Ocean Meteorology for Offshore Sailors - GRIB Files & Barometer

Prerequisites

Before this lesson you should be comfortable with the Day Skipper and Coastal Skipper meteorology syllabus:

• The Beaufort scale and its sea-state descriptions.
• The Shipping Forecast format, sea areas, gale warnings and terms such as "soon", "later", "imminent", "veering" and "backing".
• Sources of forecasts: Navtex, VHF Maritime Safety Information broadcasts from HM Coastguard, the Met Office website, inshore waters forecasts.
• Basic synoptic charts: highs, lows, isobars, fronts and the passage of a typical depression.
• Sea breezes, land breezes and other local effects.

You should also have worked through the Yachtmaster Offshore lesson on passage planning, which uses weather windows.

Learning Objectives

This lesson covers the meteorology strand of the RYA Yachtmaster Offshore syllabus (G158) at ocean-going depth. By the end you will be able to:

• Interpret a synoptic chart: estimate wind strength and direction from isobar spacing and orientation, and locate lows, highs, troughs, ridges and fronts (RYA syllabus: meteorology, interpretation of weather charts and forecasts).
• Describe the sequence of weather through a frontal depression, including the cloud, wind, pressure and visibility changes, and time the passage of fronts along your route (RYA syllabus: weather associated with fronts and depressions).
• Explain how upper-air features (500mb troughs and ridges, the jet stream) drive the development of surface lows, including explosive deepening.
• Compare GRIB model data with official text forecasts, and explain the differences between ECMWF and GFS (RYA syllabus: sources of forecasts and their limitations).
• Obtain forecasts offshore through Navtex, HM Coastguard MSI, SafetyNET, HF/SSB and satellite data, and state the limits of each.
• Use barometer trends and the Met Office tendency terms to anticipate change (RYA syllabus: use of the barometer).
• Explain how wave height, period and steepness relate, and why wind against current or tide is dangerous (RYA syllabus: sea state and swell).
• Recognise squalls, thunderstorms, fog and sea-breeze effects and say how to prepare for each.
• Describe how tropical revolving storms form, how to tell which semicircle you are in, and the avoidance tactics in each (RYA syllabus: tropical revolving storms).
• Make and justify a go, no-go or divert decision from a mix of forecast and observed data, as the examiner expects.

Why Offshore Meteorology Is Different

Close to land, local effects dominate: sea breezes, wind funnelling along valleys, the shelter of headlands. Offshore those effects disappear and the large-scale pressure pattern takes over. Two things follow:

1. With little friction over open water, winds are typically stronger offshore than at the coast for the same pressure gradient, and the sea has more fetch to build.
2. You cannot run for shelter in a couple of hours, so you must see weather coming a day or more ahead and route around it.

The Offshore skipper's aim is not to forecast the exact wind at a given minute, but to understand where the weather systems are going, recognise warning signs early, and keep the vessel inside its safe operating limits.

Synoptic Charts

Isobars and wind strength

Isobars join points of equal pressure at mean sea level, usually drawn every 4 hPa (millibars). The closer the isobars, the steeper the pressure gradient and the stronger the wind.

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In the Northern Hemisphere, air circulates anticlockwise around a low and clockwise around a high. In theory the wind blows along the isobars (the geostrophic wind), but at the surface friction slows it and turns it towards low pressure: by about 10 to 15° over the sea and 30° or more over land. Surface wind over the sea is typically about two-thirds to three-quarters of the geostrophic wind.

A geostrophic wind scale printed on the chart lets you read wind speed from isobar spacing. At UK latitudes, as a rough guide, isobars 4hPa apart and about 100nm apart give around 30 knots geostrophic (roughly 20 to 25 knots at the surface). Halve the spacing and you double the wind.

Isobar curvature matters too. Around a tight low, winds are slightly lighter than the spacing suggests; around a high, slightly stronger. Watch for kinks in the isobars, which often mark a trough or a front, and for small, tight closed lows (secondary depressions) on the trailing cold front of a main low: these can deepen fast and produce winds well above those in the main forecast.

