an AIRMET, a SIGMET contains information about more severe types of weather.
There are three types of SIGMET:
- Volcanic ash (VA or WV SIGMET)
- Tropical Cyclone (TC SIGMET)
- Other En-route weather (WS SIGMET):
- Mountain waves
- Dust or sandstorms
- Radioactive clouds
In the US, there is an additional type of SIGMET, known as a Convective SIGMET.
The information mentioned above is typically broadcast on ATIS, ATC, and VOLMET facilities. They’re each given an alphabetic designator from N through Y (except for S and T). SIGMETs are issued as necessary and remain valid for four hours. For hurricanes and volcanic ash outside of the CONUS (Contiguous United States), SIGMETS are valid for six-hour intervals.
There are three different types of Convective SIGMETs:
- Line Thunderstorm: Thunderstorms a minimum of 60 miles long, with thunderstorms affecting 40% of the convective SIGMET area’s length.
- Area Thunderstorm: Thunderstorms covering a minimum of 40% of the area concerned and exhibiting powerful radar reflections, or significant satellite or lightning readings.
- Embedded Thunderstorm: Embedded or severe thunderstorms are forecast to occur for over 30 minutes.
How to Read a SIGMET
SIGMETs are similar to METARs and TAFs but use a slightly more complicated structure. Let’s take a look at it in detail.
SIGMETs use the following abbreviations to denote specific types of weather phenomena:
- ABV: Above
- CNL: Cancel
- CTA: Control area
- FCST: Forecast
- FIR: Flight Information Region
- FL: Flight level
- FT: Feet
- INTSF: Intensify
- KT: Knots
- KMH: Kilometers per hour
- M: Meters
- MOV: Moving
- NC: No Change (in intensity)
- NM: Nautical Miles
- OBS: Observed
- SFC: Surface
- STNR: Stationary
- TOP: Top (of CB cloud)
- WI: Within (an area)
- WKN: Weakening (intensity)
- Z: Coordinated Universal Time (“Zulu time”)
This structure is divided into three sections:
- Main Body
The Header Section
The header uses the following structure:
TTAAii CCCC YYGGgg [CCx]
- TT: An identifier for the types of SIGMETs previously mentioned. VA or WV is for volcanic ash, TC is for a tropical cyclone, and WS for other.
- AA: The country or territory code.
- ii: Bulletin number
- CCCC: The ICAO location of the disseminating office.
- YY: Day of the month.
- GG: Hours in UTC.
- gg: Minutes in UTC.
- [CCx]: Indicates a correction where x is the correction number (A-Z).
The Summary Section
CCCC SIGMET [n][n]n VALID YYGGgg/YYGGgg CCCC-
- CCCC: The ICAO location of the affected area.
- SIGMET: Suppose the current SIGMET is a convective SIGMET. In that case, it will read SIG[E/C/W] CONVECTIVE SIGMET ##[E/C/W] where E/C/W designates whether the SIGMET applies to the Eastern, Central, or Western United States, and the ## denotes the number of convective SIGMETS issued for that particular region.
- [n][n]n: A sequence number of form 1′, 01, A01, etc., increased for each time the SIGMET remains active past 0001UTC or on renewals.
- VALID YYGGgg/YYGGgg: Indicates SIGMET active period (WS SIGMETs are not active for more than 4 hours). YY = day of the month, GG = hours, and gg = minutes.
- CCCC-: The ICAO location of the disseminating office followed by a hyphen.
The Body Section
The body section of a SIGMET has many variations and takes the following structure:
CCCC [FIR/CTA list] <Phenomenon> OBS/FCST [AT GGggZ] <Location> <Level> [MOV XXX xx KT/KMH] [INTSF/WKN/NC] [FCST AT <GGgg>Z <location>]=
CCCC [FIR/CTA list]: The ICAO location and region
<Phenomenon>: Denotes any of the following:
- OBSC TS: Obscured thunderstorms
- EMBD TS: Embedded thunderstorms
- FRQ TS: Frequent thunderstorms
- SQL TS: Squall line thunderstorms
- OBSC TSGR: Obscured thunderstorms with hail
- EMBD TSGR: Embedded thunderstorms with hail
- FRQ TSGR: Frequent thunderstorms with hail
- SQL TSGR: Squall line thunderstorms with hail
- SEV TURB: Severe turbulence
- SEV ICE: Severe icing
- SEV ICE (FZRA): Severe icing due to freezing rain
- SEV MTW: Severe mountain wave
- HVY DS: Heavy dust storm
- HVY SS: Heavy sandstorm
- RDOACT CLD: Radioactive cloud
In a convective SIGMET, the following additional codes may be used:
- AREA TS: Area-wide thunderstorms
- LINE TS: Thunderstorm line
- EMBD TS: Embedded thunderstorms
- TDO: Tornado
- FC: Funnel Cloud
- WTSPT: Waterspout
- HVY GR: Heavy Hail
OBS/FCST [AT GGggZ] indicates if the phenomenon is observed (OBS) or forecasted (FCST) as well as the Zulu time that it was observed or forecasted.
<Location> The meteorological phenomenon’s location, typically described with latitude and longitudinal coordinates.
<Level> indicates the altitude of the phenomenon, where:
- FLnnn or nnnnM or nnnnFT: Denotes an altitude
- SFC/FLnnn or SFC/nnnnM or SFC/nnnnFT: From the surface (ground) (SFC) to a particular altitude
- FLnnn/nnn or nnnn/nnnnFT: Between altitudes
- TOP FLnnn or ABV FLnnn or TOP ABV FLnnn: Above a certain point or cloud cover
[MOV XXX xx KT/KMH] describes whether it is a moving front, and the direction and rate of movement are given as a compass direction (XXX, e.g., “N” or “WNW”). The rate is denoted in KT (or KMH). Occasionally STNR (Stationary) may be used if no significant movement is expected.
[INTSF/WKN/NC] denotes the change in strength over time.
[FCST AT <GGgg>Z <location>] helps note where the front is expected to be at the end of the SIGMET’s validity period.
SIGMET Renewal or Cancellations
Suppose a SIGMET’s validity period is about to expire, but the weather phenomena are still expected to occur. In that case, a new sequence number will be added to the SIGMET, thereby renewing it.
The following will replace a SIGMET that is canceled during its validity period:
CNL SIGMET [n][n]n YYGGgg/YYGGgg
As a pilot, it is essential to understand SIGMETs and AIRMETs in order to make better decisions about whether a flight should be canceled, diverted, or rerouted. SIGMETs and AIRMETs keep you safe by providing information about the conditions in a particular area and along your planned route.
In addition, the ability to quickly read and understand SIGMETs and AIRMETs can help you avoid dangerous weather phenomena such as thunderstorms and severe turbulence – an extremely useful skill!
Use this article and practice reading actual SIGMETs and AIRMETS. Try to decipher them as quickly as possible. You should find that what appears to be Greek at first slowly becomes very intuitive.