MANAIR Manual of Standards and Procedures for Aviation Forecasts: chapter 6


6.1 Definition

AIRMET: Information message issued by a meteorological watch office (MWO) to advise pilots of the occurrence or expected occurrence of specified weather phenomena which may affect the safety of aircraft operations, which were not already included in a Graphic Area Forecast (GFA), and the development of those phenomena in time and space.

6.2 Domain

The message shall describe specified weather phenomena up to and including 24,000 feet (FL240) for the Canadian flight information regions (FIR) and a portion the Gander Oceanic flight information region. Refer to E.1 of Appendix E for the map of the Canadian AIRMET information domain. As with a GFA, the top of a weather phenomenon above FL240 is stated, provided that the base of such phenomenon is below FL240.

6.3 Projection

As per Technical Regulations, Volume II, Meteorological Service for International Air Navigation, section [C.3.3] 3.2.2 (WMO-No. 49), the depiction of the location of the phenomenon shall use the polar stereographic projection true at 60 degrees latitude north with lines between coordinate points as orthodromes (great circles).

6.4 Language

An AIRMET message is prepared in abbreviated English language using approved MSC abbreviations as published in the Manual of Word Abbreviations (MANAB). Abbreviations and words to be used in AIRMET messages are given in E.2 of Appendix E.

6.5 Units

The only units explicitly stated are:

  • nautical miles (NM) for a distance associated to the abbreviation “WTN” (within)
  • knots (KT) for speed
  • zulu (Z) for time in UTC, with midnight being indicated by the numerals zero and zero “00”
  • statute miles (SM) for visibility
  • feet (FT) for the height of the base and top of BKN or OVC cloud
  • heights are otherwise stated in hundreds of feet and preceded by the abbreviation “FL

6.6 Types of AIRMET

ICAO Annex 3 provides one AIRMET format which encompasses all of the specified weather phenomena. An AIRMET can be identified through the data type designator included in the WMO abbreviated heading of an AIRMET message as explained in section 6.7.

6.7 Structure of the AIRMET message

An AIRMET message consists of:

  • WMO heading: An AIRMET is preceded by an appropriate WMO heading.
  • First line: Containing location indicators of the relevant ATS unit serving the FIR and MWO, alphanumeric identification and period of validity.
  • Meteorological part: Containing meteorological information concerning the phenomenon for which the AIRMET is issued.
  • Equal sign “=”: Indicates the end of the message.

6.8 Format of AIRMET

Note: In the following text, square brackets [ ] are used to indicate an optional or conditional element, and angled brackets < > for symbolic representation of a variable element, which in the real AIRMET, accepts concrete alphanumeric values.

6.8.1 WMO header

The Manual on the Global Telecommunication System (WMO-No. 386) recommends the use of the following abbreviated headings for bulletins containing meteorological information.

Table 11: T1T2A1A2ii CCCC YYGGgg [ CCx]
Symbol Interpretation
T1 Data type designator (refer to 6.8.1.1)
T2 Data type designator (refer to 6.8.1.1)
A1A2 Geographical designator (where message applies not the issuing country) (refer to 6.8.1.2)
ii Bulletin number (refer to 6.8.1.3)
CCCC International four-letter location indicator of the centre originating or compiling the bulletins (refer to 6.8.1.4)
YYGGgg International date-time group
CCx Correction identifier (refer to 6.8.1.5)
6.8.1.1 Data type designator

T1: W for Warnings (WMO-No. 386, table A, A.II 5/2)

T2: A for AIRMET (WMO-No. 386, table B1, A.II-5/6)

6.8.1.2 Geographical designator

The geographic designator is where the meteorological information applies to, not the issuing country.

A1A2: CN for Canada (all FIRs but CZQX Oceanic) (WMO-No. 386, table C1, A.II-5/9)

A1A2: NT for North Atlantic area (CZQX Oceanic) (WMO-No. 386, table C, A.II-5/12)

6.8.1.3 Bulletin number

ii: number with two digits

  • 01-19 for global distribution
  • 20-39 for interregional distribution (as per WMO regions)
6.8.1.4 International four-letter location indicator of the centre originating or compiling the bulletins

CCCC: As standardized practice, all AIRMET messages issued by Canada will be under CWAO which refers to the Canadian Meteorological Centre/Network Operations (CMC/NETOPS) as the centre compiling the bulletins.

6.8.1.5 Correction identifier

CCx: Since there are no provisions in ICAO documentation for correcting an AIRMET, the correction identifier will not be used. Refer to 6.8.4 for details.

