# SIGMET

SIGMET, or Significant Meteorological Information (AIM 7-1-6), is a severe weather advisory that contains meteorological information concerning the safety of all aircraft. Compared to AIRMETs, SIGMETs cover more severe weather. Today, according to the advancement of technology in civil aviation, the SIGMET is sent as IWXXM model.

## Types

There are three main types of internationally recognized SIGMETs per ICAO:
* Volcanic ash (VA or WV SIGMET)
* Tropical Cyclone (TC or WC SIGMET)
* Other En-route weather phenomenon (WS SIGMET), which may consist of
** Thunderstorm types
** Turbulences types
** Mountain waves
** Icing/Sleet/Hail
** Dust or sand storms
** Radioactive Cloud
This information is usually broadcast on the ATIS at ATC facilities, as well as over VOLMET stations. They are assigned an alphabetic designator from N through Y (excluding S and T). SIGMETs are issued as needed, and are valid up to four hours. SIGMETS for hurricanes and volcanic ash outside the CONUS are valid up to six hours.

### Convective SIGMETs

For airmen in the U.S., there is an additional category of SIGMET known as a convective SIGMET. These are issued for convection over the coterminous U.S. on a scheduled basis, hourly at 55 minutes past the hour. The Convective SIGMETs are valid for 2 hours or until superseded by the next SIGMET. If it is determined that the conditions do not meet the Convective SIGMET criteria within a given region, a CONVECTIVE SIGMET... NONE is issued. Additionally, a 2 to 6-hour outlook is attached at the end of each regional bulletin.$^{:8,11}$
There are three types of convective SIGMETs:
| Type | Definition |
| Line Thunderstorm | Thunderstorms at least 60 miles long with thunderstorms affecting 40% of its length. |
| Area Thunderstorm | Thunderstorms covering at least 40% of the area concerned and exhibiting a very strong radar reflectivity or a significant satellite or lightning signature. |
| Embedded Thunderstorm | Embedded or severe thunderstorms expected to occur for more than 30 minutes. |
When shown in a graphical depiction, the convective SIGMET polygon is a "snapshot" that delineates the region of thunderstorm at the issuance time of 55 past each hour.$^{:7}$
There are also a few special issuance convective SIGMETs to cover extreme weather more common in the U.S., including:
* Tornadoes
* Hail greater than or equal to 3/4 inches in diameter
* Wind gusts greater than or equal to 50 knots
* Indications of rapidly changing conditions not sufficiently described in existing convective SIGMETs.$^{:8}$

## Structure

SIGMETs are internationally used by ICAO and WMOs using standardized abbreviations
| Abbreviation | Meaning |
| ABV | Above |
| CNL | Cancel or cancelled |
| CTA | Control area |
| FCST | Forecast |
| FIR | Flight Information Region |
| FL | Flight level |
| FT | Feet |
| INTSF | Intensify or intensifying |
| KT | Knots |
| KMH | Kilometres per hour |
| M | Metres |
| MOV | Moving |
| NC | No Change (in intensity) |
| NM | Nautical Miles |
| OBS | Observed |
| SFC | Surface |
| STNR | Stationary |
| TOP | Top (of CB cloud) |
| WI | Within (area) |
| WKN | Weakening (intensity) |
| Z | Coordinated Universal Time |
and are split into three lines:
* Header
* Summary
* Main Body
In sum, a standardized SIGMET will have the following structure:
TTAAii CCCC YYGGgg [BBB] CCCC SIGMET [n][n]n VALID YYGGgg/YYGGgg CCCC- CCCC <FIR/CTA Name> FIR <Phenomenon> OBS/FCST [AT GGggZ] <Location> <Level> [MOV XXX xx KT/KMH] [INTSF/WKN/NC] <Forecast time and forecast position>=

### Header

The header line consists of the following characters
TTAAii CCCC YYGGgg [CCx]
| Component | Description |
| TT | A data type identifier per the types of SIGMETs mentioned above. VA or WV for volcanic ash, TC for tropical cyclone, and WS otherwise. |
| AA | The two-character country or territory code of the alert. |
| ii | Bulletin number |
| CCCC | The 4-character ICAO location of the dissmenating office. |
| YY | Day of the month. |
| GG | Hours UTC. |
| gg | Minutes UTC. |
| [CCx] | When included, indicates a correction that takes the form of CCx where x is A-Z indicating the correction number. |

