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A map from the U.S. National Weather Service showing active SIGMETs
Aviation briefings

SIGMET,[1] or Significant Meteorological Information (AIM 7-1-6),[2] 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.[3]

Types

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There are three main types of internationally recognized SIGMETs per ICAO:[4]

  • Volcanic ash (VA or WA 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).[2] 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.[5]

Convective SIGMETs

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The eastern, central, and western regions of the coterminous United States have their own Convective SIGMET bulletin

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 The Convective SIGMETs are valid for 2 hours or until superseded by the next SIGMET.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.[6]: 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.[6]: 7 

There are also a few special issuance convective SIGMETs to cover extreme weather more common in the U.S., including:[7]

  • 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.[6]: 8 

Structure

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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>=

[edit]

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

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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. [a]
[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

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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

[8]

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

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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 [9]

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.[6]: 5 

Examples

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WSUS32 KKCI 071655

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[10] 

SIGC
CONVECTIVE SIGMET 83C

This is a convective weather pattern in the central region of the contiguous U.S. with sequence number 83C 

VALID UNTIL 1855Z

That is valid until 18:55 UTC 

MI IN WI IL IA LM

That covers Minnesota, Indiana, Wisconsin, Illinois, Iowa, and Lower Michigan flight regions 

FROM 30E GRR-30S GIJ-30SSW BDF-10ENE IOW-50ENE DBQ-30E GRR

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). 

AREA TS MOV FROM 25025KT. TOPS TO FL450.

Thunderstorms moving from 250 degrees (roughly WSW) at 25 knots, covering an altitude from the cloud cover to 45,000 ft.

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

SIGMET

View on Grokipedia
from Grokipedia
SIGMET, or Significant Meteorological Information, is a standardized weather advisory issued by a meteorological watch office (MWO) to provide pilots and air traffic services with concise descriptions of the occurrence or expected occurrence of specified en-route weather phenomena that may affect the safety of low-level and cruising flight operations. These advisories are disseminated internationally in accordance with (ICAO) standards, focusing on hazardous conditions such as severe or extreme , severe icing, severe mountain waves, heavy dust or sandstorms, widespread areas of thunderstorms (including embedded, frequent, or types), tropical cyclones, and clouds. SIGMETs are typically valid for four hours, though this extends to six hours for volcanic ash and tropical cyclones, and they are issued as soon as practicable when the phenomena are observed or forecast to impact a significant area, such as more than 3,000 square miles for convective activity in certain regions. In practice, SIGMETs are prepared by designated MWOs, such as the Aviation Weather Center in the United States or regional equivalents under ICAO oversight, using abbreviated in a structured format that includes the (FIR), validity period, location coordinates, intensity, and expected movement of the phenomenon. Unlike routine forecasts, SIGMETs are unscheduled and prioritized for urgent dissemination via aeronautical fixed telecommunications networks, flight information services, and , ensuring timely updates or cancellations when conditions change. They play a vital role in enhancing by enabling pilots to avoid or prepare for threats, with additional provisions for special cases like radioactive clouds from nuclear incidents or widespread heavy sandstorms reducing visibility to less than 200 meters. The criteria for issuance are precisely defined to distinguish SIGMETs from less severe advisories like AIRMETs, which address moderate conditions; for instance, SIGMETs require phenomena to pose clear hazards to at cruising altitudes, often excluding non-thunderstorm-related convective activity unless it meets regional thresholds. Globally harmonized through ICAO Annex 3, SIGMETs also support modern data exchange formats like BUFR and IWXXM for automated processing, facilitating integration into systems and contributing to reduced weather-related incidents in international .