6.8.1.6 Bulletin scheme

AIRMET messages will be issued in a set of two bulletins. The first one is aimed at international distribution while the second one will contain additional information for national purposes. Refer to E.3 of Appendix E for the full bulletin scheme, as shown in this excerpt:

FIR FIR name Type International (ICAO) National
CZYZ TORONTO AIRMET WACN04 CWAO WACN24 CWAO

6.8.2 First line--CCCC SIGMET [n]nn VALID YYGGgg/YYGGgg CCCC-

Table 12: CCCC SIGMET [ n] nn VALID YYGGgg/YYGGgg CCCC-
Symbol Interpretation
CCCC ICAO location indicator of the ATS unit serving the FIR to which the AIRMET refers
AIRMET Message identifier
[n]nn Daily alphanumeric sequence (refer to 6.8.2.1)
VALID Period of validity indicator
YYGGgg/YYGGgg Validity period of AIRMET given by date/time group of the beginning and date/time group of the end of the period (refer to 6.8.2.2)
CCCC- ICAO location indicator of the MWO that issued the message and a
 hyphen “-” without a space, to separate the preamble from text
(refer to 6.8.2.3)
6.8.2.1 Daily alphanumeric sequence
  1. An AIRMET message shall be identified by a letter:
    • Letter attribution rules and the lettering scheme are defined in section E.4.
  2. An AIRMET message letter shall be numerically sequenced as follows:
    • numbering of an AIRMET message (of the letter) begins at one
    • the number is incremented by one when updating the message, including cancellation
    • the sequence number shall correspond with the number of AIRMET messages issued for an event within a FIR since 0000Z on the day concerned
    • the numbering is thus reset at 0000Z (messages are not updated at 0000Z for the sole purpose of resetting the number)

Example: CZVR AIRMET U1 VALID 040315/040715 CWEG-

6.8.2.2 Validity period and lead time of AIRMET
Symbol Lead time - up to (hours) Validity period (hours)
WA 4 4
  1. The period of validity of an AIRMET is four hours.
  2. In the case of an AIRMET for an ongoing phenomenon:
    • the date/time group indicating the start of the AIRMET period will be rounded back to five minutes from the filing time (date/time group in the WMO heading).
  3. In the case of a AIRMET for an expected phenomenon:
    • the beginning of the validity period will be the time of the expected commencement (occurrence) of the phenomenon.
    • For an AIRMET, the lead time (the time of issuance of the AIRMET) can be up to four hours before the start of the validity period (i.e. expected time of occurrence of the phenomenon).
    • An AIRMET for an expected phenomenon is issued only for the first appearance of an event in Canadian airspace (i.e. moving in from the U.S. or onset inside a Canadian FIR). A phenomenon moving from one FIR to another is treated as an ongoing phenomenon. No forecast event AIRMET messages would be sent for the second FIR.

Example (1): For an ongoing phenomenon:

WACN05 CWAO 161220
CZUL AIRMET M1 VALID 161220/161620 CWUL-

Example (2): For an expected phenomenon (expected time of occurrence 0315Z):

WACN01 CWAO 040115
CZVR AIRMET U1 VALID 040315/040715 CWEG-

6.8.2.3 Meteorological Watch Office (MWO)

CWUL for Canadian Meteorological Aviation Center East (CMAC-E) in Montréal.

CWEG for Canadian Meteorological Aviation Center West (CMAC-W) in Edmonton.

6.8.3 Format of the meteorological part of an AIRMET

The meteorological part of an AIRMET consists of the nine elements indicated in the following table (this table has been divided into two parts due to horizontal space constraints):

Element 1
Location indicator of the FIR
Element 2
Name of the FIR
Element 3
Description of the phenomenon
Element 4
Observed or forecast
Element 5
Location
<CCCC> <name> FIR <Phenomenon> OBS [AT <GGggZ>] FCST Geographical location of the phenomenon
Element 6
Level
Element 7
Movement or expected movement
Element 8
Change in intensity
Element 9
Remark
[FL<nnn/nnn>] [SFC/FL<nnn>] [TOP FL<nnn>] MOV <direction, speed>KT
QS
INTSFYG or WKNG or NC RMK
6.8.3.1 Location indicator and name of the FIR (element 1 and 2)

Example (1): CZUL MONTREAL FIR

Example (2): CZVR VANCOUVER FIR

6.8.3.2 Description of the phenomenon (element 3)

The description of the phenomenon consists of a qualifier and a phenomenon abbreviation. AIRMET will be issued for the following phenomena:

