Understanding Weather at Sea - Beaufort Scale & Forecasting

Prerequisites

The RYA Competent Crew course has no formal entry requirements: you need no previous sailing experience, and the certificate is normally issued from age 16. This lesson does assume you have worked through the earlier lessons in this course, in particular:

• Points of sail and basic helming: you should know what "upwind", "reaching" and "downwind" mean and what the no-go zone is, because wind changes are described in those terms.
• Sail handling: you should know what reefing is. Most weather decisions at this level end in "reef early" or "stay in harbour".
• Personal safety: you should know how to put on and adjust a lifejacket and how to clip on a harness, because these are the first things you do when the weather deteriorates.
• Communication: you should know that VHF radio is used to receive Maritime Safety Information (MSI) from HM Coastguard.
• Navigation basics: you should be able to read compass directions (N, NE, E and so on) and know that a nautical mile is about 1,852 metres and a knot is one nautical mile per hour.

At Competent Crew level you are not expected to plan a passage around the weather. That is the skipper's job, and it is developed on the Day Skipper course (see the Weather Forecasting lesson). You are expected to understand the forecast the skipper reads out, to notice changes in wind, sea and sky on watch, to record them in the log, and to report them early.

Learning Objectives

This lesson covers the RYA Competent Crew syllabus topic of weather: the sources of forecasts, basic weather signs (barometer, cloud sequence, wind shifts, visibility) and the duty to observe, compare with the forecast and alert the skipper. By the end of it you should be able to:

• Name the main sources of marine weather information in the UK (Shipping Forecast, Inshore Waters Forecast, Coastguard MSI on VHF, NAVTEX, Met Office and app-based services) and say what each is for (RYA syllabus: forecast sources).
• Define the forecast terms veering, backing, gust, squall, cyclonic and the visibility terms good, moderate, poor and fog, and the timing words imminent, soon and later (RYA syllabus: understanding forecast language).
• Estimate wind strength on the Beaufort scale from what the sea looks like, and relate it to sea-state terms (RYA syllabus: Beaufort scale).
• Tell the difference between true wind and apparent wind and know which one your sails and instruments respond to.
• Read a barometer trend, use the three-hour rule and explain what a falling, rising or steady barometer suggests (RYA syllabus: basic weather signs, barometer).
• Identify cirrus, cumulus, cumulonimbus, stratus and nimbostratus cloud and describe the cloud and wind sequence of a passing front (RYA syllabus: cloud sequence ahead of a warm front).
• Recognise the main local effects: sea breeze, land breeze, acceleration zones, wind shadows and sea fog.
• Keep a weather log and make a clear, useful report to the skipper (RYA syllabus: observe conditions, compare with forecast, alert the skipper).

Why Weather Awareness Matters to Crew

A yacht is driven by the wind and moves on a surface shaped by it. Almost every decision on board depends on the weather: when to leave, how much sail to carry, whether to stay in an anchorage, whether the crew should clip on. The skipper makes those decisions, but the skipper cannot watch the sky, the sea and the barometer for 24 hours a day. Crew on watch are the skipper's eyes.

Weather rarely changes without warning. The barometer falls, high cloud thickens, the wind shifts or a squall line shows up on the horizon long before conditions become dangerous. A crew member who notices these signs and says "the barometer has dropped 4 millibars since the last log entry" or "there's a big dark cloud building to windward" gives the skipper time to act while it is still easy. Reefing in Force 4 is routine; reefing in Force 7 is hard and dangerous.

Keep this principle in mind for the whole lesson: the forecast tells you what to expect, and your own observations tell you whether it is happening. You need both.

Weather and the Accident Record

Most incidents handled by the RNLI and HM Coastguard involving leisure sailing craft have weather as a contributing factor. The common pattern is not an unforecastable storm. It is a forecast that was available, but was not read, not understood or not acted on: a Force 6 warning ignored because the morning looked pleasant, an offshore wind that made a launch ramp or anchorage a lee shore, a fog bank that was forecast, or a crew that had not dressed for the worst conditions of the day. Good crew help to break that chain.

Getting the Forecast

The Main Sources

The diagram below sets out the main families of forecast information and the instruments on board that let you check them.