Buys Ballot's Law

In the Northern Hemisphere, if you stand with your back to the true wind, low pressure is on your left, and slightly in front of you because of friction. In the Southern Hemisphere it is on your right. This lets you locate the centre of a low from your own wind, without a chart, and track it as the wind shifts.

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Frontal depressions

Most bad weather in temperate latitudes comes from frontal depressions moving broadly east or north-east across the Atlantic. The diagram shows the sequence an observer experiences as a depression passes to the north.

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An occlusion forms when the faster cold front catches the warm front and lifts the warm sector off the surface. Its weather is a blend: a band of cloud and rain, often slow-moving and prolonged.

Timing fronts

Fronts are drawn on successive analysis and forecast charts. Measure how far a front has moved between charts, divide by time, and project it along your route. Plan, where possible, to cross fronts in daylight with the crew rested and the boat reefed in advance. Cold front squalls can arrive suddenly with a 90° wind shift and a doubling of wind speed; a 40 knot gust in the dark with full sail up is a classic route to a broach or gear failure.

Upper-Air Dynamics: 500mb Charts and the Jet Stream

Surface weather is driven by what happens several kilometres up. You do not need to analyse upper-air charts in detail at Offshore level, but understanding them explains why some lows explode while others fizzle.

The 500mb chart

A 500mb chart shows the height of the 500hPa pressure surface, roughly 5,500m (18,000ft) up. The contours form a wavy pattern of troughs (dips towards the equator) and ridges (bulges towards the pole). Surface lows tend to move along this upper flow.

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• A surface low ahead of (east of) an upper trough is in an area of upper divergence and is likely to deepen.
• A surface low beneath an upper ridge tends to fill and weaken.
• The steering flow at 500mb gives a rough guide to the direction and speed of the surface system, often around half to two-thirds of the upper wind speed.

The jet stream

The polar front jet stream is a narrow band of very strong wind, often 100 to 200 knots, at around 9 to 12km. Within it are faster sections called jet streaks. Where air leaves a jet streak (the exit region) or enters it (the entrance region), upper air diverges in particular quadrants, drawing air up from the surface and lowering surface pressure.

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When a developing surface low comes under a region of strong upper divergence, it can deepen explosively. A low that deepens by 24hPa or more in 24 hours (adjusted for latitude) is called a "bomb". These can produce storm-force winds where the morning forecast showed only a modest low. A strong jet stream lying across the Atlantic towards the UK is a warning sign for an unsettled spell with rapidly developing lows.

Forecast Sources: GRIBs, Models and Text Forecasts

Numerical models

All modern forecasts start with numerical weather prediction models. The two global models a yachtsman meets most often are ECMWF (European) and GFS (American).

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ECMWF is generally regarded as more accurate in the medium range (days 3 to 7); GFS is free, widely available and updated four times daily. Regional high-resolution models (for example the Met Office UKV or ICON-EU and AROME) handle coastal and short-range detail better. When models disagree significantly, the uncertainty is real: plan for the worse outcome or wait for agreement.

GRIB files versus text forecasts

A GRIB file is raw gridded model output delivered as wind arrows, pressure and sometimes waves, rain and cloud.

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Strengths of GRIBs: easy to download at sea by satellite, they show the pattern over time and can feed routing software. Limitations:

• They show mean wind at 10m, not gusts. In squalls and behind cold fronts, gusts can be 50% or more above the mean.
• Grid spacing of 0.25° to 0.5° (15 to 30nm) smooths out anything smaller: squall lines, local acceleration around headlands, wind against tide.
• No human forecaster has checked them for known model errors.
• The timing of fronts can be several hours out.

Text forecasts (the Shipping Forecast, Navtex, Coastguard MSI broadcasts, gale warnings) are prepared by forecasters who compare models, observations and experience, and who issue explicit warnings. The professional approach is to use GRIBs for the big picture and timing trends, and the official text forecast and your own observations (barometer, sky, sea) to set the limits. Always add 25 to 30% to GRIB mean wind to estimate likely gusts, and more in unstable showery air.

The barometer

Record barometric pressure in the log every hour offshore. The tendency matters more than the value:

A rapid rise after a cold front can bring strong, gusty winds just as dangerous as the fall before it ("first rise after low, foretells a stronger blow").