  1. SFC WND SPD widespread mean surface wind speed above 30 kts
  2. Surface visibility and/or cloud less than 3SM and/or BKN or OVC below 1,000 feet AGL over a widespread area
  3. Thunderstorm (TS) and/or Towering cumulus (TCU)
    • TCU if ISOLD, OCNL, FRQ; or
    • TS if ISOLD, OCNL, with hail as necessary; or
    • Both TS and TCU if both are present and the spatial amount of the TCU exceeds that of the TS, with the cumulative spatial amount not exceeding 100%. Otherwise only the TS is reported.
  4. Turbulence (TURB) only MDT
  5. Icing (ICG) only MDT
  6. Mountain waves (MTW) only MDT

The appropriate abbreviations and combinations thereof, and their meaning are given in E.5 of Appendix E.

6.8.3.3 Indication if the phenomenon is observed or forecast (element 4)

OBS [AT <GGggZ>] or FCST

The use of “OBS” occurs if either:

  • there has been a recent1 direct observation of the phenomenon (i.e. METAR, PIREP); or
  • there has been a recent1 indirect observation of the phenomenon (i.e. remote sensing imagery) or the observation of a meteorological element (i.e. strong wind, freezing precipitation), which leads the forecaster to believe that the phenomenon is occurring.

OBS is optionally followed by the time group in the form AT GGggZ, where GGgg is the time of the observation in hours and minutes UTC. If the exact time of the observation is not known, the time is not included. No additional information will be given, (i.e. aircraft type reporting the phenomenon).

Note (1): “recent” means normally within a two-hour period preceding the issuance time of the SIGMET and in no circumstance more than three hours preceding the issuance time of the SIGMET.

The use of “FCST” occurs if either:

  • the phenomenon is not directly or indirectly observed but there is strong meteorological evidence (i.e. numerical guidance) to suggest that the phenomenon is occurring; or
  • there is an expected phenomenon. In other words there is meteorological evidence (i.e. numerical guidance) to suggest that the phenomenon will be occurring within the prescribed lead time (refer to 6.8.2.2).

When FCST is used, it is assumed that the time of the occurrence or commencement of the phenomenon coincides with the beginning of the period of validity in the first line of the SIGMET.

Example (1): OBS AT 0140Z

Example (2): FCST

6.8.3.4 Location of the phenomenon (element 5)

The location of the phenomenon is depicted as an area using coordinate points:

  • The description always begins with the abbreviation “WTN” (within).
  • The area can be described as a circle, a line or a polygon.
  • Distances are in nautical miles (NM), direction to one of the eight points of compass (octants1).

Note (1): Refer to E.6 of Appendix E for details.

Description Recommended maximum number of coordinate points
Circle1 1
Line2 4
Polygon3 7

Note (1): A radial distance is used to define the size of the circle.

Note (2): The area is defined by a distance either side of the line.

Note (3): Polygon must be closed. The last coordinate point is a repeat of the first one.

Coordinate points:

  • The international (ICAO) AIRMET message describes a coordinate point using a latitude and longitude only (resolution defined in E.6 of Appendix E)
  • The national AIRMET message describes a coordinate point using a latitude and longitude. Additionally, an equivalent description with respect to an aviation reference site3 is given and is an approximation of the coordinate point in latitude and longitude.

There are two exceptions to this rule:

  1. Any coordinate point located within Gander Oceanic FIR will be described in latitude and longitude only. These points will be limited to the resolution as defined in E.6 of Appendix E.
  2. Any coordinate point north of N7200 will be described with respect to an aviation reference site only if within a 90 nautical mile radius of that site. Otherwise the coordinate point will be represented in latitude and longitude only. This is due to the sparse number of aviation reference sites over northern Canada.
National or international Symbolic form Examples
International (ICAO) Nnnnn Wnnnnn N4502 W07345
National /Nnnnn Wnnnnn/[Distance1 Direction2] Identifier3 /N4502 W07345/25 SW CYUL

Note (1): Distance is in nautical miles (NM) with units not explicitly stated.

Note (2): Direction to the eight point compass (octants). See E.6 of Appendix E for details

Note (3): Refer to E.7 of Appendix E for the list of available aviation reference sites.