Introduction

Definition

A SIGMET, or Significant Meteorological Information, is a weather advisory issued to alert aircraft operators of potentially hazardous en-route meteorological conditions. It provides a concise description of the occurrence or expected occurrence of specified weather phenomena that may affect the safety of all types of during flight. The scope of a SIGMET encompasses significant events over a defined geographical area, typically covering en-route rather than terminal or surface conditions. These advisories address widespread hazards, such as those impacting at least 3,000 square miles in certain regions, including severe , severe icing, thunderstorms, , and dust storms reducing visibility. Unlike AIRMETs, which focus on moderate phenomena that could affect certain aircraft categories, SIGMETs are reserved for more severe and extensive threats requiring immediate attention from all pilots. The primary purpose of SIGMETs is to enhance by disseminating timely and critical meteorological information through established aviation weather services, enabling pilots to make informed decisions about route adjustments or . Issued by meteorological watch offices in accordance with international standards, such as those outlined by the (ICAO), SIGMETs ensure standardized communication of these hazards globally.

Historical Development

The development of SIGMET originated in the post- period under the (ICAO), established by the 1944 Chicago Convention to standardize international aviation practices. During , meteorological reporting for aviation relied heavily on ad-hoc military systems, with weather advisories provided through informal channels to support combat operations. Following the war, ICAO formalized these into civilian standards, adopting Annex 3—Meteorological Service for International Air Navigation—in 1948 as the initial Meteorological Codes, which included provisions for significant meteorological information to ensure flight safety. This marked the transition from fragmented military reports to a global, coordinated framework for weather advisories like SIGMET, emphasizing en-route hazards. A key milestone occurred in the 1970s with the distinction between convective and non-convective SIGMETs, driven by advances in radar technology and the need to address thunderstorm-related risks more precisely. In the United States, the established the Convective SIGMET Unit in 1978 to issue hourly forecasts for hazardous thunderstorms, influencing ICAO's broader adoption of categorized advisories. This evolution enhanced the timeliness and specificity of warnings for severe turbulence, icing, and associated with . The 1980 eruption of catalyzed the inclusion of in SIGMET criteria, as ash clouds disrupted air traffic across and highlighted the need for dedicated advisories. U.S. authorities issued the first volcanic ash SIGMETs during the event, leading to international updates in ICAO Annex 3 during the 1980s and 1990s that formalized ash detection, forecasting, and dissemination protocols. These changes established Volcanic Ash Advisory Centers to coordinate global responses. In the , further refinements addressed emerging hazards. Recent enhancements in the focus on digital dissemination via the System Wide Information Management (SWIM) framework, with ICAO Amendment 78 to Annex 3 (adopted March 2018, applicable November 2020) introducing initial provisions for advisory information and mandating the use of the ICAO Meteorological Model (IWXXM) for machine-readable SIGMETs, enabling seamless global data sharing. Subsequent amendments, such as Amendment 82 (adopted April 2025, applicable November 2025), continue to refine meteorological services, including digital formats and new advisory provisions.

Types

Convective SIGMETs

Convective SIGMETs are specialized inflight weather advisories issued for hazardous convective weather phenomena, such as severe thunderstorms, embedded thunderstorms, or lines of thunderstorms, that directly threaten across the . These advisories are produced by the Aviation Weather Center (AWC) and focus on conditions implying severe , severe icing, and low-level , distinguishing them from non-convective SIGMETs by their emphasis on thunderstorm-related activity. They are numbered sequentially from 1 to about 30 each day and cover three geographic regions: western (W), central (C), and eastern (E), excluding and . Issuance criteria require that thunderstorms affect at least 40 percent of an area of 3,000 square miles or more, or 40 percent of a line at least 60 nautical miles long, or involve severe thunderstorms characterized by of 3/4 inch (19 mm) or greater in diameter, surface gusts of 50 knots or greater, or observed or forecasted tornadoes. These thresholds ensure the advisories address widespread or intense convective activity capable of impacting multiple , with descriptions including the location, movement (typically in knots), intensity trends, and expected changes based on and forecasts. The primary aviation hazards associated with convective SIGMETs include severe turbulence from updrafts and downdrafts, icing within cumulonimbus clouds, reduced visibility due to heavy precipitation, and low-level wind shear that can lead to sudden airspeed changes and loss of control, particularly during en-route flight phases where deviations may be limited. These risks apply to all aircraft categories, with cumulonimbus tops often exceeding 40,000 feet, complicating altitude avoidance strategies. In the United States, convective SIGMETs are routinely issued hourly at 55 minutes past the hour (H+55) during the convective season, which primarily spans spring through fall (approximately April to October), with each advisory valid for up to two hours or until superseded. If no qualifying conditions exist, a "CONVECTIVE SIGMET... NONE" message is broadcast; otherwise, special unscheduled bulletins are issued for rapidly evolving threats, ensuring timely updates to support and inflight decisions.

