Prerequisites
- Competent Crew level awareness of wind, reefing and the effect of weather on a yacht (see the Weather Awareness lesson): Beaufort force, veering and backing, true and apparent wind, and the barometer log.
- Understanding of compass directions and of wind direction (always named for where it blows from).
- Familiarity with VHF use (SRC level recommended) for receiving Maritime Safety Information.
- Day Skipper Practical candidates must have meteorology to Day Skipper Shorebased level, at least 5 days and 100 miles at sea, and be at least 16.
Learning Objectives
This lesson covers the RYA Day Skipper Shorebased meteorology syllabus: weather systems and the Beaufort scale, the sources and interpretation of forecasts, the use of the barometer, and the weather elements that affect coastal passage planning. By the end of it you will be able to:
- Interpret a synoptic chart: isobars, highs, lows, ridges, troughs, fronts and pressure gradient, and estimate wind from isobar spacing (RYA syllabus: sources of information and synoptic charts).
- Apply Buys Ballot's Law and describe veering and backing (RYA syllabus: wind and pressure systems).
- Describe the weather sequence as a warm front, cold front and occlusion pass, including the cloud, pressure, wind and visibility changes (RYA syllabus: depressions and fronts).
- Describe the main air masses affecting the British Isles and the weather each brings.
- Use barometer readings and trends, and the Shipping Forecast terms for pressure change, to anticipate change (RYA syllabus: barometer).
- Recognise the main cloud types and what they foretell (RYA syllabus: cloud types and weather signs).
- Use the Beaufort scale and relate wind strength to sail plan and reefing (RYA syllabus: Beaufort scale and sea state).
- Explain sea breezes, land breezes, katabatic winds, headland effects, wind against tide and fog (RYA syllabus: local effects and visibility).
- Obtain forecasts from the Shipping Forecast, Inshore Waters Forecast, Coastguard MSI broadcasts, NAVTEX, internet services and GRIB files, and understand the terms used (RYA syllabus: forecast sources and terminology).
- Make a sound go/no-go decision when forecasts and observations disagree, with a plan B (RYA syllabus: weather in passage planning).
Why the Skipper Must Understand Weather
Most leisure-craft emergencies have weather as a cause or contributor: more wind than the crew can handle, wind against tide in a headland race, fog in a shipping lane, or a front arriving earlier than expected. A Day Skipper is expected not just to read a forecast, but to understand why the weather is changing, to watch for signs that the forecast is wrong, and to plan passages with escape routes. Forecasts describe what is likely; your barometer, the sky and the sea tell you what is actually happening.
At Competent Crew level you learned to observe and report. At Day Skipper level you must interpret and decide. The skipper's legal duty under SOLAS Chapter V (Regulation 34, which applies to all vessels including yachts) is to plan the passage taking weather information into account. This is why every Day Skipper exam and every practical assessment includes a weather question: "what is the forecast, and what does it mean for this passage?"
Pressure Systems and Isobars
The atmosphere's weight produces air pressure, measured in millibars (mb) or hectopascals (hPa), the same unit. Average sea-level pressure is about 1013 mb; UK values usually range from about 960 to 1040 mb.
Highs and lows
- Low (depression): air converges at the surface and rises; rising air cools and forms cloud and rain. In the Northern Hemisphere the wind circulates anticlockwise around a low. Depressions bring the strong winds, fronts and unsettled weather typical of the UK, and usually travel from west to east at 20 to 30 knots.
- High (anticyclone): air sinks and warms, suppressing cloud. Wind circulates clockwise, usually lighter. Highs bring settled weather, but in winter they can bring fog and in summer strong sea breezes. They move slowly and can persist for days.
- Ridge: an extension of high pressure between lows, giving a short spell of fair weather. A ridge between two depressions usually means a brief lull before the next system arrives.
- Trough: an extension of low pressure, often bringing showers or a band of rain, with a wind shift similar to a cold front.
- Col: the area of slack pressure between two highs and two lows. Winds are light and variable, and thunderstorms or fog may develop.
Isobars and pressure gradient
Isobars join points of equal pressure, usually drawn every 4 mb on UK charts.
- Close isobars mean a steep pressure gradient and strong winds; widely spaced isobars mean light winds.
- Above the friction layer the wind blows almost parallel to the isobars. At the sea surface, friction slows it and it crosses the isobars at roughly 10 to 20 degrees, inwards towards the low (more over land).
- A geostrophic wind scale printed on the synoptic chart converts isobar spacing to wind speed for a given latitude.