Description International (ICAO) National
Circle WTN 45 NM OF N4643 W07345 WTN 45 NM OF /N4643 W07345/75 N CYUL
Line WTN 45 NM OF LINE N4459 W07304 - N4855 W07253 -N5256 W06904 WTN 45 NM OF LINE /N4459 W07304/45 SE CYUL - /N4855 W07253/30 NW CYRJ - /N5256 W06904/75 W CYWK
Polygon WTN N4502 W07345 -N4907 W07331 - N5345 W06943 -N5256 W06758 - N4848 W07149 -N4508 W07206 - N4502 W07345 WTN /N4502 W07345/25 SW CYUL - /N4907 W07331/60 SE CYMT - /N5345 W06943/150 E CYAH - /N5256 W06758/45 W CYWK - /N4848 W07149/25 NE CYRJ - /N4508 W07206/25 SW CYSC - /N4502 W07345/25 SW CYUL
6.8.3.5 Flight level and extent (element 6)

FL<nnn/nnn>

SFC/FL<nnn>

TOP FL<nnn>

The location and extent of the phenomenon in the vertical is given by one or more of the above abbreviations, as follows:

  • reporting a layer “FL<nnn/nnn>,” where the lower level is reported first; this is used particularly in reporting turbulence and icing
  • reporting a layer with reference to one FL using “SFC/
  • reporting the level of the tops of the TS and/or TCU clouds using the abbreviation “TOP

Example (1): MDT TURB…FL220/270

Example (2): MDT ICG…SFC/FL030

Example (3): ISOL TS…TOP FL340

Square brackets “[ ]” indicate that element 6 is optional. Specifically, element 6 is not required for the three following phenomena:

  1. SFC WND SPD: Widespread mean surface wind speed above 30 kts.
  2. Surface visibility and/or cloud: Less than 3SM and/or BKN or OVC below 1,000 feet AGL over a widespread area.
  3. The level or extent of the phenomenon is either not required (i.e. phenomenon at the surface) or the information is contained within element 3.
6.8.3.6 Movement or expected movement (element 7)

MOV <direction, speed>KT or QS

Direction of movement is given with reference to one of the 16 points of compass (radials1). Speed is given in knots (KT). The abbreviation “QS” is used if no significant movement is expected.Example: MOV SSE 15KT

Note (1): Refer to E.6 of Appendix E for details.

6.8.3.7 Change in intensity (element 8)

The expected evolution of the phenomenon’s intensity is indicated by one of the following abbreviations:

  • INTSFYG: intensifying
  • WKNG: weakening
  • NC: no change
6.8.3.8 Remark (element 9)

The remark (RMK) is found only in the national AIRMET message. It begins on a new line. The purpose is to allow additional information of national interest to be conveyed in an AIRMET message. Items listed in the remark line will be separated by a forward slash “/”.

  • the GFA region(s) the AIRMET message applies to (refer to Figure 6)
  • cross-referencing AIRMET messages when a phenomenon straddles one or several FIR boundaries (refer to Figure 6)
  • for a phenomenon that has moved out of an FIR, the cancelled AIRMET message will refer to the continuing AIRMET message in neighbouring FIR(s) within Canada’s AOR (refer to Figure 7)

Symbolic form: RMK [GFACN<nn>] / [CCCC <name> FIR SIGMET[n]nn]1

Note (1): Refer to 6.8.3.1 and 6.8.2.1 for the description.

Figure 6. See long description below.

Description of image

AIRMET phenomenon with a spatial coverage extending over two flight information regions, Edmonton and Winnipeg.

Figure 6: AIRMET phenomenon covering two FIR

The above phenomenon requires two AIRMET messages, one per FIR. The description will be identical in both messages (1) and (2):

Message (1):

WACN22 CWAO 161220
CZEG AIRMET B1 VALID 161220/161620 CWEG-
CZEG EDMONTON FIR
«meteorological part»
RMK GFACN32/CZWG WINNIPEG FIR AIRMET A1=

Message (2):

WACN23 CWAO 161220
CZWG AIRMET A1 VALID 161220/161620 CWEG-
CZWG WINNIPEG FIR
«meteorological part»
RMK GFACN32/CZEG EDMONTON FIR AIRMET B1=

Figure 7. See long description below.

Description of image

AIRMET phenomenon having moved southeastward from the Edmonton flight information region, in time such that the spatial coverage is solely in one flight information region, Winnipeg.