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The Shipping Forecast is issued by the Met Office and broadcast on BBC Radio 4 (long wave 198 kHz and FM) four times a day, at about 0048, 0520, 1201 and 1754 UK clock time (check the current BBC schedule, because the times and the long-wave service are occasionally changed). It covers the large sea areas around the British Isles (Dover, Wight, Portland, Plymouth, Biscay, Fastnet, Shannon, Lundy, FitzRoy, Sole and so on) for the next 24 hours. It always follows the same order: gale warnings, general synopsis, then for each area wind, sea state, weather and visibility, followed by reports from coastal stations. Because it covers large offshore areas, it may not show local effects close to the coast.

The Inshore Waters Forecast covers coastal waters out to 12 nautical miles, divided into stretches of coast (for example "Lyme Regis to Lands End including the Isles of Scilly"). It gives a 24-hour forecast and an outlook for a further 24 hours. For most coastal day sailing this is the forecast that matters most. It is broadcast on Radio 4 after the late Shipping Forecast and is on the Met Office website.

HM Coastguard Maritime Safety Information (MSI) broadcasts on VHF. The Coastguard announces on Channel 16 that a broadcast is coming, then reads the Inshore Waters Forecast, gale warnings and strong wind warnings on a working channel (for example Channel 62 or 86, depending on the station). Broadcasts are made at fixed times, typically every three to four hours depending on the station; the times and channels are listed in the nautical almanac and on the Coastguard pages of the Reeds or Admiralty publications. In an area with poor phone signal, this is your best source. Urgent gale warnings are also announced on Channel 16 at the next silence period.

NAVTEX is a dedicated receiver that prints or displays text forecasts, navigational warnings and gale warnings automatically on 518 kHz. Many cruising yachts carry one. It works without a phone signal and keeps a written record. Each message carries a header identifying the station, the message type and a serial number, so you can tell if you have missed one.

Online and app services (Met Office, XCWeather, Windy, PredictWind and others) show wind, waves and pressure as maps, often hour by hour. Many of these use GRIB files, raw computer-model output. They are very useful for planning, but they are produced by a computer model without a human forecaster and they usually show average wind, not gusts. Treat them as a guide and check them against the official forecast.

Harbour offices and marinas post a daily printout of the forecast and often the local barometer reading. Ask the harbour master about local effects such as an entrance that becomes dangerous in strong onshore wind.

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Image: Christopher M. Klaus (Argonne National Laboratory), public domain, via Wikimedia Commons

The picture above is useful to keep in mind throughout this lesson: it shows where each cloud type sits in the sky. Cirrus, cirrostratus and cirrocumulus are high; altostratus and altocumulus are in the middle; stratus, stratocumulus, cumulus and the base of nimbostratus and cumulonimbus are low.

Cross-Referencing

No single source is perfect. Good practice before any passage:

1. Read the Inshore Waters Forecast and any gale or strong wind warnings in force.
2. Look at a model-based app to see how the wind is expected to change during the day.
3. Look at the sky and the sea, and note the barometer reading in the log.
4. During the passage, listen to the Coastguard MSI broadcasts and record any changes.

If the sources disagree, plan for the worse of them.

Strong Wind and Gale Warnings

The Met Office issues a graded series of warnings. You should know the first two well.

Gale warnings use timing words: imminent means within 6 hours of the time of issue, soon means 6 to 12 hours, and later means more than 12 hours. For a typical 9 to 12 metre training yacht, Force 6 is already a serious day's sailing, and a gale warning means the boat stays in harbour.

Forecast Language

The Shipping and Inshore forecasts use a fixed vocabulary, and each word has a precise meaning. Examiners and instructors expect you to understand them exactly.

Wind Direction and Change

Wind direction is always given as the direction the wind is coming from. A "south-westerly" blows from the south-west towards the north-east.

To remember it: veering goes the way the clock hands go. In the northern hemisphere the wind usually veers as a weather front passes, and it usually backs as a depression approaches from the west.

Wind Strength Terms

The forecast gives the mean wind speed. A forecast Force 5 (about 17 to 21 knots) will typically gust to 25 or 30 knots, which is Force 6 or 7, and the gusts in showers and near land can be more. Plan the sails for the gusts, not just the mean.

Weather and Sea State Terms

The weather element of the forecast uses simple words: fair, showers, rain, drizzle, thundery showers, snow, fog patches. "Showers" means short, heavy, intermittent rain from cumulus clouds, often with gusty winds. "Rain" means continuous rain from layer cloud. "Thundery showers" tell you cumulonimbus clouds are likely.

Visibility Terms

Mist and haze both reduce visibility. Mist is caused by water droplets and usually means moderate or poor visibility. Fog is the most dangerous because other vessels may not see you until they are very close. In fog the crew's jobs include lookout, listening for fog signals, and helping the skipper sound the correct fog signal (see the COLREGs lesson).