Non-Convective SIGMETs

Non-convective SIGMETs are in-flight advisories issued for significant meteorological phenomena that are not associated with thunderstorms but pose hazards to all types of , particularly at higher altitudes. These advisories alert pilots to conditions such as severe icing, severe or extreme turbulence, widespread dust or sandstorms, clouds, and tropical cyclones that can affect flight safety over large areas. Unlike convective SIGMETs, which address rapidly evolving thunderstorm activity, non-convective SIGMETs focus on more persistent or widespread hazards that require route planning adjustments to mitigate risks. Specific phenomena warranting non-convective SIGMETs include severe , defined as icing at a rate sufficient to cause ice protection systems to fail or accumulate ice in unprotected areas, typically ≥0.25 inches per hour, which exceeds the capabilities of typical de-icing equipment and necessitates immediate diversion. Severe , characterized by large abrupt altitude or attitude changes that may temporarily render the aircraft uncontrollable, and extreme turbulence, involving violent oscillations causing structural stress, are also covered, often linked to or mountain waves. Widespread duststorms or sandstorms that reduce to less than 3 statute miles, volcanic ash clouds capable of causing or abrasion , and tropical cyclones with sustained winds of 35 knots or greater over tropical waters are additional triggers, generally requiring observed or forecasted impacts over at least 3,000 square miles, except for volcanic ash and tropical cyclones where issuance is based on any significant occurrence. These hazards have profound aviation impacts, including performance degradation and structural damage from accumulation that can reduce lift by up to 50% with just 1/2 inch buildup, potential aircraft disintegration from severe , and catastrophic engine ingestion risks from . Dust and sandstorms impair navigation and landing, while tropical cyclones introduce strong winds and low visibility that disrupt en-route and terminal operations, often affecting high-altitude jet routes. Issuance requires the phenomena to be observed via pilot reports, , or , or reliably forecasted, with details on , extent, intensity, movement, and evolution provided to enable pilots to avoid the affected . Validity is typically 4 hours, extendable to 6 hours for volcanic ash and tropical cyclones, ensuring timely updates as conditions change.

Issuance

Criteria for Issuance

SIGMETs are issued based on criteria outlined in ICAO Annex 3, Appendix 6, for significant meteorological phenomena that pose hazards to en route within a (FIR) or control area (CTA). These hazards must be either observed or forecast to occur, directly affecting at cruising levels, and require specified thresholds of intensity and spatial extent to warrant issuance. Phenomena are limited to those explicitly listed, such as thunderstorms, , icing, and , ensuring that only conditions with widespread or severe impact trigger alerts. For convective phenomena, issuance criteria emphasize thunderstorms and associated . Thunderstorms qualify based on descriptors such as obscured (OBSC), embedded (EMBD), frequent (FRQ, affecting more than 75% of the area with minimal separation), or (SQL) types, particularly when accompanied by severe , , or strong winds. Descriptors such as embedded (EMBD) cumulonimbus clouds, frequent (FRQ) coverage exceeding 75%, or (SQL) with significant are used when these conditions are widespread or continuous. Tropical cyclones trigger SIGMETs when maximum surface winds reach or exceed 34 knots (63 km/h) near the center, including forecasts of position and intensity changes. Non-convective criteria focus on hazards like turbulence, icing, and visibility-impairing events. Turbulence SIGMETs are issued for moderate (MOD) conditions with eddy dissipation rate (EDR) between 0.20 and 0.45, or severe (SEV) with EDR of 0.45 or greater, whether in-cloud or clear-air (CAT). Icing requires moderate non-convective intensity or severe levels, including freezing rain (FZRA). Volcanic ash clouds of any concentration necessitate immediate issuance, specifying heights and movement. Sandstorms or duststorms qualify if visibility is reduced to less than 200 meters with sky obscured (heavy); or to 200-600 meters, or less than 200 meters without sky obscured (moderate). Mountain wave turbulence triggers alerts for moderate downdrafts of 1.75–3.0 m/s or severe downdrafts of 3.0 m/s or more. SIGMETs address both observed and forecast phenomena, with validity periods up to 4 hours for most events (extended to 6 hours for and tropical cyclones). Observed conditions are reported as current (e.g., at a specific time) and expected to persist, while forecasts cover expected onset within the validity period (up to 4 or 6 hours); longer-term forecasts are issued via Volcanic Ash Advisories (VAAs). If criteria are not met for any listed phenomena, no SIGMET is issued; instead, a "NIL" or equivalent message may be broadcast periodically to confirm the absence of significant hazards.

