GFA icing, turbulence & freezing level chart symbols
The icing, turbulence and freezing level chart is the second GFA chart (TC AIM MET 4.12).
- Shows: icing and turbulence areas (type, intensity, bases, tops), forecast freezing level, surface synoptic features.
- Read with the clouds and weather chart, same valid time.
| Icing | Turbulence | |
|---|---|---|
| Colour (when in colour) | Blue | Red |
| Plotted intensity | Moderate or severe only | Moderate or severe only |
| Bases/tops | Hundreds of ft ASL | Hundreds of ft ASL; surface-based in ft AGL |
| Per-layer label | Type: RIME, MX (mixed), CLR (clear) | Cause abbreviation (below) |
| Severe shown as | Denser stippling, normally nested inside moderate area | Higher-density hatching, normally nested inside moderate area |
Icing:
- Light icing never plotted: goes in comments box, with time of occurrence if only part of the period.
Turbulence cause abbreviations:
| Abbreviation | Cause |
|---|---|
MECH |
Mechanical |
L LVL WS |
Low-level wind shear |
LEE WV |
Lee/mountain waves |
L LVL JET |
Significant low-level jet |
CAT |
Clear air turbulence |
WV= wave, not wind. PIREP/WVfield reports wind: different report (TC AIM MET Table 15.1).- Two turbulence layers stacked at different altitudes: lower hatches up-to-the-right, higher hatches down-to-the-right.
Low-level jet (L LVL JET): arrow points downwind, labelled speed = peak core value.
- Plotted at peak core speed 50 kt or more.
- May also be plotted at 35 to 45 kt if it brings significant turbulence or shear.
- Not normally plotted above 6,000 ft ASL (exception: over high terrain).
- Height never labelled, only speed and direction.
Freezing level contours:
- Dashed lines, nearest 2,500 ft, labelled in hundreds of ft ASL:
| Label | SFC | 25 | 50 | 75 | 100 |
|---|---|---|---|---|---|
| Height | Surface | 2,500 ft | 5,000 ft | 7,500 ft | 10,000 ft |
- Multiple freezing levels: plot only the lowest, unless a higher one matters to safety (e.g. freezing precipitation aloft).
- Layer staying above freezing: closed area labelled AFL (above-freezing layer).
- Significant time change: comments box, e.g.
FZLVL 20 LOWERING TO SFC AFT 03Z.
Trap: MECH/L LVL WS/LEE WV/L LVL JET/CAT name cause, not intensity; intensity is colour/hatching density (moderate vs. severe); nothing below moderate plotted. Plot only the lowest freezing level, unless a higher one matters to safety.
References
- TC AIM MET 4.12 (Icing, Turbulence and Freezing Level Chart)
- TC AIM MET Table 15.1 (Aviation Forecast Abbreviations)