Timing Terms

The Met Office uses these words consistently: imminent (within 6 hours), soon (6 to 12 hours) and later (more than 12 hours from the time of issue). Note the time of issue: a forecast "imminent" at 0520 that you hear at 0900 may already have arrived. Other timing words you will hear are at first, for a time, becoming and perhaps.

Reading a Forecast Line

Read a forecast one element at a time, always in the order wind, sea state, weather, visibility. For example:

"Portland: Southwest 5 to 7, occasionally gale 8 later. Rough or very rough. Rain then squally showers. Moderate or poor, occasionally good."

• Wind: from the south-west, Force 5 to 7 (17 to 33 knots), with the mean wind occasionally reaching Force 8 (34 knots or more), more than 12 hours after issue.
• Sea state: rough or very rough, wave heights of 2.5 to 6 metres.
• Weather: rain first, then squally showers behind a cold front.
• Visibility: moderate or poor (1,000 metres to 5 miles), occasionally good in the clearer air after the front.

This forecast would keep almost every training yacht in harbour.

Estimating Wind Strength: The Beaufort Scale

The Beaufort scale describes wind strength by what it does to the sea. It lets you estimate wind force without instruments and check that your wind instrument is telling the truth. The diagram shows Forces 0 to 7, the range you will usually sail in on a training course.

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How to Estimate the Force

1. Look upwind at the open water, not at the sheltered water close to you.
2. Count the white horses. None means Force 3 or less. A few scattered means about Force 3 to 4. Most waves breaking means Force 5 or more.
3. Look for spray blowing off the tops of the waves (Force 5 and above) and streaks of foam lying along the wind direction (Force 7 and above).
4. Check against the wind instrument, remembering that the instrument may show apparent wind (see below).

Rule of thumb: sea state needs time and distance to build up. Close to a windward shore, or in the first hour of a new wind, the sea will look smaller than the wind strength. Wind blowing against the tide makes the sea look rougher, with steeper and shorter waves.

Practise every day of your course: guess the force, then check the anemometer. You will improve quickly.

Conversions to Remember

• A Beaufort force is roughly the mean wind speed in knots divided by 5, plus a little. Force 4 is 11 to 16 knots, Force 5 is 17 to 21, Force 6 is 22 to 27: each step adds about 5 to 6 knots.
• Wind force on the rig rises with the square of wind speed. Doubling the wind speed from 10 to 20 knots means four times the force on the sails. That is why a boat that is happy at Force 4 can be hopelessly overpowered at Force 6, and why the right time to reef feels "early".

Sea State

Sea state describes the waves, not the wind. Forecasts give it as a word, and each word covers a range of wave heights. Wave height is measured from trough to crest.

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The table shows typical open-sea values. Local conditions change them a lot:

• Wind against tide makes waves steeper and closer together. A Force 5 against a 2-knot spring tide off a headland can give a short, breaking sea that is much worse than "moderate" suggests.
• Shallow water makes waves steepen and break, especially over banks and bars.
• Swell is made by distant wind and can arrive before the wind does. A long swell on a calm day is often the first sign of a deep depression far away.
• Fetch and duration: the longer the wind has blown, and the greater the distance of open water it has blown over, the bigger the waves. An offshore wind of Force 6 in a sheltered bay may give almost flat water close to the land, but a mile or two out the sea will be rough.

A forecast of "moderate, occasionally rough" with a Force 6 means a hard, wet day for new crew. Expect seasickness and wear full foul-weather gear.

Reading the Wind on Board

Wind Direction Indicators

On board you read the wind from several clues. Learn to use them together. The diagram below shows the most common ones.

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• Flags and smoke stream away from the wind, pointing downwind. Use flags on moored boats and chimney smoke on shore when entering harbour.
• Ripples and dark patches on the water show where gusts are. A dark, ruffled patch moving towards you is a gust on its way, often called a catspaw. Seeing one 30 seconds before it arrives lets the helm prepare and the crew ease a sheet.
• The masthead wind vane (often a Windex) points into the wind. When the boat is moving, it shows apparent wind.
• Telltales (strips of wool or ribbon on the shrouds and sails) show the local airflow. Telltales on the shrouds help the helm judge the wind angle; telltales on the sails show whether the sail is trimmed correctly.
• Your face: turn your head until the wind feels the same on both ears and cheeks. You are then facing the apparent wind.
• Moored boats and anchored boats lie head to wind (or head to tide, if the tide is stronger), so a harbour full of boats pointing the same way shows the wind direction in a glance.

