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Reading the weather on board
You do not need meteorology. You need three observations that together make a picture, and the habit of making them before the decision arrives.
The barometer
It is not the reading that counts but the change. A fall of more than about one hectopascal an hour over several hours announces rising wind. So read it three times a day and put the value in the log, otherwise you have a number instead of a trend.
The cloud
High cirrus thickening into a milky veil is the classic warm front warning: the wind will back or veer, rain follows, visibility drops. Towering cumulus in the afternoon means something else: short, hard gusts from shifting directions.
Estimating Beaufort
From force four white horses are widespread, from force six foam blows in streaks along the wind and spray begins to fly. That estimate matters more than the reading at the masthead, because it still works when the instrument fails.
The forecast
Listen to the shipping forecast for your sea area before letting go, not afterwards. And hold the forecast against your own observation: where the two disagree, that is the most interesting information of the day.
Weather for sailing: planning the evening before
Good weather planning for sailors starts the night before, not on the pontoon. Look at the overall pattern: where are the highs and lows, which way are they moving, and is a front due to cross your area? A surface pressure chart shows this at a glance. The closer the isobars, the lines of equal pressure, the more wind you can expect.
Next comes the timing. When does the wind fill in, when does it veer or back, when does it ease? Arrange the passage so the hardest leg falls in the quietest spell, and choose a bolt hole in case the forecast is wrong.
Do not forget local effects. On warm days the sea breeze often freshens noticeably in the afternoon. Between islands, round headlands and where the wind funnels between hills, it can blow much harder than in open water. Those differences rarely appear in a broad forecast. Pilot books and local sailors know them.
The weather forecast for sailors: sources and how to listen
The marine forecast is written for people at sea. It gives wind direction and Beaufort force, sea state, visibility and warnings, area by area. Learn the names of the sea areas you sail in and you will find the right line quickly.
Besides apps and websites there are sources that work without a phone signal. Coastguard and coast radio stations broadcast forecasts and warnings on VHF, and NAVTEX delivers safety messages and weather as text, internationally on 518 kHz. Offshore it is often the only source that reliably gets through.
Write the forecast down rather than just listening. Wind, direction, trend and warnings belong in the log. Writing makes contradictions stand out, and the crew can check later what was actually forecast.
Using the barometer when sailing: trend, tendency and warnings
The barometer measures air pressure in hectopascals, which are the same as millibars. What matters is not a single reading but how fast and which way it is moving. That is why many forecasts give the pressure tendency over three hours. A slow, steady fall suggests an approaching low, a rapid fall a deep one and strong wind soon.
If pressure rises quickly after a low has passed, the weather is not over yet. Behind a cold front the wind often shifts, with hard gusts and showers. The old sailor's saying runs: quick rise after low, sure sign of stronger blow. It is a rule of thumb rather than a law, but it is right often enough.
An electronic barograph records the trace, and many plotters and weather stations show a curve. A traditional aneroid does the job too, provided you read it at set times. Give the glass a light tap before reading so the needle does not stick.
Wind on the Beaufort scale: forces, knots and what you see
The Beaufort scale divides wind into forces from 0 to 12, each with a range in knots and a picture on the water. Force four is roughly 11 to 16 knots, force six roughly 22 to 27 knots, force eight roughly 34 to 40 knots. In British shipping forecasts a gale warning is issued for force eight or more.
Forecasts usually give the mean wind. Gusts come on top of that, sometimes well on top in showers and thunderstorms. So plan for the gusts, not the average. A boat that sails nicely in force five has to stay safe in gusts of force seven.
Remember that the sea state does not depend on wind alone. Wind against tide builds short, steep seas, and shallow water makes them more confused. The same breeze can mean very different conditions from one place to the next.
Watching the weather underway: cloud, wind and weather apps
Most weather apps show model data, often as GRIB files. They are excellent for the big picture but smooth out local gusts, thunderstorms and funnelling. Use two different models: where they agree, the situation is usually predictable. Where they differ widely, be cautious.
Underway your own observation is the best check. By Buys Ballot's law, in the northern hemisphere, if you stand with your back to the wind the low pressure lies on your left. That tells you roughly where the low is and which way the wind is likely to shift.
Keep an eye out for towering clouds with anvil tops. They bring sudden strong wind from changing directions, often before the rain arrives. Reef early and secure anything loose on deck as soon as one is heading your way.
- The evening before, check the pressure chart and marine forecast.
- Before letting go, listen to the latest forecast and log it.
- Note the pressure and compare it every few hours.
- Watch wind and cloud and set them against the forecast.
- If things differ, decide early: reef, change course or head for shelter.
Frequently asked questions
Which weather sources matter most for sailors?
The marine forecast for your sea area, coastguard warnings and NAVTEX. Apps add detail but do not replace official forecasts and warnings.
How fast does the barometer have to fall before I should worry?
As a rule of thumb, a fall of about one hectopascal an hour over several hours is a clear sign of rising wind. Follow the trend over hours rather than reacting to one reading.
What does force six mean for a cruising yacht?
Force six is roughly 22 to 27 knots. For most cruising boats that means sailing reefed, with white horses everywhere and a building sea.
Can I rely on weather apps?
For the broad picture, often yes. Local gusts, thunderstorms and funnelling are only roughly captured. Compare several sources with what you see around you.
How do I recognise a thunderstorm at sea?
By tall, towering cloud with a dark base and often an anvil shaped top. A strong gust front usually arrives before the rain. Reef early and keep clear if you can.
What is a gale warning?
In British shipping forecasts it is issued when winds of force eight or more, or gusts of 43 knots or more, are expected in a sea area. It is a signal to be in port or well prepared.
What this has to do with insurance
After weather damage the question is what you knew and when. A log with pressure, wind and the forecast you heard answers that on its own and shows the decision was made on the picture available at the time. It is the cheapest evidence there is on board.