Estimating the wind from isobar spacing
The geostrophic wind is the wind that would blow parallel to straight isobars, if friction did not exist. It depends on the isobar spacing and the latitude. The table gives rough values at 50 degrees North (the latitude of the English Channel) for 4 mb isobars:
| Isobar spacing | Geostrophic wind | Typical surface wind at sea (about two-thirds to three-quarters) |
|---|---|---|
| 60 nm | 50 knots | 35 to 38 knots (Force 8) |
| 90 nm | 34 knots | 23 to 25 knots (Force 6) |
| 120 nm | 25 knots | 17 to 19 knots (Force 5) |
| 180 nm | 17 knots | 11 to 13 knots (Force 4) |
| 240 nm | 13 knots | 9 to 10 knots (Force 3) |
Two corrections matter. The geostrophic wind is stronger at lower latitudes for the same spacing (about 20 per cent stronger at 40 degrees North than at 50, and about 12 per cent weaker at 60 degrees), which is why the geostrophic scale on the chart has a latitude axis. And the surface wind is reduced by friction: about two-thirds to three-quarters of the geostrophic speed over open sea, about half over land. Curved isobars around a low reduce the wind slightly (cyclonic curvature) and around a high increase it a little (anticyclonic curvature).
Worked example. On a chart at 50 degrees North you measure 4 mb isobars 90 nautical miles apart across the Channel. The geostrophic wind is about 34 knots. At sea the surface wind will be about 70 per cent of this: 34 x 0.7 = about 24 knots, Force 6, with gusts to 30 knots or more. The direction will be about 15 degrees backed from the isobars. A Day Skipper should read this as a Force 6 day, unsuitable for a crew new to the boat.
Buys Ballot's Law
In the Northern Hemisphere, if you stand with your back to the wind, the low pressure is on your left (and slightly ahead). Facing the wind, the low is on your right. This tells you where the depression lies and how it will move relative to you.
For example: you are in the English Channel and the wind is south-westerly. With your back to the south-west, the low is on your left, to the north-west or north. That is consistent with a depression passing north of you, with its warm front already ahead and its cold front to follow. In the Southern Hemisphere the law is reversed.
Veering and backing
- Veering: the wind shifts clockwise (e.g. south-west to west to north-west).
- Backing: the wind shifts anticlockwise (e.g. west to south-west to south).
In the UK, a backing wind with falling pressure usually means a depression approaching; a veering wind usually means a front has passed. A wind that backs and the barometer that falls together are one of the most reliable warnings you have.
The track of a depression and where you are
Where the depression's centre passes relative to you decides what the wind does:
- If the low passes to the north of you (the usual case in the UK), the wind backs ahead of the warm front, veers at the warm front, and veers again, sharply, at the cold front.
- If the low passes to the south of you, the wind backs steadily through north, to north-west and west, and does not veer. The weather is cold, dull and unsettled, and the winds are often strong.
- If the low passes right over you, the wind falls light and variable in the centre and then reverses direction.
If the wind veers while the barometer falls, you are probably on the right-hand side of a depression's track in the Northern Hemisphere; if it backs while the barometer falls, you are likely on the left-hand side. (In tropical storms the same rule is used to find the "dangerous semicircle", a topic taught at higher levels.)
Air Masses
Weather depends on the air mass involved as well as on the pressure pattern. An air mass is a large body of air with fairly uniform temperature and humidity, taking its character from where it came from. The British Isles lie where several meet.
| Air mass | Source and route | Weather in the UK |
|---|---|---|
| Polar maritime (Pm) | From the north-west, over a cold sea | Cold, showery, unstable; cumulus and cumulonimbus; very good visibility between showers |
| Returning polar maritime (rPm) | Polar maritime that has been south and returned north | Warmer, more stable; showers fewer, stratocumulus |
| Tropical maritime (Tm) | From the south-west, over a warm sea | Mild, humid, cloudy; stratus, drizzle, hill fog; sea fog over cold water |
| Arctic maritime (Am) | From the north | Very cold, snow showers in winter, excellent visibility |
| Polar continental (Pc) | From the east or north-east in winter | Very cold and dry in winter; cloudy and cold over the North Sea |
| Tropical continental (Tc) | From the south in summer | Hot, hazy, and sometimes thundery |
The cloud and visibility on a given day are strongly influenced by the air mass: unstable (cold air over a warm surface) gives heaped cloud, showers and gusty winds but excellent visibility; stable (warm air over a cold surface) gives layer cloud, drizzle, fog and poor visibility but steadier winds.
Fronts and the Life of a Depression
A front is the boundary between two air masses of different temperature. A typical Atlantic depression forms on the polar front, where cold polar air meets warm tropical air. It starts as a small wave, deepens, develops a warm front ahead and a cold front behind, enclosing a warm sector, and then the cold front catches the warm front and the depression occludes and fills. The whole life cycle takes about three to five days, and depressions often travel as a "family" of several, with ridges of higher pressure and a brief spell of fair weather between each.