Responsibilities and Procedures

SIGMETs are issued by Meteorological Watch Offices (MWOs), which are designated by contracting states to a specific (FIR) or portion thereof, ensuring comprehensive coverage of en-route airspace. These offices operate 24 hours a day to monitor and report significant meteorological conditions that could impact . In the United States, for example, the Aviation Weather Center (AWC) functions as the primary MWO for the contiguous states and adjacent coastal waters, coordinating with regional centers for broader coverage. The issuance procedures begin with continuous monitoring of weather phenomena using tools such as , , and models to identify potential hazards. Once a phenomenon is detected, MWO meteorologists validate it against established meteorological criteria to determine if a SIGMET is warranted. If criteria are met, the SIGMET is prepared in the required abbreviated plain language format and disseminated without delay. Dissemination occurs primarily through the aeronautical fixed service (AFS), including the AFTN and increasingly digital formats like IWXXM via the System Wide Information Management (SWIM) framework, ensuring global accessibility. To reach in-flight , SIGMETs are relayed by air traffic services (ATS) units via methods such as (ATIS), controller-pilot data link communications, or voice broadcasts, with priority given to urgent cases. Issuance frequency varies by type and urgency: non-convective SIGMETs are issued on an unscheduled basis as soon as practical when a qualifying is observed or forecast, while convective SIGMETs follow a routine hourly schedule (at 55 minutes past the hour) in designated regions, supplemented by special unscheduled bulletins for rapidly evolving threats. Coordination among MWOs is essential, particularly at boundaries, where adjacent offices exchange information to provide seamless handoffs and prevent gaps or duplications in coverage. Finally, MWOs monitor ongoing conditions and cancel SIGMETs when the associated dissipates or no longer meets the criteria, notifying users through updated messages.

Format

The header of a SIGMET message serves as the initial segment that provides essential identification details, allowing pilots, air traffic controllers, and meteorologists to rapidly assess the message's origin, issuance timing, validity, and geographic scope for relevance to specific flights. This structure follows international standards outlined by the (ICAO), ensuring uniformity across global aviation weather services while accommodating regional variations. Standard components of the SIGMET header include the location indicator, which is a (WMO) abbreviated heading identifying the issuing meteorological watch office (MWO) or aviation weather center, such as "WSUS31 KKCI" for the Aviation Weather Center in Kansas City. The follows, formatted as DDHHMM (day of the month, hour, and minute in UTC), for example "201200" indicating issuance on the 20th at 12:00 UTC. The valid period is then specified with "VALID DDHHMM/DDHHMM", denoting the start and end times of the message's effectiveness, such as "VALID 111200/111600" for a four-hour window from 12:00 to 16:00 UTC on the 11th. These elements collectively establish the temporal framework, with SIGMETs for phenomena other than or typically valid for up to four hours and volcanic ash or tropical cyclone variants extending to six hours. The (FIR) identifier appears next, using a four-letter ICAO code to delineate the volume covered, such as "KZSE" for the Seattle FIR in the United States or "EGRR-" for London's FIR in . The message type is explicitly stated as "SIGMET" or abbreviated as "" in the WMO header, with convective variants labeled "CONVECTIVE SIGMET" to distinguish thunderstorm-related hazards. A sequence number, like "01" in "SIGMET 01", tracks daily issuances within the FIR. Overall, the header's design facilitates efficient parsing in high-stakes environments, transitioning seamlessly to the descriptive body while prioritizing through clear, standardized metadata.