Figure 7: SIGMET phenomenon covering only one FIR

The above phenomenon is a continuation of Figure 6. In other words, the area has moved southeastward with time. AIRMET A2 is an update of A1 while AIRMET B2 is a cancellation of B1 since the phenomenon no longer affects the Edmonton FIR. The messages would look like this:

Message (1):

WACN22 CWAO 161530
CZEG AIRMET B2 VALID 1615300/161620 CWEG-
CZEG EDMONTON FIR CNCL AIRMET B1 161220/161620
RMK GFACN32/CZWG WINNIPEG FIR AIRMET A2=

Message (2):

WACN23 CWAO 161530
CZWG AIRMET A2 VALID 161530/161930 CWEG-
CZWG WINNIPEG FIR
«meteorological part»
RMK GFACN32=

6.8.4 Correcting an AIRMET

In the event of an AIRMET message transmitted with an error, a correction is sent by updating the AIRMET according to 6.8.5.

6.8.5 Updating of AIRMET

An updated AIRMET, when issued, automatically replaces the previous AIRMET in the same series (i.e. the previous AIRMET with the same letter):

  • An AIRMET must be updated every four hours (from date/time group in the WMO heading, refer to 6.8.1); however,
  • a forecaster may update an AIRMET at any time if it is considered necessary.

For rules regarding the alphanumeric sequence, refer to 6.8.2.1.

Example (1): For an ongoing phenomena:

WACN05 CWAO 161220
CZUL AIRMET M3 VALID 161220/161620 CWUL-

Updated four hours later (i.e. before 1620Z) as:
WACN05 CWAO 161605
CZUL AIRMET M4 VALID 161605/162005 CWUL-

Example (2): For an expected phenomenon (expected time of occurrence 0315Z):

WACN01 CWAO 040115
CZVR AIRMET U1 VALID 040315/040715 CWEG-

Updated at 0245Z as event occurred earlier (30 minutes) than expected:
WACN01 CWAO 040245Z
CZVR AIRMET U2 VALID 040245/040645 CWEG-

6.8.6 Cancelling an AIRMET

Cancelling an AIRMET is required when:

  • during the validity period of an AIRMET, the phenomenon for which the AIRMET had been issued is no longer occurring or no longer expected to occur (forecast AIRMET);
  • during the validity period of an AIRMET, the phenomenon for which the AIRMET had been issued strengthens, such that a SIGMET is now required. Refer to 6.8.9 for details; and
  • during the validity period of an AIRMET, the new issue of the Graphical Area Forecasts (GFA) has been transmitted and now includes the phenomenon. Refer to 6.8.8 for details.
6.8.6.1 WMO header

Same data type designator. Refer to 6.8.1.

6.8.6.2 First line--CCCC SIGMET [n]nn VALID YYGGgg/YYGGgg CCCC-
Table 13: CCCC SIGMET [ n] nn VALID YYGGgg/YYGGgg CCCC-
Symbol Interpretation
CCCC ICAO location indicator of the ATS unit serving the FIR to which the AIRMET refers
SIGMET Message identifier
[n]nn Daily alphanumeric sequence (refer to 6.8.2.1)
VALID Period of validity indicator
YYGGgg/YYGGgg Validity period of AIRMET given by date/time group of the beginning and date/time group of the end of the period
CCCC- ICAO location indicator of the meteorological watch office (MWO) originating the message and hyphen “-” without a space, to separate the preamble from text (refer to 6.8.2.3).
6.8.6.2.1 Daily alphanumeric sequence

Number incremented by one. Refer to 6.8.2.1 for details.

6.8.6.2.2 Validity period of SIGMET

In the case of a AIRMET for an ongoing phenomenon:

  • the date/time group indicating the start of the AIRMET period will be rounded back to five minutes from the filing time (date/time group in the WMO heading) while the date/time group indicating the end of the AIRMET period remains the same (as the AIRMET it cancels)
  • if the validity period has expired, the date/time group indicating the start and end of the AIRMET period will all be the same, and those will be rounded back to five minutes from the filling time

In the case of a AIRMET for an expected phenomenon:

  • the validity period remains unchanged.
6.8.6.3 Second line

The second line of a cancelled AIRMET consists of six elements identified in the following table:

Element 1
Location indicator of the FIR
Element 2
Name of the FIR
Element 3
Cancellation
Element 4
AIRMET message being cancelled
Element 5
Validity period of AIRMET message being cancelled
Element 6
Remark
<CCCC> <name> FIR CNCL AIRMET [n]nn <YYGGgg/YYGGgg> RMK
6.8.6.3.1 Location indicator and name of the FIR (element 1 and 2)

Refer to 6.8.3.1.

6.8.6.3.2 Cancellation (element 3)

Indicated by the abbreviation “CNCL”.

