SPECI trigger criteria: the ten lettered conditions (a) to (j)
A SPECI fires between hourly METARs when one of ten lettered conditions (TC AIM MET 8.3.1) is met, measured against the preceding routine or special observation.
Ceiling (a) and visibility (c): SPECI when value falls below, or climbs back up to, a step.
| Ceiling | Visibility |
|---|---|
| 1,500 ft | 3 SM |
| 1,000 ft | 1 1/2 SM |
| 500 ft | 1 SM |
| 400 ft* | 3/4 SM* |
| 300 ft | 1/2 SM |
| 200 ft* | 1/4 SM* |
| 100 ft* | |
| lowest published minimum | lowest published minimum |
- * steps: only at aerodromes with precision approaches, down to and including published minima there.
- Lowest published minimum step: any aerodrome with one, not only precision-approach.
Sky condition (b): separate from ceiling. SPECI when a layer aloft appears below 1,000 ft (or below highest minimum for an IFR straight-in landing/takeoff) and none was reported below that height in the previous report.
Categorical criteria:
| Letter | Condition | SPECI when |
|---|---|---|
| (d) | Tornado, waterspout, funnel cloud | Observed; disappears from sight; reported by public from a reliable source |
| (e) | Thunderstorm | Begins; increases to heavy; ends (end-SPECI waits 15 min with no activity) |
| (f) | Precipitation | Any of 11 named types, or ice crystals, begins, ends, or changes intensity; up to 15 min leeway after a type ends. Steady/shower switch at same intensity (e.g. -RA to -SHRA): no SPECI |
| (g) | Freezing fog | Its beginning or end only; not fog or other obstructions in general |
| (h) | Wind | 2-min mean speed suddenly doubles previous value AND exceeds 30 kt (both, not either); or direction change meets separately defined wind shift criteria |
| (i) | Temperature | Only at a published list of roughly two dozen major Canadian aerodromes: rise of 5°C or more from previous value of 20°C or higher, or fall to 2°C or lower |
| (j) | Volcanic eruption | Any eruption; no ash-cloud or distance threshold stated |
The list is a minimum: MET 8.3.2.1 lets the observer add a special observation on own initiative for any condition judged important to flight safety or efficiency.
Trap: ceiling is any crossing of the fixed ladder; sky condition is a NEW layer below 1,000 ft not reported last time.
References
- TC AIM MET 8.3.1(a) through (j)
- TC AIM MET 8.3.2.1