Body

The body of a SIGMET message contains the primary descriptive content regarding the hazardous meteorological , utilizing abbreviated to convey essential details for . It begins with an indicator of whether the information is based on observations (prefixed by "OBS AT" followed by the time in UTC) or forecasts (prefixed by "FCST AT" followed by the time), ensuring pilots can assess the immediacy of the threat. The phenomenon itself is specified using standardized ICAO codes to denote type and intensity, such as "SEV " for severe turbulence, "SEV ICE" for severe icing, "OBSC TS" for obscured thunderstorms, or "VA " for volcanic ash , with one phenomenon per message to maintain clarity. These codes draw from ICAO Annex 3, Table A6-1, which lists approved abbreviations for phenomena like severe mountain waves ("SEV MTW"), heavy dust or sandstorms ("HVY DS" or "HVY SS"), and tropical cyclones ("TC"). Location and spatial extent form the core of the body's geographical description, typically defining the affected area within the relevant (FIR) using coordinates in degrees and minutes (e.g., "N2020 W07005"). Polygonal areas are outlined with 4 to 7 points connected clockwise and prefixed by "WI" (within), such as "WI N4255 E02055 – N4245 E02115 – N4230 E02030 – N4250 E02000," while linear features may use "ALNG" (along) or corridors like "APRX 50KM WID LINE." Sectors relative to lines or entire can also be referenced, with vertical extent specified in flight levels (e.g., "FL180/FL450") or altitudes (e.g., "SFC/10000FT"), omitting height details if the phenomenon is unambiguously surface-based, such as widespread dust storms. Movement is indicated by direction (using 16-point notation, e.g., "ESE") and speed (e.g., "MOV ESE 20KT" for 20 knots eastward) or "STNR" for stationary, providing pilots with predictive information. Forecast evolution within the body includes expected changes in intensity, using codes like "INTSF" (intensifying), "WKN" (weakening), or "NC" (no change), often tied to a future position (e.g., "FCST AT 1800Z PSN N2330 W06315") for phenomena like tropical cyclones. For rapid updates, descriptions remain brief to prioritize timeliness, omitting redundant elements such as movement if a forecast position is provided instead. All elements adhere to ICAO's prohibition on unnecessary descriptive material, ensuring the body remains concise while covering the phenomenon's scope and dynamics.

Renewal and Cancellation

SIGMETs have defined validity periods that commence at the time of issuance and vary by phenomenon type to ensure timely updates for . SIGMETs for phenomena other than or tropical cyclones, including convective SIGMETs for embedded or frequent thunderstorms, are valid for a maximum of 4 hours (with regional variations, such as 2 hours ). Special cases, including clouds and tropical cyclones, extend to 6 hours to account for their prolonged impact. Renewal occurs through the issuance of a new SIGMET message when the weather phenomenon is expected to continue beyond the original validity period, based on ongoing meteorological re-evaluation. This new message uses an updated sequence number in the header, such as incrementing from "SIGMET 1" to "SIGMET 2," and may amend the body with revised location, intensity, or movement details if conditions have evolved. The renewal must be issued before the expiry of the previous message to maintain continuous coverage, up to every 4 hours for SIGMETs other than volcanic ash or tropical cyclones. Cancellation is required when the specified hazard no longer exists or is not anticipated within the , preventing outdated information from influencing . A dedicated cancellation message, formatted as "CNL SIGMET [sequence] [original validity period]," is issued promptly, including the originating meteorological watch office identifier and the affected area. For instance, if no significant weather activity has been observed in monitored areas during the hour prior to H+55 (55 minutes past the hour), a "CNL" or equivalent "NONE" advisory may be broadcast to confirm absence of hazards. Amendments for errors or significant changes also involve cancelling the original SIGMET before issuing the corrected version, ensuring at least a one-minute separation between messages.