Getting Weather Information Offshore

The RYA expects you to know every route by which a forecast reaches a yacht, and to know what each one is good and bad at.

An examiner will ask what you would do if your sources disagreed. The answer is to plan for the more severe forecast, give the most weight to the one that is best verified against what you are actually seeing (barometer, sea state, sky), and, offshore, to leave margin rather than rely on one source.

The UK Met Office forecast products you should recognise

• Shipping Forecast: general synopsis, then area forecasts giving wind direction and force, weather and visibility, in the fixed order. Terms are strictly defined: imminent means within 6 hours, soon means 6 to 12 hours, later means more than 12 hours; good visibility is more than 5 nautical miles, moderate 2 to 5nm, poor 1000m to 2nm and fog less than 1000m.
• Gale warning: issued for mean wind of Force 8 or more, or gusts of 43 to 51 knots. Severe gale is Force 9, storm Force 10 or above, and a strong wind warning Force 6 or 7.
• Surface analysis and forecast charts (T+24, T+48, T+72, T+96): show isobars, fronts and the centres of highs and lows with their pressures. Compare the T+0 chart with the latest observed barometer to check that the model is on track.
• Ocean wave charts: show significant wave height and swell direction and period. Remember that significant wave height is the average of the highest third of waves, so individual waves will be higher: the highest wave you meet in a few hundred is about twice the significant height.

Squalls, Thunderstorms, Fog and Offshore Hazards

Squalls and thunderstorms

A squall is a sudden increase of wind of at least 16 knots, rising to 22 knots or more, lasting at least a minute. Squalls occur along cold fronts, under cumulonimbus, in showery post-frontal air, and in the tropics under convective cloud. A squall line along a cold front can bring a 90 degree wind shift and a doubling of wind speed in under a minute, then a chaotic sea.

Drills for a squall:

1. Reef or drop sail early, when the cloud base darkens or you see a dark shelf or rain curtain approaching. Reef before you need to.
2. Turn the boat so that the sails can be depowered easily: on a reach or run it is easier to bear away and ease sheets than to try to round up.
3. Clip on, put on foul-weather gear and lifejackets, and secure loose gear below.
4. Note the time and the wind shift in the log. A radar will usually show the squall as a patch of heavy return, giving a rough bearing and speed of movement.
5. After the squall, expect a confused sea for a time, and a shifting wind that may settle in a new direction.

In thunderstorms, lightning is an additional danger. Disconnect and stow portable electronics in the oven or a metal box (a rough Faraday cage), keep crew away from the mast and metal rigging, and do not touch two metal items that could form a circuit. A bonded mast and a lightning conductor give the best protection but do not guarantee it.

Fog

Fog is a navigational and collision hazard rather than a wind hazard, but the meteorology matters because the type of fog tells you when it will go.

• Advection fog: warm moist air flows over a cold sea (the typical UK spring and early summer fog, and the Grand Banks fog where the Gulf Stream meets the Labrador Current). It can persist for days and needs a change of air mass, usually wind change, to clear it.
• Radiation fog: forms over land on clear calm nights and may drift over coastal waters at dawn. It clears with the morning sun.
• Frontal fog: associated with warm fronts and warm sectors, where drizzle saturates the air.
• Sea fog (steam fog or Arctic sea smoke): cold air over warm water in autumn and winter.

In fog, follow the COLREGs: sound signals, safe speed, radar and AIS watch, and a lookout posted forward. Rule 19 applies when vessels are not in sight of one another in or near restricted visibility.

Local effects that survive offshore

Offshore you are mostly free of sea breezes but not of orographic effects. Wind accelerates round headlands and between islands (the funnelling or Venturi effect), producing a wind up to 30 to 50% stronger than the open-sea wind. Wind also accelerates down the lee of high mountainous islands and, in the tropics, the trade wind may be forced through channels between islands such as the Windward Islands. Allow for this when your route passes through the gap between islands.