Warm front
Warm air rises gently over the colder air ahead of it, over a wide area. The slope of the front is gentle (about 1 in 150), so the cloud arrives well in advance. As it approaches:
- cirrus appears 12 to 24 hours ahead (often 300 to 600 miles ahead of the surface front), thickening into cirrostratus (sometimes with a halo round the sun or moon);
- cloud thickens and lowers to altostratus, where the sun looks as if seen through frosted glass;
- nimbostratus brings continuous moderate rain, lowering cloud base and decreasing visibility;
- the wind backs and increases; the barometer falls steadily.
As the front passes, the wind veers (often south to south-west), the rain eases to drizzle, temperature rises, and the pressure fall slows or stops. The warm sector has low stratus, drizzle, mist or fog, and moderate visibility.
The frontal cloud timeline
Cold front
Cold air undercuts the warm sector steeply (about 1 in 50), so changes are quicker and more violent:
- a band of heavy rain, often with cumulonimbus, squalls and possibly thunder;
- a sharp veer of the wind (south-west to west or north-west) and a sudden increase in strength and gustiness;
- the barometer, steady in the warm sector, rises sharply after the front;
- behind it: colder air, excellent visibility, cumulus and showers.
A cold front typically moves faster than a warm front (25 to 35 knots against 15 to 20), so it catches up. The band of rain is narrow (30 to 60 miles), so the worst of it may be over within an hour or two. There is sometimes a squall line ahead of the cold front: a line of cumulonimbus with a violent wind shift. Reef before it arrives.
Occluded front
The cold front moves faster than the warm front and eventually catches it, lifting the warm sector off the surface. The occlusion brings weather combining both fronts, with layered cloud and a band of rain, and marks a mature, often slowing depression. The diagram shows the plan view of an occluded depression and cross-sections through both types of occlusion.
Image: Pierre cb, public domain, via [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Front_occlus_trowal_en.png)
In a cold occlusion (left cross-section) the air behind the cold front is colder than the air ahead of the warm front, so it undercuts both and the warm air is lifted. In a warm occlusion (right cross-section) the air behind is milder than the air ahead, so it rides up over the cold air in front. At sea the difference is academic: expect a long period of rain, falling visibility, a wind that veers once as the occlusion passes, and then clearer, showery weather.
Summary of a passing depression
| Ahead of warm front | Warm sector | Cold front and after | |
|---|---|---|---|
| Pressure | Falling steadily | Steady or falling slowly | Rising sharply |
| Wind | Backing, increasing | Veered, steady | Veers sharply, gusty, then may ease |
| Cloud | Ci, Cs, As, Ns | St, Sc | Cb at front; then Cu |
| Weather | Rain becoming continuous | Drizzle, mist | Heavy rain, squalls; then showers |
| Visibility | Good, becoming poor | Moderate to poor, fog | Poor in rain; then very good |
| Temperature | Cool | Warmer, mild | Drops sharply |
The Barometer
The barometer is the yacht's most important weather instrument. Its trend matters far more than its absolute value.
The Shipping Forecast uses these terms for the change over the past 3 hours:
| Term | Change in 3 hours |
|---|---|
| Steady | Less than 0.1 mb |
| Rising or falling slowly | 0.1 to 1.5 mb |
| Rising or falling | 1.6 to 3.5 mb |
| Rising or falling quickly | 3.6 to 6.0 mb |
| Rising or falling very rapidly | More than 6.0 mb |
Rules of thumb:
- A fall of more than 1 mb per hour (3 mb in 3 hours) signals an active depression and strong winds: reef and reconsider plans.
- A steady fall over 6 hours or more warns of deterioration, even if the forecast is fair.
- A rapid rise after a low often brings strong, gusty winds behind the cold front: "first rise after low foretells a stronger blow".
- Long foretold, long last; short notice, soon past.
- A very low reading (below about 980 mb) with a rapid fall suggests a deep, vigorous depression close to you.
- Small, regular changes of about 1 mb twice a day (the diurnal variation) are normal and are not a trend.
Aneroid and digital barometers, and the barograph
Most yachts carry an aneroid barometer, which should be tapped lightly before reading. A barograph draws a continuous trace on a paper chart, from which the trend is obvious at a glance. Many modern chart plotters and instruments log pressure digitally. Whichever you have, check it against a reliable reading (the harbour master's, or the pressure in the Shipping Forecast coastal reports) when you are in harbour, and adjust it if it is out. An instrument that reads 5 mb out is still useful for trends, but the absolute pressure would mislead you if you compared it to a chart.