Examples

Convective SIGMET Example

A representative example of a convective SIGMET is as follows: CONVECTIVE SIGMET 1E VALID UNTIL 1555Z NC SC FL GA AND CSTL WTRS FROM 30SSE CLT-160SE ILM-140ENE OMN-60E TLH-ABY-30SSE CLT AREA SEV EMBD TS MOV FROM 21015KT. TOPS ABV FL450. TORNADOES...WIND GUSTS TO 60KT POSS. TS ASSOCD WITH TROPICAL STORM ALBERTO. This message was issued on June 21 at 1355Z by the Aviation Weather Center for the Eastern U.S. region, with a validity period of two hours until 1555Z. The header identifies it as the first convective SIGMET of the day for the eastern area (denoted by "1E"), covering North Carolina, South Carolina, Florida, Georgia, and adjacent coastal waters. The body delineates the affected location through a polygon defined by points relative to VHF omnidirectional range (VOR) stations, such as 30 nautical miles south-southeast of Charlotte VOR (CLT), 160 nautical miles southeast of Wilmington VOR (ILM), 140 nautical miles east-northeast of Jacksonville VOR (OMN), 60 nautical miles east of Tallahassee VOR (TLH), and Albany VOR (ABY), closing back to 30SSE CLT. It describes an area of severe embedded thunderstorms (SEV EMBD TS) moving from 210 degrees at 15 knots, with cloud tops above flight level 450 (approximately 45,000 feet MSL). Additional hazards include possible tornadoes and wind gusts to 60 knots, linked to Tropical Storm Alberto. Issued during an episode of widespread convective activity associated with Tropical Storm Alberto in the Southeast U.S., this SIGMET highlights risks to (IFR) operations, including severe turbulence, icing, low-level , and potential structural damage from hail or gusts, necessitating route deviations or altitude changes for affected flights. Pilots decode the location descriptors by referencing sectional or en route charts to identify VOR radials and distances—for instance, "30SSE CLT" indicates 30 nautical miles along the 157-degree radial from the Charlotte VOR—enabling precise plotting of the hazardous area for avoidance planning.

Non-Convective SIGMET Example

A representative example of a non-convective SIGMET for severe icing is as follows: ABCD 111030 SIGMET 001 FOR SEV ICE VALID 111030/111430 BIKF BIKF 5545N02000W - 5745N01800W - 5745N02200W - 5545N02000W SEV ICE INTMTS BTN FL050 AND FL220 MOV NE 15KT. This message follows the standardized ICAO format for non-convective SIGMETs, where the header includes the issuing office identifier (ABCD), issuance time (111030 UTC on ), SIGMET number (001), phenomenon type (SEV ICE for severe icing), and validity period (111030/111430 UTC, or four hours). The body specifies the affected (BIKF, Reykjavik Oceanic FIR in ), delineates a polygonal area using latitude-longitude coordinates (forming a over the North Atlantic), describes the as severe icing occurring intermittently (INTMTS) between flight levels 050 and 220 (approximately 5,000 to 22,000 feet), and notes movement northeast at 15 knots. Such SIGMETs are typically issued for hazards like severe icing due to supercooled large droplets (SLD), which form in stratiform clouds over the North Atlantic and pose significant risks to aircraft on transatlantic jet routes by rapidly accumulating ice that can overwhelm de-icing systems. The intermittent nature (INTMTS) indicates the icing may not be continuous across the area, allowing pilots to interpret the advisory for tactical route deviations, such as climbing above FL220 or rerouting around the polygon to minimize exposure.