True Wind and Apparent Wind

True wind is the wind blowing over the earth's surface, the wind you would feel standing still on a pontoon or at anchor. Forecasts always give true wind.

When the boat moves, it makes its own wind. Motor at 6 knots on a calm day and you feel a 6-knot breeze on your face from straight ahead. This is the boat wind (or "induced wind"), equal in speed to the boat and blowing from directly ahead.

Apparent wind is the wind you actually feel on a moving boat: the true wind and the boat wind combined by vector addition. It is the wind that fills the sails, moves the telltales and the Windex, and is shown by the wind instrument unless it is set to show true wind.

Key facts:

• Upwind and on a close reach, apparent wind is stronger than true wind. Sailing upwind at 6 knots in a 15-knot true wind, you feel about 20 knots.
• Downwind, apparent wind is weaker than true wind. Running at 6 knots before an 18-knot true wind, you feel only about 12 knots.
• On every point of sail except dead downwind, apparent wind comes from further forward than the true wind. The faster the boat goes, the further forward the apparent wind moves.
• Sails are always trimmed to the apparent wind.

The table gives rough figures for a 6-knot yacht in a 15-knot true wind:

Worked example: the downwind trap. You are running downwind in a forecast Force 5 (about 18 knots). The boat does 6 knots, so you feel only about 12 knots and the sailing feels easy. The skipper decides to turn and beat back to harbour. Now the apparent wind is about 22 to 24 knots and the boat heels hard. Nothing changed in the weather; only your direction changed. This is why experienced skippers reef before turning upwind, and why you should never judge the wind strength while running before it.

Barometric Pressure

What the Barometer Tells You

A barometer measures air pressure in millibars (mb), also written hectopascals (hPa), the two being numerically the same. Normal pressure around the UK is about 1013 mb. High pressure (1020 mb and above) usually brings settled weather and lighter winds. Low pressure (depressions, below about 1000 mb) brings cloud, rain and stronger winds. UK pressure normally ranges between about 960 and 1040 mb.

The trend matters more than the reading. That is why the barometer is recorded in the log every hour on passage. The diagram summarises how to read it.

The Three-Hour Rule of Thumb

Compare the reading now with the reading three hours ago (from the log):

As crew, any fall of 3 mb or more in three hours should be reported to the skipper straight away. Tap the barometer gently before reading an analogue one, because the needle can stick. Always read it at the same height and in the same place, and note any correction the skipper has for the instrument (setting the pointer to the true pressure from a reliable source when in harbour).

Barometer and Wind Together

A falling barometer on its own is a warning; combined with other signs it becomes a forecast.

Worked example. The log shows 1012 mb at 0900, 1010 mb at 1000, 1008 mb at 1100 and 1006 mb at 1200. That is 6 mb in three hours, at the top of "falling quickly". Combined with thickening high cloud from the west, it means a depression is approaching fast. The right call is to tell the skipper, put in a reef while the wind is still moderate, and check the latest Coastguard broadcast.

Worked example: a diurnal cycle. The barometer also rises and falls by about 1 mb twice a day in a regular pattern (higher at about 1000 and 2200, lower at about 0400 and 1600 in the UK). Do not mistake this small, regular change for a trend. A fall of 1 mb between 1000 and 1600 on its own means little; a fall of 4 mb on top of it means a lot.

Clouds as Weather Signs

Clouds often warn you before the barometer or the wind does. At Competent Crew level you need to recognise five types, and to be able to put them in the order in which they arrive ahead of a depression.

Cirrus: High, Wispy Cloud

Cirrus is very high (above about 6,000 m) and made of ice crystals, so it looks like thin white streaks, often called "mares' tails". Scattered cirrus in a blue sky is fair weather. Cirrus that thickens and spreads into a milky sheet (cirrostratus, sometimes with a halo around the sun or moon) is often the first sign of an approaching warm front, perhaps 12 to 24 hours away. "Mares' tails and mackerel scales make tall ships take in their sails." Mackerel sky is cirrocumulus or altocumulus, small rounded cloudlets in rows, which often goes with a change in the weather.

Cumulus: Heaped Cloud

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Cumulus clouds are white and puffy with flat bases, like cauliflowers. Small, scattered fair-weather cumulus on a sunny afternoon is harmless and often comes with a sea breeze. When cumulus grows tall, the air is unstable and showers are likely. Under every big cumulus expect a gust and a change of wind direction as it passes.