Reading the Chart Like a Skipper: The Weather Routine

On passage, a disciplined routine is more useful than occasional close study:

1. Before departure: look at the synoptic chart and the trend over the last three days, at least three days of forecast charts, GRIBs from two models, the official text forecast and the long-range outlook. Decide your weather window with margin (see the passage planning lessons).
2. Every watch change: log position, barometer (and trend), wind direction and force, sea state, swell, cloud and visibility. These are your own observations and form the baseline.
3. Twice a day: download or receive fresh forecasts and compare them to what you actually saw. Where the model is running late or early, adjust.
4. When the barometer falls faster than forecast: treat it as a warning, whatever the forecast says. Take the earlier action: reef, secure, divert.
5. When the wind veers sharply: a cold front has passed or you are in a trough. Prepare for gusts, heavy showers and a rapid sea state change.

A useful rule is the 3-hour rule: if in 3 hours the barometer has fallen by more than 6 hPa or the wind has increased by two Beaufort forces, your plan is no longer valid and you must re-evaluate the passage.

Waves, Swell and Steepness

Wave danger is about steepness rather than height alone. Steepness is wave height divided by wavelength. Wavelength in deep water is proportional to the square of the period: approximately 1.56 times period squared, in metres. So a 12-second swell has a wavelength of about 225m, while a 5-second wind wave is about 39m long.

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A 4m swell with a 12-second period has a steepness of about 1 in 56: the boat rides up and over comfortably. A 4m wave with a 5-second period has a steepness of about 1 in 10, close to the breaking limit of about 1 in 7. Breaking waves are what knock boats down; the force in a breaking crest can be many times that of a non-breaking wave of the same height.

Other causes of dangerous seas:

• Crossed seas: swell from a distant system crossing a local wind sea produces confused, pyramid-shaped seas with unexpected breaking crests. Common after a cold front, when the new wind sea builds across the old swell.
• Shoaling: as waves run into water shallower than about half their wavelength they slow, steepen and may break. Banks and the edge of the continental shelf (for example south-west of Ireland and in the Bay of Biscay) can produce dangerous seas.
• Wind against current or tide.

Wind against current

When wind blows against a current, the waves slow down, their length shortens and their height increases. A modest breeze against a strong current can produce short, steep, breaking seas out of proportion to the wind.

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The NASA visualisation below shows how complex the real surface currents of the North Atlantic are. The Gulf Stream (warm colours, left) is a fast, narrow river of water, and it breaks into meanders and eddies as it heads north-east. Each edge of a meander has current running in a different direction, so wind-against-current conditions can change within a few miles.

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Image: Greg Shirah, NASA Scientific Visualization Studio, public domain, via Wikimedia Commons

The Gulf Stream with a northerly wind and the Agulhas Current with a south-westerly gale are notorious examples. Around the UK, the same physics applies on a smaller scale in tidal races (Portland, Alderney, the Lizard) and over banks, where a Force 5 against a spring tide can create seas that would be dangerous for a small yacht. Plan passages to cross such areas with wind and tide together, and in strong winds keep well clear of races.

Tropical Revolving Storms

Tropical revolving storms (TRS) are called hurricanes in the Atlantic and north-east Pacific, typhoons in the north-west Pacific, and cyclones in the Indian Ocean and South Pacific. They are outside the normal operating area for most Yachtmaster Offshore passages but an Offshore skipper must understand them, as many go on to sail to the Caribbean, the Azores or beyond.

Formation

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A TRS needs:

1. Sea-surface temperatures above about 26.5°C, through a reasonable depth of ocean.
2. A latitude of at least about 5° from the equator, so that the Coriolis effect can start rotation. They rarely form within 5° of the equator.
3. Low vertical wind shear, so the developing storm is not torn apart.
4. A pre-existing disturbance, such as an easterly wave off West Africa.
5. Moist air through the mid-levels of the atmosphere.

The storm is a heat engine powered by latent heat released as water vapour condenses. Winds exceed 64 knots in a hurricane, with a calm eye 10 to 30nm across surrounded by the eyewall, where the strongest winds and heaviest rain occur. North Atlantic hurricane season runs officially from 1 June to 30 November, peaking from August to October. The simplest and only reliable tactic for a yacht is to not be there: plan passages outside the season and keep in touch with the National Hurricane Center's advisories.