6.8.6.3.3 AIRMET message being cancelled (element 4)

The abbreviation “AIRMET” followed by the alphanumeric sequence of the message being cancelled.

6.8.6.3.4 Validity period of AIRMET message being cancelled (element 5)

A reference to the valid period for the AIRMET being cancelled.

6.8.6.3.5 Remark (element 6)

Refer to 6.8.3.8.

Example (1): For an ongoing phenomenon:

WACN05 CWAO 161220
CZUL AIRMET M3 VALID 161220/161620 CWUL-
CZUL MONTREAL FIR
<<meteorological part>>
Cancelled as:
WACN05 CWAO 161430
CZUL AIRMET M4 VALID 161430/161620 CWUL-
CZUL MONTREAL FIR CNCL AIRMET M3 161220/161620=

Example (2): For an ongoing phenomenon:

WACN05 CWAO 161220
CZUL AIRMET M3 VALID 161220/161620 CWUL-
CZUL MONTREAL FIR
<<meteorological part>>
Cancelled late as the validity period had expired:
WACN05 CWAO 161630
CZUL AIRMET M4 VALID 161630/161630 CWUL-
CZUL MONTREAL FIR CNCL AIRMET M3 161220/161620=

Example (3): For an expected phenomenon (expected time of occurrence 1500Z):

WACN05 CWAO 161220
CZUL AIRMET M1 VALID 161500/161900 CWUL-
CZUL MONTREAL FIR
<<meteorological part>>
Cancelled before the expected time of commencement of the phenomenon as:
WACN05 CWAO 161430
CZUL AIRMET M2 VALID 161500/161900 CWUL-
CZUL MONTREAL FIR CNCL AIRMET M1 161500/161900=

6.8.7 Test AIRMET

There may be occasions when test AIRMET messages are transmitted by the meteorological watch office (MWO). The test AIRMET messages will be identifiable by the letter “T” in the alphanumeric sequence (refer to 6.8.2.1). Additionally, the statement “THIS IS A TEST” will be added at the beginning and end of the message.

Example (1):

International (ICAO)

WACN03 CWAO 162225
CZWG AIRMET T1 VALID 162225/170225 CWEG-
CZWG WINNIPEG FIR THIS IS A TEST ISOLD TS OBS WTN 20NM OF LINE N4929 W09449 - N5104 W09348 - N5209 W09120 TOP FL340 MOV E 15KT NC THIS IS A TEST=

National

WACN23 CWAO 162225
CZWG AIRMET T1 VALID 162225/170225 CWEG-
CZWG WINNIPEG FIR THIS IS A TEST ISOLD TS OBS WTN 20NM OF LINE /N4929 W09449/25 SW CYQK - /N5104 W09348/CYRL - /N5209 W09120/60 NW CYPL TOP FL340 MOV E 15KT NC THIS IS A TEST
RMK GFACN33=

Example (2):

International (ICAO)

WACN03 CWAO 162300
CZWG AIRMET T2 VALID 162300/170225 CWEG-
CZWG WINNIPEG FIR THIS IS A TEST CNCL AIRMET T1 162225/170225 THIS IS A TEST=

National

WACN23 CWAO 162300
CZWG AIRMET T2 VALID 162300/170225 CWEG-
CZWG WINNIPEG FIR THIS IS A TEST CNCL AIRMET T1 162225/170225 THIS IS A TEST
RMK GFACN33=

6.8.8 Relationship to GFA

An AIRMET is issued to advise pilots of the occurrence or expected occurrence of specified weather phenomena which may affect the safety of aircraft operations, which were not already included in the Graphic Area Forecast (GFA). Once issued, an AIRMET message automatically amends the current GFA. If the phenomenon is included in the GFA, then there is no requirement to issues an AIRMET.

6.8.9 Relationship to SIGMET

A SIGMET is always issued for its specified weather phenomena. On the other hand, an AIRMET is issued for its specified weather phenomena if not included in the Graphic Area Forecast (GFA).

In a situation where a phenomenon described in a SIGMET decreases in intensity to now meet the AIRMET criteria (e.g. SEV TURB becomes MDT TURB), the SIGMET shall then be cancelled. An AIRMET shall be issued, if and only if the phenomenon was not forecast in the current GFA. If the phenomenon was forecast in the currently valid GFA, the issuance of an AIRMET is not required.

Conversely, a phenomenon that had resulted in the issuance of an AIRMET may increase in intensity to the SIGMET criteria. In this situation, the AIRMET shall be cancelled while a SIGMET shall be issued.

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