Logging the barometer
Record the pressure in the log at least every 3 hours (and every hour if it is moving), along with wind direction and strength, cloud, and visibility. A plotted trace makes trends obvious that single readings hide.
Clouds
Clouds show what the air is doing. Learn to recognise the main types and what they mean. The diagram shows where each type sits in the sky.
Image: Christopher M. Klaus (Argonne National Laboratory), public domain, via [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Cloud_types.jpg)
Clouds are grouped as high (cirrus, cirrocumulus, cirrostratus: base above about 6,000 m, ice crystals), medium (altostratus, altocumulus: 2,000 to 6,000 m) and low (stratus, stratocumulus, and the bases of nimbostratus, cumulus and cumulonimbus: below 2,000 m). The prefixes tell you the form: cirro means high, alto means medium, strato means layered, cumulo means heaped and nimbo means rain-bearing.
Cirrus is high, wispy ice-crystal cloud. Isolated cirrus is harmless; increasing, thickening cirrus spreading from the west is the first warning of a warm front 12 to 24 hours away. Altostratus is a grey mid-level sheet with the sun dimly visible; rain likely within a few hours.
Nimbostratus is thick, dark, featureless rain cloud; continuous rain and poor visibility at the front itself. Stratocumulus is a low, lumpy grey layer; dull, sometimes drizzle, common in settled or post-frontal air; rarely dangerous.
Cumulus with flat bases and limited growth signals a good sailing day. Towers that keep building show instability and developing showers. Cumulonimbus is a towering cloud with an anvil top. Expect squalls of 30 knots or more from a sudden direction, heavy rain or hail, and lightning. Reef before it arrives; watch for a dark, lowering base and a line of white water on the surface. The squall usually arrives before the rain.
Thunderstorms and squalls
A cumulonimbus has a strong downdraught that hits the sea and spreads out as a gust front, which can arrive several miles ahead of the cloud with a sudden wind of 30 to 50 knots from a direction quite different from the gradient wind. The rain follows. A line of cumulonimbus along a cold front produces a squall line. Thunderstorms in the UK are most common in summer in hot, humid, unstable air (tropical continental or tropical maritime air, perhaps with a col or a trough), and on cold fronts. Very rarely, waterspouts form beneath cumulus congestus or cumulonimbus. Keep away.
Response: reef early, secure the boat, wear lifejackets and clip on, and prepare to alter course so as to avoid the worst of the cell if the skipper judges it possible, bearing in mind that the cloud may be moving at 20 knots or more. If the cloud is moving across your bearing it will miss you; if its bearing is steady you are on a collision course, much as with a ship.
The Beaufort Scale
Forecasts give wind in Beaufort forces. A skipper must be able to estimate force from the sea state and relate it to the boat's sail plan.
| Force | Description | Knots | Sea state | Typical sail plan (RYA course notes) |
|---|---|---|---|---|
| 0 to 1 | Calm, light air | 0 to 3 | Mirror-like, ripples | Drifting conditions |
| 2 | Light breeze | 4 to 6 | Small wavelets | Full mainsail and large genoa |
| 3 | Gentle breeze | 7 to 10 | Large wavelets, occasional crests | Full sail |
| 4 | Moderate | 11 to 16 | Small waves, frequent white horses | Reduce headsail size |
| 5 | Fresh | 17 to 21 | Moderate waves, many white horses, some spray | Reef the mainsail |
| 6 | Strong | 22 to 27 | Large waves, white foam crests everywhere: "yachtsman's gale" | Reef the main and reduce the headsail |
| 7 | Near gale | 28 to 33 | Sea heaps up, spray, breaking waves, foam blown in streaks | Deep-reefed main, small jib |
| 8 | Gale | 34 to 40 | Moderately high waves, breaking crests, spindrift | Deep-reefed main, storm jib |
| 9 | Severe gale | 41 to 47 | High waves, dense foam streaks, spray affects visibility | Trysail and storm jib |
| 10 | Storm | 48 to 55 | Very high waves with long overhanging crests; sea white with foam | Survival conditions |
Remember that forecasts give mean wind speed: gusts can be 30 to 50 per cent higher, especially in showers and near land. For most Day Skipper crews in a 30 to 40 foot yacht, Force 5 is the comfortable limit for a passage, Force 6 demands an experienced crew, and a gale warning means stay in harbour.
Sea state and wave height
Forecast sea states are smooth (0.2 to 0.5 m), slight (0.5 to 1.25 m), moderate (1.25 to 2.5 m), rough (2.5 to 4 m), very rough (4 to 6 m), high (6 to 9 m) and very high (9 to 14 m). The same wind can give very different seas. A 20-knot offshore wind gives almost flat water in the lee of the land but a rough sea a few miles out; the same wind against the tide gives a steep, breaking sea.