Cumulonimbus: The Thunder Cloud

Cumulonimbus is a cumulus that has grown right up to the top of the lower atmosphere, often 10,000 m or more, and spread out at the top into a flat anvil. It brings heavy rain or hail, thunder and lightning, and squalls: sudden winds that can reach 40 knots or more for a few minutes, from a new direction. Cumulonimbus clouds are common just behind a cold front and in summer thunderstorms. The gust usually arrives just before the rain, as the cold downdraught spreads out from under the cloud; a line of white water on the sea ahead of a dark cloud base warns you it is coming.

When you see one coming, especially if it is upwind:

1. Tell the skipper immediately.
2. Reef or reduce sail before it arrives.
3. Secure loose gear on deck and close hatches.
4. Put on lifejackets and clip on.
5. In lightning, keep away from the mast, shrouds and other metal fittings, and stay low in the cockpit.

Lightning

A yacht with an aluminium mast is not especially likely to be hit, but a strike is serious: it can damage electronics, compasses and the hull, and injure people. Thunder is heard about 3 seconds per kilometre behind the flash (about 5 seconds per mile), so you can estimate how far away a storm is. If lightning is close: stay low and in the cockpit or below, avoid touching metal at both ends of the boat at once (for example, do not touch a shroud and the backstay together), and disconnect and stow hand-held electronics if the skipper directs. Do not swim or hang over the side.

Stratus and Nimbostratus: Layer Cloud

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Stratus is a low, grey, even sheet. It brings drizzle and poor visibility and may reach the sea as fog. Nimbostratus is a thicker, darker layer that gives steady, continuous rain for hours. Both are typical of a warm front.

Cloud Quick Reference

Cloud Sequence of a Passing Depression

When a typical Atlantic depression passes north of the UK, a crew member on watch might see this sequence:

1. Cirrus thickening from the west; barometer starting to fall; wind backing slightly and increasing.
2. Cloud lowers to a grey sheet (altostratus, then nimbostratus); steady rain begins; barometer falling steadily; wind increasing.
3. Warm front passes: rain eases to drizzle, wind veers (for example south to south-west), barometer fall slows or stops. Low stratus, drizzle and poor visibility in the "warm sector".
4. Cold front passes: a line of heavy showers or cumulonimbus, squalls, wind veers sharply (for example south-west to north-west), temperature drops, barometer starts to rise quickly.
5. Behind the cold front: clear, cold air with good visibility and cumulus showers, gusty winds that slowly ease.

Note that the wind veers at both fronts in the northern hemisphere. Knowing this sequence helps you understand forecast phrases such as "south-west 5 to 7, veering north-west later".

Weather Lore: What Works and What Doesn't

Sailors' sayings can be useful, but only some of them have a physical basis.

Use lore as a prompt to check the barometer and the forecast, never as a replacement.

Local Effects

Forecasts describe the general wind over a sea area. Near land, the wind is changed by temperature differences and by the shape of the land. These local effects explain why the wind in a bay can be very different from the forecast.

Sea Breeze

On a sunny day, the land warms up much faster than the sea. Warm air rises over the land, and cooler air flows in from the sea to replace it. This is the sea breeze, an onshore wind.

• It usually starts in late morning, is strongest in mid-afternoon and dies around sunset.
• It typically reaches Force 3 to 4, and occasionally Force 5 near the coast.
• It is strongest on clear, sunny days with light general wind, especially in spring and early summer when the sea is still cold.
• Signs: a line of cumulus building over the land, and a calm patch offshore in the late morning before it fills in.
• It reaches a few miles to 10 or more miles offshore, depending on how strongly the land is heating.
• In the northern hemisphere the sea breeze tends to veer (turn clockwise) through the afternoon.

Why it matters: a calm morning forecast can become a brisk afternoon onshore wind. A sea breeze blowing into a harbour entrance changes the way you return to your berth, and it can turn an anchorage into a lee shore.

Land Breeze

At night, the land cools faster than the sea. Air over the land becomes cooler and heavier and flows down and out to sea. This is the land breeze, an offshore wind.

• It is usually weaker than a sea breeze, often Force 1 to 2, but it can be stronger where it flows down valleys and fjords.
• It is strongest near dawn and can help an early departure.
• An anchorage that was sheltered all day can have the boat swinging round at night.