The satellite photograph below, taken from the International Space Station, shows the structure of a mature hurricane: the clear central eye, the ring of towering cloud (the eyewall) where the strongest winds and heaviest rain occur, and spiral rain bands curling in towards the centre.

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Image: Mike Trenchard, NASA Johnson Space Center, public domain, via Wikimedia Commons

Warning signs

• An unusual swell from the direction of the storm, sometimes days ahead.
• Barometer: the normal tropical diurnal variation (about 3hPa twice daily) is disrupted. A pressure 3hPa below the seasonal mean is a warning; 5hPa below means a storm is very likely nearby.
• Cirrus radiating from a point on the horizon, then thickening cloud.
• Increasing wind, often from an unusual direction.

Semicircles

Looking along the direction the storm is moving, the Northern Hemisphere right-hand semicircle is the dangerous semicircle and the left-hand is the navigable semicircle. In the Southern Hemisphere they are reversed.

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The right-hand side is dangerous for two reasons. The storm's forward speed adds to the rotational wind speed, so winds and seas are highest there. And the wind tends to blow a vessel ahead of the storm, into its path. In the navigable semicircle the forward speed is subtracted and the wind tends to blow you away from the track.

Finding which semicircle you are in (Northern Hemisphere)

Use Buys Ballot's Law to find the bearing of the centre (back to the wind, the centre is on your left and a little forward; in a TRS face the wind and the centre is about 90 to 135° to your right, depending on distance). Then watch how the wind changes over a period of hours:

In the Southern Hemisphere the rules are reversed: backing means dangerous semicircle.

Avoidance tactics (Northern Hemisphere)

• Dangerous semicircle: put the wind on the starboard bow (close-hauled on starboard tack) and make as much way as possible away from the centre.
• Navigable semicircle: put the wind on the starboard quarter and make maximum speed away from the centre.
• In the path ahead of the storm: put the wind on the starboard quarter and run for the navigable semicircle.

In the Southern Hemisphere substitute "port" for "starboard". Check frequently that the tactic is working: the barometer should stop falling and the wind should ease.

Worked Example: Day Two of a Three-Day Passage

You are 90nm from your destination on a three-day passage across the western approaches. Your 0600 log entries:

• Barometer 1006 hPa, down 9 hPa in the last 3 hours (falling very rapidly).
• Wind south-easterly Force 5, backing slowly.
• Cirrostratus with a halo around the sun this morning, now thickening altostratus.
• GRIB (GFS) shows 25 knots mean from the south in 12 hours. The 0520 Shipping Forecast gives "south 6 to gale 8, occasionally severe gale 9 later" for your sea area.
• The destination harbour has a tidal gate that opens in 18 hours. A bail-out port with an all-weather entrance is 8 hours away on a reach.

Analysis. All the signs point the same way: a warm front approaching from the west, ahead of a deep low. A barometer falling more than 6 hPa in 3 hours indicates a gale is likely. The GRIB mean of 25 knots, plus 30% for gusts, already gives 32 knots, and the official forecast is more severe than the GRIB, which is typical: forecasters have seen the model under-forecasting the deepening.

Options. Continuing means arriving at the destination around the peak of the gale, after the crew has been up all night, and possibly having to wait offshore for the tidal gate. Diverting reaches shelter before the worst conditions on a comfortable point of sail.

Decision. Divert now, while the crew is still fresh and the wind still moderate. Reef in advance of the front, brief the crew, and log the reasoning. Cross-check the bail-out port's entrance in a southerly gale before committing.

Worked Example: Locating the Low and Planning the Crossing

You are in the Western Approaches at 49 degrees N, 10 degrees W, heading east to Falmouth, 180nm away at 6 knots. At 0900 the wind is south-south-west Force 5 and the barometer reads 1004 hPa, having fallen 5 hPa in 3 hours. At 1200 the wind is south-west Force 6 and the barometer is 999 hPa. At 1500 the wind is west-south-west Force 7 and the barometer is 995 hPa, still falling. The 1700 Shipping Forecast for your area reads "south-west 7 to severe gale 9, veering west, rain then showers, moderate or good".