Reefing thresholds
The exact figures depend on the boat (check the owner's manual) but the principle is universal: reef before you need to. If you are wondering whether to reef, do it now; reefing is easier, safer and quicker in lighter wind. Reef before going out of a sheltered harbour, before dark, before a forecast increase, and before a squall reaches you. Shake out reefs gradually.
As an approximate guide for a modern 35-foot cruiser, full sail is comfortable up to about 15 knots apparent wind upwind, the first reef is needed at around 18 to 20 knots, the second at about 24 to 26, and a storm jib and heavily reefed main (or trysail) at 35 knots. Remember that the apparent wind upwind is stronger than the true wind by roughly the boat speed, so a true Force 4 can already be 20 knots apparent when you are beating.
Local Effects
The forecast covers a sea area of hundreds of miles. Close to the coast, the land changes the wind.
Sea breeze
On a sunny day with light gradient wind, the land heats faster than the sea, warm air rises over the land and cooler sea air flows in to replace it. A sea breeze typically begins late morning, peaks mid-afternoon at Force 3 to 4 (sometimes Force 5), extends 10 to 15 miles out to sea, veers through the afternoon (in the Northern Hemisphere), and dies around sunset. Signs: cumulus forming over land in the morning, a calm patch offshore as the gradient wind and sea breeze cancel.
The sea breeze adds to the gradient wind when the gradient wind is onshore (it can then reach Force 5 or 6) and opposes it when the gradient wind is offshore; if the offshore gradient wind is more than about 10 to 12 knots the sea breeze will not develop at all. On a coast with an offshore wind, the sea breeze may fill in only a mile or two from the shore.
Land breeze
At night the land cools faster than the sea, and a light offshore breeze (Force 1 to 3) flows out from the shore. It is usually weaker than the sea breeze, strongest around dawn.
Katabatic wind
At night, slopes cool and the dense cold air drains downhill and out of valleys and estuaries. In UK waters katabatic winds are usually modest (Force 2 to 4) but they can make an anchorage below a steep valley uncomfortable; in mountainous areas such as Norway and parts of the Mediterranean they can reach gale force. The reverse process by day produces an anabatic (upslope) wind.
Headlands and funnelling
Wind accelerates around headlands and is funnelled between islands and along valleys and straits, often by a full Beaufort force more than the open sea. Combine that with a tidal race and wind against tide, and a headland can be dangerous when the open sea is merely lively. Give headlands a wide berth, or time your passage for slack water or wind with tide.
Other effects of the coast on wind: friction over land slows the wind and turns it, so within several miles of a coast both the strength and the direction differ from the open-sea wind, and the difference is greatest where the wind blows off high or rough land; high land deflects and funnels the flow, and can cause violent gusts down steep slopes; and a lee shore (a shore towards which the wind blows) is the most dangerous coastline for a sailing vessel, because you cannot sail away from it if the engine fails or the wind increases.
Wind against tide
When the wind blows against the tidal stream, waves become steeper and shorter. A Force 5 against a 2 to 3 knot spring stream can produce seas more typical of Force 7. Plan passages to have wind and tide together in exposed areas. The effect is worst over shoals and off headlands where the stream is accelerated: a tidal race that is gentle at slack water, or with wind and tide together, can become impassable when the stream turns against a fresh wind.
Fog
Fog forms when air is cooled to its dew point, the temperature at which it is saturated. The two common types at sea differ in cause and duration.
- Radiation fog forms over land on clear, calm nights under high pressure, as the ground cools. It can drift into harbours and estuaries in the early morning, but usually burns off by mid-morning or midday and rarely extends far out to sea.
- Advection (sea) fog forms when warm, moist air flows over colder sea, commonly in spring and early summer around the south-west of the UK. It can cover large areas, persist for days, and is not cleared by sunshine; only a change of air mass or a stronger wind clears it (typically about Force 6 or more).
- Frontal fog and hill fog occur in the warm sector of a depression when moist tropical maritime air is forced up hills, and as low cloud and drizzle at the warm front.
- Steam fog (sea smoke) occurs in very cold air over a warmer sea.
Indicators that fog may form: the air temperature and the sea temperature converge, or the air temperature and the dew point (from a sling psychrometer or a good instrument) are within 1 or 2 degrees of each other and the gap is closing; a warm humid southerly airstream moving over cold sea; clear, calm night over land near the coast.
The Shipping Forecast describes visibility as: good (more than 5 miles), moderate (2 to 5 miles), poor (1000 m to 2 miles), and fog (less than 1000 m). In fog you must proceed at a safe speed, sound the correct fog signals (Rule 35), and keep a proper lookout, with radar if you have it. Consider whether to anchor out of the shipping lane or to divert to port.