Acceleration Zones and Wind Shadows

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When wind meets an obstacle, it has to go round or over it:

• Acceleration zones: wind squeezed between islands, through a gap in the hills or around a headland speeds up. It can be 30 to 50 per cent stronger than the forecast. This is often called "headland effect" or "funnelling". Rounding a headland, expect a sudden increase and a change of direction, and reef before you get there.
• Wind shadows: downwind of high land, cliffs, large buildings or big ships, the wind is much lighter and gusty. Anchorages in the lee of land are sheltered for this reason. Be careful, though: as you sail out of the shadow, the full wind returns suddenly.
• Downdrafts off high land (sometimes called "williwaws" in extreme cases) can bring violent gusts down a steep shore even when the anchorage looks calm.
• Sails and close-hauled boats also make wind shadows. A large yacht or ship passing upwind of you can take away your wind for a moment.

Fog

Most fog around the UK is sea fog (advection fog). It forms when warm, moist air moves over colder sea, most often in spring and early summer. Unlike land fog, it can form or persist in a fresh wind and may last all day. Radiation fog forms over land on clear, calm nights and can drift into harbours and estuaries at dawn, but it usually burns off by late morning.

Warning signs of sea fog: warm, humid southerly air, a forecast of "fog patches", and visibility dropping in drizzle. When visibility closes in, the skipper will want: an extra lookout in the bow, navigation lights on, the radar reflector up, the correct fog signal, lifejackets on, a position fix written in the log, and the engine ready. Crew on lookout should stand in the bow away from the noise of the engine, listen for fog signals and report the bearing and any change in loudness of every sound.

Weather and Comfort: Looking After the Crew

Weather is not only about the boat. A cold, wet, tired and seasick crew makes mistakes.

• Layers: wear a base layer that wicks moisture, an insulating fleece or wool layer, and a windproof and waterproof outer layer. Add a hat and gloves. On a summer day in the Solent it can still be 10 degrees colder on the water than on the pontoon.
• Wind chill: moving air strips heat from the body. A 15-knot wind on a 10 degree day feels close to freezing on wet skin.
• Sun and dehydration: sunburn, heat exhaustion and dehydration are as common as cold in summer. Use sunscreen, a hat and sunglasses, and drink regularly, especially on long reaches in light winds.
• Seasickness: worse in rough weather and with cold and fatigue. Take tablets early, stay on deck in the fresh air and look at the horizon.
• Food and drink: hot drinks and high-energy snacks keep the crew warm and sharp in bad weather.

Weather and Your Duties as Crew

What does weather awareness look like in practice?

• Before leaving: listen when the skipper briefs the forecast. Ask about anything you do not understand. Dress for the worst conditions forecast, not the conditions on the pontoon.
• On watch: write the barometer, wind direction and force, sea state, visibility and cloud cover in the log each hour, or as the skipper directs.
• Report changes straight away: a falling barometer, a wind shift, a building cloud, a squall line, visibility dropping, or the boat feeling overpowered.
• Reef early: it is always easier to shake out a reef than to put one in when it is too late. "If you are thinking about reefing, it is time to reef."
• Prepare the boat: in deteriorating weather, secure gear below, close hatches and portholes, check the cockpit drains, and have foul-weather gear and lifejackets on before they are needed.

The Weather Log

A simple hourly weather log entry looks like this:

The log is a legal record, a memory aid and a forecasting tool. Seeing three columns move together (pressure down, wind backing and increasing, cloud thickening) is far more convincing than any single reading.

Making a Good Report

Skippers value short, factual reports. Say what you see, what has changed and since when, and what you suggest. Compare these two:

• Poor: "It's getting a bit windy and the sky doesn't look nice."
• Good: "The barometer has fallen 4 millibars in three hours to 1006. The wind has backed from south-west to south and gone from Force 4 to Force 5. There is a thickening grey sheet from the west. I think we should consider a reef."

Worked Example

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Scenario: Reading an Inshore Forecast on a Day Sail

You are crew on a 10-metre yacht in the Solent planning a day sail out to the Needles and back. At 0800 the skipper reads you the Inshore Waters Forecast for "Selsey Bill to Lyme Regis" for the next 24 hours:

"Southwest 4 or 5, increasing 6 at times later. Slight or moderate. Showers, rain later. Good, occasionally moderate later."

The barometer reads 1009 mb and is down from 1012 mb at 0500.

Step 1: wind. South-west Force 4 to 5 is 11 to 21 knots. "Increasing 6 at times later" means Force 6 (22 to 27 knots) is possible for spells more than 12 hours after the time of issue. The forecast was issued at 0500, so "later" is after about 1700.