Step 1: where is the low? Stand with your back to the south-west wind. In the Northern Hemisphere the low is on your left and a little in front, that is to the north-west or north. As the wind veers from south-south-west through south-west to west-south-west, the low is passing to the north of you and moving east. That is the pattern of the warm sector, followed by the approach of the cold front.

Step 2: what is the pressure telling you? A fall of about 4 to 5 hPa in each of the last three 3-hour periods is "falling quickly". The Shipping Forecast has mentioned severe gale 9. The pressure trend plus veering wind implies the cold front is about to pass or has just passed within a couple of hours.

Step 3: what comes next? With the pressure still falling, the cold front has not yet arrived. When it passes, expect a sharp veer to north-west, heavy showers, squalls and a sharp pressure rise of perhaps 5 hPa or more in an hour. The wind may increase further as the barometer rises ("first rise after low, foretells a stronger blow").

Step 4: decision. Reef down now to the third reef and storm jib, clear the decks, brief the watch, get everyone clipped on and into lifejackets. Because the wind is south-west to west and your course is east, you are on a broad reach to a run: sea state is a concern, and the new north-west wind sea will cross the old south-west swell after the front. Keep sea room: the Cornish coast is a lee shore in west and north-west gales. Plan to make a safe offing and heave-to or run off under a small headsail rather than push towards Falmouth in the dark with a full crew fatigue load.

Step 5: lessons for the log. Record the times, the barometer readings, wind directions and the reason for the decisions. The examiner asking "what did you do and why" expects precisely this commentary.

Worked Example: Isobar Spacing to Surface Wind

On a surface chart at 50 degrees N the isobars are 4 hPa apart. Using the geostrophic wind scale you measure a spacing of 150nm between isobars near your position.

• A 4 hPa difference over 150nm is a gradient of roughly 0.027 hPa per nm, about 1.6 hPa per degree of latitude. Reading the scale for 50 degrees N, this corresponds to a geostrophic wind of about 25 knots.
• Surface wind over the sea is about two-thirds to three-quarters of geostrophic, so 17 to 19 knots mean (Force 5). It will be backed 10 to 15 degrees from the isobar direction.
• Gusts can be 30 to 50% above the mean, so 22 to 28 knots, which equals Force 6 gusts.
• If the same isobars at the next chart are 75nm apart, the geostrophic wind doubles to about 50 knots, and the surface mean wind reaches 34 to 38 knots (Force 8). Gusts in showers will exceed 45 knots.

This shows why tightening isobars deserve more attention than the central pressure of the low.

Examiner Expectations

• You will be asked to interpret a synoptic chart: say where the fronts are, what weather they will bring, how fast they move and what you would do.
• You will be asked how you would get the forecast offshore and how you would verify it.
• You will be asked a "what if" about a falling barometer: always give the numbers (pressure fall per 3 hours, what the term means, what you would do).
• Know Buys Ballot's Law cold, and be able to apply it for both hemispheres.
• For TRS, give the semicircle rules, the avoidance tactics for both hemispheres, and say clearly that the best tactic is to avoid the season and the area entirely.

Common Mistakes

• Treating GRIB data as a forecast. It is unchecked model output showing mean wind; add gusts and compare with the official text forecast.
• Reading pressure but not tendency. The rate of change is the key short-term indicator.
• Ignoring secondary depressions. Small lows on trailing cold fronts can deepen quickly and are easily missed on GRIBs.
• Underestimating wind against current or tide. Even moderate winds against strong streams create steep breaking seas.
• Mixing up the TRS semicircles. In the Northern Hemisphere: veering wind, dangerous right-hand semicircle, wind on the starboard bow.
• Looking only at wave height. Period and steepness determine danger.
• Not reefing before a cold front. Squalls and wind shifts arrive suddenly.
• Forgetting that significant wave height is an average. Individual waves reach twice the stated height.
• Relying on one forecast source. Compare at least two: a model and an official text forecast.
• Ignoring wind acceleration around islands and headlands. The wind can be 30 to 50% stronger than the open-sea forecast.