Forecast Sources
| Source | How to get it | Notes |
|---|---|---|
| Shipping Forecast | BBC Radio 4 (long wave and FM) four times daily; Met Office website | Covers sea areas; wind, sea state, weather, visibility for the next 24 hours, plus a general synopsis |
| Inshore Waters Forecast | Radio 4 after the late Shipping Forecast; Met Office website; Coastguard VHF | Within 12 miles of the coast; more relevant for day sailing |
| Coastguard MSI broadcasts | VHF, announced on Channel 16, at fixed times through the day | Inshore forecast, gale warnings, strong wind warnings, navigation warnings |
| NAVTEX | Dedicated receiver on 518 kHz | Text forecasts, gale warnings and navigation warnings |
| Internet and apps | Met Office, other national services, model-based apps | Graphical forecasts, rain radar, satellite images, synoptic charts |
| GRIB files | Downloaded model data | Wind, pressure, waves in fine detail; model output, not observation |
| Harbour offices, marinas | Posted daily | Usually printouts of the above; the RYA notes list most harbour and marina offices as a routine source |
| Local radio stations | Marine forecasts at published times | Often after the news; times in the almanac |
| Text message (SMS) and recorded telephone forecasts | Subscription or premium-rate services | Details in the almanac or the RYA Weather Forecasts booklet (G5) |
| Telephone and fax services | Marinecall and similar | Text and graphical forecasts, may be charged |
| Weather stations and buoys | Coastal observations in the Shipping Forecast, online | Real-time wind and pressure to check against the forecast |
Understanding forecast terms
- Timing: imminent = within 6 hours of the time of issue; soon = 6 to 12 hours; later = more than 12 hours.
- Gale warnings: issued when mean winds of Force 8 or more, or gusts of 43 to 51 knots, are expected. Severe gale: Force 9 or gusts of 52 to 60 knots. Storm: Force 10 or gusts of 61 to 68 knots.
- Strong wind warnings: issued for inshore waters when the average wind is expected to be Force 6 or 7. Force 6 is often called the "yachtsman's gale".
- Visibility: good, more than 5 miles; moderate, 2 to 5 miles; poor, 1,000 m to 2 miles; fog, less than 1,000 m.
- Fair: no significant precipitation.
- Backing is the wind changing anticlockwise (north-west to south-west); veering is clockwise (north-east to south-east).
- General synopsis: how and where the weather systems are moving.
- Sea breeze: in fair weather with a light to moderate offshore gradient wind, warm air rises over the land, cools, descends and blows onshore, generally up to Force 4 by mid-afternoon. Land breeze: on a clear night the air cools over the land and flows downhill and out to sea, particularly from river estuaries, usually no more than Force 2 to 3 except near mountains.
- In the Southern Hemisphere the circulations are reversed: winds blow clockwise round a low and anticlockwise round a high, and Buys Ballot's law puts the low on your right with your back to the wind.
- Wind "becoming cyclonic" means a depression passing close, with large and variable changes in direction.
- The Shipping Forecast order: gale warnings, general synopsis, then for each area wind, sea state, weather, visibility.
- The general synopsis gives the position, central pressure, and movement of the main pressure systems, with the time they are expected at those positions, for example: "Low Rockall 987 moving east 20 knots, expected Fastnet 992 by 1800 tomorrow".
Synoptic charts and satellite and radar images
A synoptic chart is a map of the pressure pattern, with the fronts marked, for a given time. Fronts are marked by lines with symbols: a warm front has red semicircles (and a continuous red line), a cold front has blue triangles, an occluded front has alternating purple triangles and semicircles. The chart also gives the pressure at the centre of highs and lows, the movement (a direction and speed in knots), and usually wind arrows with feathers (each full feather is 10 knots, half a feather 5, a pennant 50). Forecast charts (T+24, T+48 and so on) show how the pattern is expected to evolve.
Satellite images show cloud: infra-red images show the temperature of the cloud tops (white is high and cold, so a bright comma or frontal band is a deep system) and visible images show cloud in daylight. Rain radar shows where rain is falling, and which way it is moving, and is useful for judging whether a shower line will reach you in the next hour.
Triangulating sources
No single forecast is perfect. Compare at least two independent sources; when they disagree, plan for the worse. Then compare forecasts with what you actually observe. If the GRIB shows 15 knots and you have 25, the model is wrong, and probably wrong for the next few hours too. Models smooth out local effects such as headlands, sea breezes and showers.
