SIGMET & AIRMET area, coordinate points & movement
SIGMET/AIRMET area = coordinate points, two formats:
| Format | Content | Example |
|---|---|---|
| ICAO | Lat/long only | N4643 W07345 |
| National | Lat/long plus tag from aviation reference site: /lat long/distance octant site |
/N4643 W07345/75 N CYUL (75 NM north of Montreal) |
National format drops the tag by location:
| Location | Distance from nearest reference site | Tag |
|---|---|---|
| Inside Gander Oceanic FIR | - | Dropped |
| North of N72°00' | More than 90 NM | Dropped |
| North of N72°00' | Inside 90 NM | Still used |
Identifier only after slash: point is at that site, e.g. /N6228 W11427/CYZF = Yellowknife.
Area always starts with WI (within). Distances in nautical miles. Shape direction: one of 8 compass octants (N, NE, E, SE, S, SW, W, NW).
| Shape | ICAO | National | Rule |
|---|---|---|---|
| Circle | WI <radius> NM OF <lat long> |
WI <radius> NM OF /lat long/dist octant site |
National distance/octant/site tag locates the centre, not the edge |
| Line | WI <width> NM WID LINE BTN <pt> - <pt> - ... |
same, each point tagged | Corridor of stated width through each point in order given |
| Polygon | WI <pt> - <pt> - ... - <pt> (no radius/width) |
same, each vertex tagged | Must be closed: last coordinate repeats first (national: same tag) |
Movement (coded separately from shape):
- Direction: one of 16 compass radials, speed in knots, e.g.
MOV NE 20KT. STNR: no significant movement expected.
Fixed-format special cases:
| SIGMET | Area | Other fixed parts |
|---|---|---|
| Radioactive cloud | Always a circle, radius up to 15 NM from source | Vertical: SFC to upper limit of FIR. Movement always STNR |
| Tropical cyclone | WI <radius> NM OF CENTRE (radius about current centre, not coordinates) |
Ends with forecast centre at end of validity: FCST <time>Z TC CENTRE <lat long> |
- Volcanic ash SIGMET, like tropical cyclone: location given at end of forecast period (TC AIM MET 6.7 NOTE).
Trap: assuming every national point has a tag (Gander Oceanic FIR and far-north points over 90 NM from a site have none); reading MOV radials (16) as shape octants (8), or vice versa.
References
- TC AIM MET 5.3 (AIRMET coordinate points)
- TC AIM MET 5.7-5.7.3 (AIRMET location: circle, line, polygon)
- TC AIM MET 5.9 (AIRMET movement)
- TC AIM MET 5.16 (AIRMET examples, site identifier point)
- TC AIM MET 6.2 NOTE 6 (radioactive cloud SIGMET)
- TC AIM MET 6.3 (SIGMET coordinate points)
- TC AIM MET 6.7-6.7.3 (SIGMET location: circle, line, polygon)
- TC AIM MET 6.9 (SIGMET movement)
- TC AIM MET 6.16 (SIGMET examples, tropical cyclone)