Summary

• Offshore winds are stronger and seas build higher than near the coast; plan with the large-scale pattern.
• Tight isobars mean strong winds; wind blows anticlockwise and slightly inward around a NH low.
• Know the cloud, wind, pressure and visibility sequence through warm front, warm sector, cold front and occlusion.
• Upper troughs, ridges and jet streaks control whether surface lows deepen or fill.
• GRIBs show mean wind on a coarse grid; text forecasts include forecaster judgement and warnings. Use both, plus the barometer.
• Steepness, not height, makes waves dangerous: short periods, crossing seas, shoaling and wind against current all steepen waves.
• TRS form over warm tropical seas; avoid the season. NH: dangerous right-hand semicircle, wind veers, wind on starboard bow; navigable left-hand semicircle, wind backs, wind on starboard quarter.

Check Your Understanding

1. In the Northern Hemisphere you stand with your back to a south-westerly wind. Where is the centre of the low?

Answer: On your left and slightly forward: roughly to the north-west or north-north-west of you.

2. Describe the changes in wind and pressure as a cold front passes.

Answer: The wind veers sharply, often with squalls and gusts, and pressure, which may have been steady or falling in the warm sector, rises sharply. Heavy rain with cumulonimbus gives way to clearer air with showers and very good visibility.

3. A surface low is forecast to move under the forward (eastern) side of an upper-level trough. What is it likely to do?

Answer: Deepen, because upper divergence ahead of the trough removes air from the column and lowers surface pressure.

4. A GRIB file shows 22 knots mean wind in a showery north-westerly airstream. What gusts would you plan for?

Answer: At least 30% more, about 29 knots, and in unstable showery air gusts can be 50% or more above the mean, so 33 knots or more in squalls. Plan sail area accordingly.

5. Why is a 3m wave with a 5-second period more dangerous than a 3m swell with a 10-second period?

Answer: Wavelength is proportional to the square of the period: about 39m for 5 seconds and 156m for 10 seconds. The short wave is four times steeper (about 1 in 13 compared with 1 in 52), close to breaking, and breaking waves cause knockdowns.

6. The barometer has fallen 7 hPa in 3 hours. What does this tell you?

Answer: Falling very rapidly: a gale or storm is likely soon. Reef, secure the boat, review your bail-out options and consider your position relative to the approaching low.

7. In the Northern Hemisphere, near a TRS, the barometer is falling and the wind is veering steadily. Where are you and what should you do?

Answer: In the dangerous (right-hand) semicircle. Put the wind on the starboard bow and make maximum way away from the centre.

8. What are the five conditions needed for a TRS to form?

Answer: Sea surface temperatures above about 26.5°C; latitude more than about 5° from the equator for Coriolis effect; low vertical wind shear; a pre-existing disturbance; and moist air in the mid-troposphere.

9. Why might you avoid crossing a tidal race with a Force 5 blowing, even though Force 5 is well within your boat's limits?

Answer: Wind against a strong tidal stream shortens and steepens the waves, creating breaking seas much more dangerous than the wind strength suggests. Cross with wind and tide together, or wait for slack water.

10. Name four ways to receive a weather forecast 300nm from land, and give one limit of each.

Answer: Navtex (range limited to about 250 to 400nm from the station, text only); SafetyNET via Inmarsat-C (needs a receiver, text only); HF/SSB weatherfax and voice nets (needs radio, time and skill, reception varies); satellite phone GRIB download (costs per megabyte, mean wind only and no forecaster check). Your own barometer and observations are a fifth source.

11. What does the Shipping Forecast mean by "imminent", "soon" and "later"?

Answer: Imminent means within 6 hours of the forecast issue time, soon means 6 to 12 hours, later means more than 12 hours.

12. A line of dark cumulonimbus is approaching from the north-west and you are on a broad reach under full sail. What do you do?

Answer: Reef or reduce sail before it arrives, make sure the crew are clipped on and in foul-weather gear, bear away to depower, note the time and wind shift in the log, and expect a sharp veer and a gusty sea after the squall passes.