GRIB files
GRIB files are compact files of numerical weather-model output: wind, pressure, wave height and so on, on a grid. They are read by programs such as OpenCPN, qtVlm and Windy and are often loaded over a satellite link. Know their limits: the grid spacing (often 0.25 to 0.5 degrees, about 15 to 30 miles) is too coarse to show local effects; most show the 10-metre mean wind and not gusts; they start from the time the model ran, which may be 6 to 12 hours old; and different models (for example GFS and ECMWF) can disagree, which in itself is a sign of uncertainty.
Decision Making Under Uncertainty
Before every passage, ask:
- What is the forecast wind strength and direction for the whole passage, and what are the gusts?
- Is the boat and crew fit for the worst forecast conditions?
- Is there wind against tide at any headland or in any entrance?
- What is the visibility, and is fog possible?
- Where are the ports of refuge if it deteriorates, and are they safe to enter in the forecast wind?
- What is the trend: is the next system arriving early or late?
Underway, keep checking the barometer, sky and wind. If the forecast says fair but the barometer is falling steadily and cirrus is thickening, trust your instruments: reef early, brief the crew, and consider an earlier port of refuge. Shelter over schedule: arriving late or not at all is always better than an accident.
A go/no-go framework
| Factor | Go | Think hard | No-go |
|---|---|---|---|
| Forecast mean wind (typical 35 ft yacht, mixed crew) | Force 4 or less | Force 5 | Force 6 or more, or any gale warning |
| Wind direction against your harbour or shelter | Offshore or along shore | Onshore, strengthening | Onshore with a dangerous entrance |
| Barometer | Steady or rising slowly | Falling slowly | Falling quickly (more than 3 mb in 3 hours) |
| Visibility | Good | Moderate, fog possible | Fog forecast in a shipping lane |
| Crew | Experienced, rested | Mixed experience | Tired, seasick, or short-handed |
| Ports of refuge | Several, easy | One, or tidal | None within range |
A single "no-go" is enough to stay in harbour. Several "think hard" results mean you plan with a margin: leave early, reef at the pontoon, or choose a shorter or more sheltered route.
Worked Example: Reading the Signs
You leave at 0800 for a 30 mile coastal passage. The forecast for your area: south-west 4, increasing 5 to 6 later, rain later, good becoming moderate.
- 0900: barometer 1014 mb, steady. Cirrus increasing from the west. Wind SW 3 to 4.
- 1000: 1012 mb (falling 2 mb per hour). Wind backing to SSW and increasing to Force 5. Sky thickening to altostratus; the sun appears watery.
- Diagnosis: a warm front is approaching faster than "later" (more than 12 hours) suggested. Falling 2 mb per hour with a backing wind indicates the front within about 4 hours; expect lowering cloud and rain by about 1400, wind Force 6, visibility becoming poor.
- Actions: reef now to the first reef, brief the crew, put on foul-weather gear and lifejackets, fix the position and calculate the ETA. With 18 miles to go at 5.5 knots, you would arrive about 1320, before the worst, but the destination entrance faces south-west and will be rough with wind against the ebb from 1500. You decide to continue but nominate a closer, well-sheltered port 8 miles away as an alternative, to be chosen if the wind reaches Force 6 before 1200.
- Brief: "Pressure is falling 2 mb an hour and the wind is backing. That is a warm front coming in early. Expect rain and Force 6 by early afternoon. We are reefing now, and if it is Force 6 by noon we will divert."
Worked example: planning from a synoptic chart
It is Tuesday evening. You plan to sail west along the Channel on Wednesday, from Poole to Weymouth (about 30 miles), leaving at 0700. The synoptic chart for 0600 Wednesday shows a depression 990 mb centred south-west of Ireland, moving north-east at 20 knots, with a warm front just reaching Cornwall and the cold front 200 miles behind it. The isobars across the Channel are 4 mb and about 100 miles apart.
- Wind estimate. At 50 degrees North, 100 miles spacing gives a geostrophic wind of about 30 knots. At the surface at sea, about 70 per cent gives roughly 21 knots (Force 5), from the south-west and backed from the isobars. Gusts will be Force 6.
- Timing. The warm front is about 200 miles from Dorset at 20 knots, so arrives in roughly 10 hours, say 1600. Cirrus is likely already overhead at dawn. The cold front follows several hours later, probably during the night, and brings the veer and squally showers.
- Passage. Poole to Weymouth heading west to south-west means beating into the Force 5 wind and likely wind against the east-going tide at times. Weymouth is protected from the south-west by Portland, but Portland Race lies ahead.
- Decision. The forecast is within a Force 5, "think hard" zone with deterioration expected in the afternoon. A skipper with a mixed crew might decide to stay in Poole, or to go east instead with the wind on the beam or behind, or to leave early, reefed, with a firm deadline for arrival before 1400 and a plan B in Lulworth or Swanage Bay depending on the wind.
