GFA clouds and weather chart: symbols
The clouds and weather chart is the first of two GFA charts. Forecasts: cloud layers, surface-based phenomena, visibility, weather, obstructions to vision, plus driving synoptic features.
Clouds:
- Bases and tops shown from surface to 24,000 ft ASL.
- Convective tops (TCU, ACC, CB) shown even above 24,000 ft.
- Cirrus never depicted.
- Type named only if significant. Convective types (CU, TCU, ACC, CB) always stated when present.
Sky condition and border:
| Sky condition | Border |
|---|---|
BKN or OVC (organized cloud area) |
Scalloped border (brown) |
SKC, FEW or SCT (no organized area, vis above 6 SM) |
No border |
Layer separation:
| Separation | Stated as |
|---|---|
| Less than 2,000 ft | One summation amount, LYRS added right after |
| 2,000 ft or more | Each layer separately, own bases and tops |
Heights:
- Hundreds of feet ASL unless noted (
45= 4,500 ft). - AGL heights carry
CIGandAGL, noted in the comments box. - Vertical visibility into surface-based (obscured) layer: hundreds of feet AGL.
LCA CIGS 3-5 AGL= local obscured ceilings, vertical visibility 300-500 ft AGL.
Weather:
- Listed immediately after visibility.
- Obstructions to vision (
BR,HZ) named only when visibility forecast 6 SM or less.
Area/line coding:
| Phenomenon | Area | Line |
|---|---|---|
| Showery / intermittent precipitation | Hatched | Dashed green |
| Continuous precipitation | Stippled | Solid green |
| Obstruction to vision, no precipitation, vis <= 6 SM | - | Dashed orange |
| Freezing precipitation | Red | Solid red |
Coverage terms by cloud family:
| Family | Table | Term | Meaning | Coverage | Spacing |
|---|---|---|---|---|---|
| Non-convective | 4.5 | LCA |
local | 25% or less | - |
| Non-convective | 4.5 | PTCHY |
patchy | 26-50% | - |
| Non-convective | 4.5 | XTNSV/INTMT |
extensive/intermittent | greater than 50% | - |
| Convective | 4.4 | ISOL |
isolated | 25% or less | Individual features |
| Convective | 4.4 | OCNL |
occasional | 26-50% | Well-separated features |
| Convective | 4.4 | FRQ |
frequent | greater than 50% | Little or no separation between adjacent clouds |
- Convective spacing in words: TC AIM MET 5.2, MET 6.2 Note 5.
Synoptic features:
- Motion 5 kt or more: arrow with speed value.
- Under 5 kt:
STNR(stationary). - Isobars (equal mean sea level pressure): every 4 hPa from 1,000 hPa reference.
Surface wind:
- Barbs with speed value routinely at 20 kt sustained or more.
- Lower sustained speed shown at forecaster's discretion if gusts expected to cause moderate mechanical turbulence.
- Gusts: letter
Gplus peak gust speed in knots.
Trap: LCA and ISOL both mean 25% or less, but LCA is non-convective (clouds, precipitation) and ISOL is convective (CBs, showers); mixing tables gives right percentage, wrong cloud family.
References
- TC AIM MET 4.11 (Clouds and Weather Chart)
- TC AIM MET 4.11 Table 4.4 (convective clouds and showers)
- TC AIM MET 4.11 Table 4.5 (non-convective clouds and precipitation)
- TC AIM MET 5.2
- TC AIM MET 6.2