Step 2: sea. "Slight or moderate" is waves of 0.5 to 2.5 metres. In the Needles Channel, a south-west wind against the ebb tide will make the sea much rougher than this.

Step 3: weather and visibility. Showers now, with rain later, suggesting a front approaching from the west. Visibility good (more than 5 nm), becoming moderate (2 to 5 nm) in rain.

Step 4: barometer. A 3 mb fall in three hours is "falling", which agrees with the forecast of increasing wind and rain.

Step 5: what the crew can expect. The outbound leg to the Needles is upwind into a south-west wind, so the apparent wind will feel stronger than the forecast. The skipper will probably put in a reef before leaving and plan to be back in harbour before late afternoon, ahead of the Force 6 and the rain. Crew should wear full foul-weather gear, take seasickness tablets early if prone, and keep an eye to the west for heavy showers.

Step 6: on watch. At 1200 you note the barometer at 1006 mb, high cloud thickening, and a large shower cloud with a dark base to windward. You report all three to the skipper. That is exactly what a good crew member does.

Scenario: The Afternoon Sea Breeze

A calm, hazy June morning in a south-facing bay, with a forecast of "variable 2 or 3". At noon cumulus is building over the hills behind the town and the sea is flat calm a mile offshore. By 1400, a south-easterly Force 4 has filled in from the sea. Your anchorage, open to the south-east, is now a lee shore with a short chop. The cause is a sea breeze, not a forecasting error. The crew's job is to note the change, check the anchor is holding, and be ready to help the skipper move to a more sheltered spot.

Scenario: The Squall Line

You are on the helm in the late afternoon, sailing close-hauled in a south-westerly Force 4 two days after a cold front has gone through. To the west a line of dark cumulus has built, with a ragged black base and a smudge of rain beneath it. The sea ahead of it has a ruffled, whitened look.

1. You tell the skipper straight away: "Dark cloud to windward, about two miles off, with rain under it, and a line of white water in front."
2. The skipper calls for a reef. The crew put it in and the other crew put on lifejackets and clip on.
3. Five minutes later the squall arrives: the wind goes from Force 4 to Force 7 for about ten minutes, and veers 30 degrees to the north-west. The boat is under control because she was reefed in time.
4. When it passes, the wind eases to Force 5 and the skipper keeps the reef in, because more showers are following.

The lesson: it was the reporting and the early reef that made the squall routine rather than frightening.

Common Mistakes

The diagram summarises the most frequent errors and how to correct them.

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• Relying on one forecast, once. A forecast heard at breakfast may be out of date by lunchtime. Check the latest MSI broadcast or NAVTEX before leaving and during the passage.
• Judging the wind while running downwind. Downwind, apparent wind feels much lighter than true wind. Reef before turning upwind.
• Mixing up true and apparent wind. Forecasts give true wind; sails and the Windex respond to apparent wind. Know which one your instrument is showing.
• Getting the wind direction backwards. A "northerly" comes from the north.
• Confusing veering and backing. Veering is clockwise; backing is anticlockwise.
• Ignoring the barometer. One reading tells you little; the change over three hours tells you a lot. Log it every hour.
• Dismissing the sky. Thickening cirrus and building cumulonimbus are warnings, often before any instrument changes.
• Anchoring in an acceleration zone or in an anchorage that will become a lee shore when the sea breeze arrives.
• Forgetting wind against tide. The sea state in a tidal race can be far worse than the forecast suggests.
• Dressing for the pontoon. It is always colder and wetter at sea, and wind chill is serious even in summer.
• Misreading forecast timing. "Later" means more than 12 hours from issue, and "imminent" means within 6 hours. Check when the forecast was issued.
• Treating the mean wind as the maximum. Gusts are often 30 to 50 per cent higher than the forecast mean.
• Staying silent. Not reporting something because "the skipper must have noticed" is a very common failing. Say it.