Common Mistakes
- Relying on a single forecast. Compare at least two.
- Ignoring the barometer because the forecast was fair.
- Forgetting gusts. A Force 5 mean can gust Force 7 in showers.
- Treating GRIB output as fact. It is a model, and it smooths local effects.
- Ignoring wind against tide at headlands and entrances.
- Reefing too late.
- Not checking forecast timing words: "later" can still be this afternoon.
- No plan B: no port of refuge identified before leaving.
- Wrong wind direction convention: a south-westerly blows from the south-west.
- Forgetting that Buys Ballot is hemisphere-dependent: the rule is reversed in the Southern Hemisphere.
- Taking the geostrophic wind as the surface wind. Friction reduces it over the sea by about a quarter to a third, and more over land.
- Mistaking a ridge for lasting good weather. The lull between two depressions in a family is short.
Summary
- Lows bring cloud, rain and wind circulating anticlockwise; highs bring settled weather circulating clockwise. Close isobars mean strong wind: at 50 degrees North, 4 mb isobars 100 nm apart give about 30 knots geostrophic.
- With your back to the wind in the Northern Hemisphere, the low is on your left.
- Air mass matters: polar maritime is showery and clear, tropical maritime is cloudy and foggy.
- Warm front: cirrus, altostratus, nimbostratus, steadily falling pressure, backing wind; then veer and drizzle. Cold front: squalls, heavy rain, sharp veer, rapid pressure rise, then clear skies and showers. Occlusion: layered cloud, one veer, mature depression.
- Watch the barometer trend: more than 1 mb per hour falling means strong wind is coming.
- Know the Beaufort scale and reef early.
- Local effects (sea breezes, katabatics, headland acceleration, wind against tide) can make conditions much worse than the area forecast.
- Radiation fog clears by day; advection fog persists.
- Use several forecast sources, compare them with observations, and always have a plan B.
Check Your Understanding
- What do closely spaced isobars on a synoptic chart indicate?
Answer: A steep pressure gradient and therefore strong winds.
- In the Northern Hemisphere, you stand with your back to the wind. Where is the low pressure?
Answer: On your left (and slightly ahead), which is Buys Ballot's Law.
- Describe the cloud sequence ahead of a warm front.
Answer: Cirrus, thickening to cirrostratus, then altostratus, then nimbostratus with continuous rain, the cloud lowering and thickening as the front approaches.
- What changes in wind and pressure mark the passage of a cold front?
Answer: A sharp veer and an increase in wind with squalls, followed by a rapid rise in pressure, then colder, clearer air with showers.
- The barometer has fallen 4 mb in the last 3 hours. What term would the Shipping Forecast use, and what should you do?
Answer: Falling quickly (3.6 to 6.0 mb in 3 hours). Expect strong winds soon: reef, secure the boat, brief the crew and review ports of refuge.
- What is a sea breeze, and when is it strongest?
Answer: A wind blowing from sea to land on sunny days as the land heats and air rises over it. It usually peaks mid-afternoon and dies around sunset.
- Why is a forecast Force 5 more dangerous off a headland at spring tides?
Answer: The wind accelerates around the headland, and with wind against a strong tidal stream the seas become steep and short, possibly forming a tidal race.
- How does advection fog differ from radiation fog?
Answer: Advection fog forms when warm moist air moves over a cold sea; it can be widespread and persist for days regardless of sunshine. Radiation fog forms over land on clear calm nights and usually clears by midday.
- What does "imminent" mean in a gale warning?
Answer: Within 6 hours of the time of issue.
- The GRIB forecast for your area was 12 knots but you are experiencing 22 knots. What should you conclude?
Answer: The model is underestimating the wind. Trust your observations, expect it to remain stronger than forecast, reduce sail and reassess the passage plan.
- On a chart at 50 degrees North the 4 mb isobars are 120 nautical miles apart. Estimate the surface wind at sea.
Answer: Geostrophic wind about 25 knots; surface wind about two-thirds to three-quarters of this, so roughly 17 to 19 knots, Force 5, with gusts of Force 6.
- The wind has been veering while the barometer rises sharply, and the sky shows towering cumulus with showers. What has happened?
Answer: A cold front has passed. The wind veers and is gusty, the pressure rises quickly, and polar maritime air brings cumulus, showers and very good visibility between them. Squalls and strong gusts are likely for several hours.
- What is the difference between a warm occlusion and a cold occlusion?
Answer: In a cold occlusion the air behind the cold front is colder than the air ahead of the warm front, so it undercuts and lifts the warm sector. In a warm occlusion the air behind is milder than the air ahead, so it rides up over the colder air ahead. At sea both give prolonged layered cloud and rain, a single veer, and then showery weather.