International Standards

ICAO Guidelines

The International Civil Aviation Organization (ICAO) establishes the global standards for Significant Meteorological Information (SIGMET) through Annex 3 to the Convention on International Civil Aviation, titled Meteorological Service for International Air Navigation. In Chapter 7 of this annex, SIGMET is defined as a form of en-route information provided to warn aircraft of potentially hazardous weather phenomena that could affect the safety of international flights at cruising levels. This framework ensures that SIGMETs are issued by designated Meteorological Watch Offices (MWOs) to support safe air navigation across international airspace. Key requirements outlined in Annex 3 mandate that MWOs shall issue SIGMET messages as soon as practicable following the observation or reliable forecast of qualifying phenomena, covering the entire (FIR) or a specified sub-area where the hazard is anticipated. SIGMETs must explicitly address volcanic ash clouds and , with special provisions for coordination with Volcanic Ash Advisory Centres (VAACs) and Tropical Cyclone Advisory Centres (TCACs), respectively; these are issued as soon as practicable after detection and remain valid for up to 6 hours, with updates at least every 6 hours. For other phenomena, such as severe turbulence or icing, the validity period is limited to a maximum of 4 hours, and messages are cancelled once the conditions cease. Standardization of SIGMET formats is detailed in Appendix 6 of Annex 3, which provides abbreviated plain-language code templates to ensure consistency and brevity in messaging. These templates include elements such as the location indicator (e.g., FIR identifier), validity period, phenomenon description, position, movement, and forecast details. Dissemination occurs via the (AFTN) or the (AMHS), reaching air traffic services units, aircraft operators, and relevant advisory centres within 10 minutes of issuance. Recent updates to Annex 3, incorporated in the 21st edition via Amendment 82 (applicable from 27 November 2025), enhance provisions for climate-related hazards by refining criteria for SIGMET issuance on severe and integrating requirements for digital exchange using the ICAO Meteorological Information Exchange Model (IWXXM). Key enhancements include provisions for quantitative volcanic ash forecasts and improved digital exchange using IWXXM. These revisions promote better alignment with global data-sharing protocols, facilitating real-time meteorological information across international networks.

Regional Variations

In North America, SIGMETs for the contiguous United States are issued by the National Weather Service's Aviation Weather Center (AWC) in Kansas City, Missouri, covering the continental area divided into eastern, central, and western regions specifically for convective SIGMETs, which are updated hourly to address severe thunderstorms and associated hazards. Separate SIGMETs are issued by the Alaska Aviation Weather Unit for the Anchorage Flight Information Region and by the Weather Forecast Office in Honolulu for Hawaii, reflecting the diverse meteorological environments across domestic territories. In Canada, SIGMETs are prepared and disseminated by Meteorological Watch Offices under NAV CANADA in coordination with Environment and Climate Change Canada, ensuring coverage for Canadian airspace including transboundary routes with the United States. Volcanic ash hazards are managed through Volcanic Ash Advisory Centers (VAACs) in Washington, D.C., and Anchorage, which issue Volcanic Ash Advisories (VAAs) integrated into SIGMETs for affected regions. In , SIGMET procedures are standardized under the EUR SIGMET and Guide (5th edition, 2023), issued by ICAO's European and North Atlantic Office, which emphasizes harmonization across 30 flight information regions through coordination by EUMETNET, the collaborative network of 33 European national meteorological and hydrological services. This guide prioritizes interoperability for high-traffic transatlantic routes within the North Atlantic Organized Track System, where SIGMETs for and icing are issued with enhanced lead times to support efficient rerouting. In the region, the ICAO Regional SIGMET Guide (11th edition) tailors issuance to the area's geophysical risks, with a strong focus on due to the Ring of Fire's concentration of active volcanoes, requiring Meteorological Watch Offices to issue WV SIGMETs promptly based on advisories from regional VAACs such as , Darwin, and Anchorage. In monsoon-influenced areas like , non-convective SIGMET validity is capped at 4 hours to account for rapidly evolving phenomena such as tropical cyclones and heavy precipitation, necessitating frequent renewals for ongoing hazards. Dissemination integrates with meteorological networks, enabling seamless sharing of SIGMETs among member states' service providers via the Aeronautical Fixed Service and regional hubs. Across regions, challenges arise from language standardization, where ICAO mandates abbreviated in English using approved abbreviations for all international SIGMETs, though some states provide supplementary domestic versions in local languages to aid national operators. Coverage gaps persist in remote polar regions, where limited ground observations require bilateral coordination between adjacent flight information regions, such as those spanning the , to ensure comprehensive SIGMET issuance despite sparse . These variations adapt core ICAO guidelines to local conditions while maintaining global .

