Weather windows and go/no-go decisions
1The lesson in plain words
Set your limits before you read the forecast. Go only if the window is long enough with a margin, and plan around wind against tide.
A weather window
A weather window is a period long enough, with conditions within the limits of your boat and crew, to complete the passage with a margin. For an 8-hour passage, look for several more hours of acceptable weather, and check what follows the window: you may need shelter at the other end.
Set your limits first
Decide your go/no-go criteria before you read the forecast, so that hope does not decide for you. For example: no more than force 5 for this crew, no strong headwind on the long leg, visibility above 2 M, and no gale warning for the area. Gusts are often a third to a half above the mean wind, and more in squalls (sudden bursts of strong wind). Use forecast gust values where available; rough mean-to-gust ratios are not upper limits.
Wind against tide
Wind blowing against a tidal stream raises short, steep seas, worst off headlands, over shallow banks and in tidal races (areas of fast, turbulent stream). A force 4 can become dangerous for a small boat there. Avoid wind against tide at headlands and races. At bars (shallow banks across entrances), also check swell, breaking seas, depth and local advice: slack water or wind with tide does not guarantee a safe crossing. The Tidal streams unit covers timing.
Sources and updates
Use official forecasts: the national met service's coastal and inshore forecasts, coastguard VHF broadcasts and NAVTEX (official maritime safety text broadcasts). Weather apps built from one computer model can miss local effects such as sea breezes and wind speeding up round headlands, so compare more than one source. Know when the next forecast is issued and get it at sea. The Meteorology module explains forecasts.
Watch the trend
Before and during the passage, compare the wind, sky, swell and pressure trend with the forecast and your limits. Falling pressure can be forecast; it alone does not prove an error. If conditions worsen sooner than expected, get an update and reassess your remaining margin. Divert early if your limits or safe arrival are threatened, while a planned refuge is still safe to enter.
2Key data
- Window length
- Passage time plus a margin, and a look at what follows
- Gusts
- Often a third to a half above the mean wind
- Wind against tide
- Short steep seas: worst off headlands, banks and races
- Official sources
- Met service forecasts, coastguard VHF, NAVTEX
3Key points
- 1
Set go/no-go limits before you read the forecast.
- 2
A weather window must cover the whole passage plus a margin.
- 3
Plan for gusts above the forecast mean wind.
- 4
Avoid wind against tide at headlands, bars and races.
- 5
Compare conditions with the forecast and your limits; divert early if limits or safe arrival are threatened.
4Frequent mistakes
Wrong: Read the forecast first, then decide what is acceptable.
Right: Set your limits first. Otherwise a keen crew talks itself into conditions it would never have chosen.
Wrong: Force 4 is always comfortable sailing.
Right: Against a strong tidal stream off a headland, force 4 can raise short, steep and dangerous seas.
Wrong: One weather app is enough.
Right: Use official forecasts and compare sources. Single-model apps can miss local winds and timing.
5Practice case & exam keys
Your 30 M passage takes 6 hours at 5 kn over the ground. Your crew’s wind limit is force 4. SW force 4 lasts until 1800, with force 6 from 2100. The headland is 15 M away, with a NE-going stream from 1000 to 1600. Your destination and refuge remain sheltered. You can leave at 0800.
Show the answer
Leaving at 0800 puts you at the headland about 1100 and in harbour about 1400, four hours before the force-4 period ends. SW wind blows towards NE, with the stream. Go only if the latest forecast, gusts, sea state, visibility, boat and crew meet your limits and arrival shelter remains usable. Set an abort check before the headland and divert early to a refuge still safe to enter if timing or conditions threaten your margin.
Exam keys
- Weather window: long enough for the whole passage plus a margin.
- Go/no-go criteria are set before reading the forecast.
- Wind against tide makes short, steep seas, worst off headlands and bars.
- Use official forecasts, compare sources, get updates and watch the trend.
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.
My notes for this unit
No notes