### Summary

The first line of the broadcast is a summary line consisting of the following characters
CCCC SIGMET [n][n]n VALID YYGGgg/YYGGgg CCCC-
| Component | Description |
| CCCC | The 4-character ICAO location of the affected area. |
| SIGMET | Indicates that this is a SIGMET broadcast. |
| [n][n]n | A sequence number of the form 1', 01, A01, etc. which is incremented for each time the SIGMET remains effective past 0001UTC or upon renewals. Helps indicate how long the SIGMET has been active. |
| VALID YYGGgg/YYGGgg | Indicates the period the SIGMET is active (WS SIGMETs can not be active for more than 4 hours), where YY is the day of the month, GG is the hour, and gg is the minute. |
| CCCC- | The 4-character ICAO location of the dissmenating office followed by a hyphen. |
       Notes  [a] – if it a convective SIGMET, then this will read SIG[E/C/W] CONVECTIVE SIGMET ##[E/C/W]. Where E/C/W indicates whether it's over the Eastern, Central, or Western United States, and ## indicates the number of the convective SIGMET issued for that region.

### Body

The main body of a SIGMET can be much more variable, and consists of
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] is again the 4-character ICAO location, followed by the affected flight or control regions.
<Phenomenon> is a code describing the meteorological phenomena as follows:
| Code | Description |
| 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 duststorm |
| HVY SS | Heavy sandstorm |
| RDOACT CLD | Radioactive cloud |
If it is a convective SIGMET, the following codes may appear
| Code | Description |
| 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 whether the phenomenon is observed (OBS) or forecasted (FCST), and the Zulu hour and minute that it was observed or will be forecasted.
<Location> is a general description of location of the meteorological phenomenon, typically utilizing latitude and longitudinal coordinates.
<Level> helps denote the altitude that the phenomenon will be occurred, and can be expressed as
| FLnnn or nnnnM or nnnnFT | At a particular altitude |
| SFC/FLnnn or SFC/nnnnM or SFC/nnnnFT | From the surface (SFC) to a particular altitude |
| FLnnn/nnn or nnnn/nnnnFT | Between certain altitudes |
| TOP FLnnn or ABV FLnnn or TOP ABV FLnnn | Above a certain point or cloud cover |
[MOV XXX xx KT/KMH] if it is a moving front, the direction and rate of movement given as a compass direction (XXX, e.g. "N" or "WNW"), and the rate is given in KT (or KMH). Sometimes STNR (Stationary) may be used instead 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 and Cancellation

If when the validity period is due to expire but the phenomenon is expected to persist, a new sequence number is added to the SIGMET to renew it.
If during the validity period of a SIGMET, the SIGMET is to be cancelled, the following replaces the SIGMET message
   CNL SIGMET [n][n]n YYGGgg/YYGGgg
In the United States, non-convective SIGMETs have a valid period no more than 4 hours. If the phenomena continue, a new SIGMET will be reissued at least every 4 hours. If an amendment is needed, a new SIGMET will be issued using the next series number. Corrections to SIGMETs are issued as necessary, and are identified with COR.$^{:5}$

## Examples

An en-route weather phenomenon in the U.S., issued by the Aviation Weather Center in Kansas City, MO on the 7th of August, at 16:55 UTC
This is a convective weather pattern in the central region of the contiguous U.S. with sequence number 83C
That is valid until 18:55 UTC
That covers Minnesota, Indiana, Wisconsin, Illinois, Iowa, and Lower Michigan flight regions
A description of the area of the storm, where GRR (Grand Rapids, MI airport), GIJ (Niles, MI VORTAC), BDF (Bradford, IL VORTAC), IOW (Iowa City, IA airport), and DBQ (Dubuque, IA airport) are ConTrol Areas (CTAs).
Thunderstorms moving from 250 degrees (roughly WSW) at 25 knots, covering an altitude from the cloud cover to 45,000 ft.