Mean wind, gusts and what models miss
1The lesson in plain words
In these files, wind is a mean at 10 m over a grid box. Gusts can be stronger, and local effects may be missed. Pair them with official forecasts.
Mean wind
The usual GRIB wind is the 10 m wind: the mean wind 10 m above the surface, averaged over a grid box. The mean wind and gusts are explained in the Local winds unit. Gusts are short bursts above the mean, and the GRIB arrow does not show them.
Gusts
Many models also provide a gust parameter: ask for it. Without it, a rough rule over open sea is to add about 20–40 % to the mean for gusts. More in unstable, showery air, near cold fronts and in the lee of land. Example: a mean of 18 knots suggests gusts of about 22–25 knots, perhaps 30 or more in a squall.
What coarse models miss
Coarse global models smooth out small features. They often miss sea breezes, coastal acceleration round headlands and through straits, wind shadows behind islands, and the exact strength and timing of fronts. Their coarse grids cannot reliably resolve individual thunderstorms or squalls. Regional models help near the coast but still miss much of it.
No warning status
These files contain model output, not forecaster-edited marine bulletins or official warnings. Read official forecasts and warnings first, then use model output for additional detail, keeping its limits in mind.
On passage
Compare observed true mean wind with the forecast for the same place and UTC. If conditions differ, reassess your margins and obtain updated official forecasts and model output. Do not assume the discrepancy will disappear or persist unchanged.
2Step by step
Using GRIB files in a passage decision
- 1
Read the official forecast and warnings for the whole route first.
They carry official warnings and a forecaster's judgement; these files do not.
- 2
Download the latest run from two models, with mean wind and gusts.
Agreement between models raises confidence.
- 3
Use forecast gusts where available. Apply the rough 20–40% allowance to mean wind only when gust output is absent; assess local effects separately.
Neither modelled gusts nor a fixed allowance guarantees the strongest wind you may meet.
- 4
Check spread and exceedance probabilities at relevant places and times.
Spread shows disagreement; exceedance probabilities estimate threshold risk.
- 5
Compare forecast conditions with your go/no-go limits, leaving a margin.
Limits agreed beforehand help you avoid an optimistic decision.
- 6
At sea, compare the forecast with real wind, sky and barometer.
A mismatch calls for reassessment, not an assumption about later conditions.
3Key data
- GRIB wind
- 10 m mean wind over a grid box
- Gust rule of thumb
- Mean + about 20–40 % over open sea; more in showers
- Worked gusts
- 18 kn mean → about 22–25 kn gusts
- Often missed
- Sea breezes, coastal acceleration, wind shadows, squalls
4Key points
- 1
GRIB wind is a 10 m grid-box mean; ask for the gust parameter.
- 2
Without it, add about 20–40 % for gusts over open sea, more in showers.
- 3
Coarse models miss sea breezes, coastal acceleration, wind shadows and squalls.
- 4
These files carry no official warnings: read official forecasts first.
5Frequent mistakes
Wrong: The GRIB shows 15 knots, so the boat will see 15 knots.
Right: That is a mean over a grid box. Gusts can exceed it, and local effects may strengthen the wind.
Wrong: No gale on the GRIB means no gale warning.
Right: Warnings come only from official forecasts. Check them every time.
Wrong: Add 30 % and the gusts are covered.
Right: The rule is rough. Squalls and coastal effects can be far stronger.
6Practice case & exam keys
Your GRIB shows a north-westerly of 16 knots mean along a coast with a high cape and a 3 NM strait between an island and the mainland. The official forecast says force 5 to 6, locally 7 near capes. Your crew prefers the GRIB. What wind do you plan for?
Show the answer
Plan for the official force 5–6, with force 7 (28–33 knots mean) near capes and higher gusts. The 16-knot model mean may smooth out local effects. Check gust output and updated warnings, then compare those conditions with your boat and crew limits before committing.
Exam keys
- GRIB wind: 10 m mean over a grid box; use the gust parameter.
- Rule of thumb: add about 20–40 % for gusts; more in unstable air.
- Models miss sea breezes, coastal acceleration, shadows, squalls; fronts' timing errors.
- No warning status: always pair with official forecasts.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
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