Summary

• The main UK sources are the Shipping Forecast, the Inshore Waters Forecast, Coastguard MSI on VHF, NAVTEX and online model services. Cross-check them and plan for the worse one.
• Wind direction is where the wind comes from. Veering is clockwise; backing is anticlockwise. In the northern hemisphere the wind veers as fronts pass.
• A gust lasts seconds; a squall lasts minutes and comes with shower cloud. Forecasts give the mean wind.
• Visibility: good more than 5 nm, moderate 2 to 5 nm, poor 1,000 m to 2 nm, fog less than 1,000 m. Timing: imminent within 6 hours, soon 6 to 12 hours, later more than 12 hours.
• The Beaufort scale lets you estimate wind force from the sea. White horses begin around Force 3 to 4, spray at Force 5, and foam streaks at Force 7. Wind force on the rig rises with the square of the wind speed.
• Apparent wind is what you feel and what the sails use. It is stronger upwind and weaker downwind than the true wind.
• The barometer trend matters: a fall of more than 3 mb in three hours is a warning sign to report.
• Cirrus thickening means a front is coming. Tall cumulus means showers. Cumulonimbus means squalls and thunder. Stratus and nimbostratus mean drizzle and steady rain.
• Sea breezes blow onshore on sunny afternoons; land breezes blow offshore at night. Wind accelerates round headlands and between islands and drops in the lee of land.
• Crew duty: observe, log and report changes early, and reef early.

Check Your Understanding

1. The forecast says "south-west 5, veering north-west 6 later". Describe the change in wind direction and strength, and say whether "veering" is clockwise or anticlockwise.

Answer: The wind will shift from south-west to north-west, which is a clockwise change, so it is veering. It will increase from Force 5 (17 to 21 knots) to Force 6 (22 to 27 knots), more than 12 hours after the forecast was issued.

1. What is the difference between a gust and a squall?

Answer: A gust is a short increase in wind lasting a few seconds. A squall is a sudden, strong increase lasting several minutes, usually linked to a shower cloud or cumulonimbus, often with heavy rain and a change in wind direction.

1. You look upwind and see small waves with fairly frequent white horses, but no spray blowing off the crests. Roughly what Beaufort force is it, and what wind speed is that?

Answer: About Force 4, a moderate breeze of 11 to 16 knots.

1. You are motoring at 6 knots in a flat calm. What does the masthead wind vane show and why?

Answer: It shows a 6-knot wind from dead ahead. There is no true wind, so the apparent wind is entirely boat wind, made by the boat's own movement.

1. Running downwind, the wind feels light and pleasant. The skipper plans to turn and beat home. Why might you suggest reefing first?

Answer: Downwind, the boat's speed is subtracted from the true wind, so the apparent wind feels lighter than it is. Turning upwind adds the boat's speed, so the apparent wind will increase sharply and the boat may be overpowered. It is safer to reef before turning.

1. The log shows the barometer at 1015 mb at 1200 and 1010 mb at 1500. What does this suggest and what should you do?

Answer: A 5 mb fall in three hours is falling quickly, which suggests a depression is approaching and strong winds are likely. Report it to the skipper at once, check the latest forecast, and expect to reef and prepare the boat.

1. On a sunny, calm morning you notice cumulus building over the land. What wind change might you expect in the afternoon, and from which direction?

Answer: A sea breeze: an onshore wind blowing from the sea towards the land, typically Force 3 to 4, strongest in mid-afternoon and dying around sunset.

1. A cloud ahead has a flat, spreading anvil top and a dark base with heavy rain beneath it. What is it and what actions should the crew take?

Answer: A cumulonimbus. Tell the skipper, reduce sail before it arrives, secure gear and close hatches, put on lifejackets and clip on, and keep clear of metal rigging if there is lightning. Expect a sudden squall and a wind shift.

1. Why might the wind be much stronger than forecast as you round a headland or pass between two islands?

Answer: The wind is squeezed and funnelled by the land, which speeds it up (an acceleration zone). The increase can be 30 to 50 per cent above the forecast, often with a change of direction, so reef before arriving.

1. What does "visibility poor" mean in a UK forecast?

Answer: Visibility between 1,000 metres and 2 nautical miles. Below 1,000 metres it is called fog.

1. A gale warning is broadcast at 0600 with the word "soon". Between what times might the gale arrive?

Answer: "Soon" means 6 to 12 hours after the time of issue, so between about 1200 and 1800. "Imminent" would mean within 6 hours, and "later" more than 12 hours.

1. List, in order, the cloud types and the changes in barometer and wind that you would expect as a warm front approaches from the west.

Answer: Cirrus, then cirrostratus (perhaps with a halo), then altostratus, then nimbostratus with steady rain. The barometer falls steadily, and the wind backs and increases. When the front passes, the rain eases to drizzle, the wind veers and the pressure fall slows or stops.

1. What are the five things you should say when reporting a weather change to the skipper?

Answer: What you have observed (for example the pressure reading), how much it has changed, over what time, any related changes in wind, sea or sky, and what you suggest (for example a reef). Short, factual reports let the skipper act early.