References

  1. https://www.bazl.admin.ch/dam/bazl/it/dokumente/Fachleute/Regulationen_und_Grundlagen/icao-annex/icao_annex_3_meteorologicalserviceforinternationalairnavigation.pdf.download.pdf/icao_annex_3_meteorologicalserviceforinternationalairnavigation.pdf
  2. https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap7_section_1.html
  3. https://skybrary.aero/articles/sigmet
  4. https://applications.icao.int/postalhistory/annex_3_meteorological_service_for_international_air_navigation.htm
  5. https://repository.library.noaa.gov/view/noaa/33564/noaa_33564_DS1.pdf
  6. https://avo.alaska.edu/pdfs/B2047_p293to296.pdf
  7. https://www.icao.int/sites/default/files/APAC/Documents/edocs/APAC-Regional-SIGMET-Guide-11th-edition_MET_SG28_vSubmitted.pdf
  8. https://community.wmo.int/en/activity-areas/aviation/news/2025-09-05
  9. https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_00-45E.pdf
  10. https://www.faa.gov/documentlibrary/media/advisory_circular/ac-0045g_chg1_fulldocument.pdf
  11. https://www.weather.gov/media/directives/010_pdfs/pd01008011curr.pdf
  12. https://www.icao.int/sites/default/files/EURNAT/Documents/EUR%20and%20Nat%20Docs/EUR%20Documents/EUR%20Documents/014%20-%20EUR%20SIGMET%20and%20AIRMET%20Guide/EUR-Doc-14-EN-5th-Ed-2023-rev-Dec23-clean.pdf
  13. https://www.preventionweb.net/news/how-satellite-data-make-flying-safer
  14. https://www.icao.int/sites/default/files/ESAF/APIRG/APIRG-24/Appendices/Appendix-3H-AFI-SIGMET-Guide.pdf
  15. https://www.hko.gov.hk/en/aviat/outreach/40th/sig-coord.htm
  16. https://www.icao.int/sites/default/files/EURNAT/Documents/EUR%20and%20Nat%20Docs/EUR%20Documents/EUR%20Documents/014%20-%20EUR%20SIGMET%20and%20AIRMET%20Guide/EUR-Doc-14-EN-5th-Ed-2023-rev-Dec23-track-changes.pdf
  17. https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_00-45H_CHG_2.pdf
  18. https://www.icao.int/sites/default/files/EURNAT/Documents/EUR%20and%20Nat%20Docs/EUR%20Documents/EASPG%20Reports/EASPG3/App_ZB-EUR-Doc-014-4th-Edition.pdf
  19. https://www.faa.gov/documentlibrary/media/advisory_circular/ac_00-45g_chg_1-2.pdf
  20. https://aircrafticing.grc.nasa.gov/1_1_2_5.html
  21. https://www.faa.gov/documentlibrary/media/advisory_circular/ac_91-74b.pdf
  22. https://wmo.int/media/news/aviation-news-2025-09-05-icao-publishes-new-editions-of-annex-3-and-pans-met
  23. https://community.wmo.int/en/activity-areas/wis/iwxxm
  24. https://www.faa.gov/documentlibrary/media/advisory_circular/ac_00-45h_chg_1.pdf
  25. https://avmet.navcanada.ca/en/sigmet.aspx
  26. https://eumetnet.eu/
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