Advanced Meteorology for Coastal Skippers - Synoptic Charts

Prerequisites

You should already be comfortable with Day Skipper meteorology:

• The Beaufort scale and its sea-state descriptions.
• The shipping forecast format, the sea areas and the terms used (imminent, soon, later; veering and backing; gale warnings; visibility terms).
• Basic ideas of highs, lows and fronts, and the sea breeze.
• Where to get forecasts: Met Office shipping and inshore waters forecasts on BBC Radio 4, Coastguard Maritime Safety Information (MSI) broadcasts on VHF, NAVTEX, the internet and weather fax.

At Coastal Skipper / Yachtmaster Coastal level you are expected to interpret a synoptic chart yourself, plot the movement of weather systems, recognise the onboard signs of approaching weather, compare several sources and make and defend a go or no-go decision. This lesson is part of the RYA Coastal Skipper / Yachtmaster Offshore shorebased course, whose written papers include taking and interpreting forecasts, plotting weather systems and weather predictions using a barometer (pass mark 60 per cent).

Learning Objectives

By the end of this lesson you will be able to do the following. The RYA shorebased syllabus items covered are named in brackets.

• Interpret isobar spacing on a synoptic chart and estimate geostrophic and surface wind strength and direction (syllabus: taking and interpreting forecasts; plotting weather systems).
• Describe the sequence of weather through a depression, including warm, cold and occluded fronts, and predict a depression's movement from the chart (syllabus: plotting weather systems).
• Recognise the main air masses affecting the British Isles and the weather each brings (syllabus: weather systems).
• Identify the main cloud types and use them, with the barometer and the wind, to anticipate approaching weather (syllabus: weather predictions using a barometer; observation of weather).
• Use the barometer and pressure tendency as a decision tool (syllabus: weather predictions using a barometer).
• Decode shipping forecast terminology exactly, including gale warnings, timing and visibility terms (syllabus: taking and interpreting forecasts).
• Compare GRIB, text and commercial forecasts and handle disagreement between sources.
• Explain local effects: sea breezes, land breezes, katabatic and anabatic winds, headland and funnelling acceleration, wind against tide, squalls and thunderstorms.
• Identify fog types, forecast fog from the dew point and plan for poor visibility.
• Make a go or no-go decision for a coastal passage from the available weather information and set abort criteria.

Synoptic Charts and Isobars

A synoptic chart shows surface pressure as isobars, usually at 4 hPa (millibar) intervals, together with fronts. Wind blows roughly along the isobars, turned in towards low pressure by surface friction, and its speed is proportional to the pressure gradient: the closer the isobars, the stronger the wind.

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Key rules

• In the Northern Hemisphere, wind circulates anticlockwise around a low and clockwise around a high.
• Buys Ballot's law: in the Northern Hemisphere, if you stand with your back to the wind, low pressure is on your left. Equivalently, facing the wind, the low is on your right and slightly behind you.

• Over the sea, the surface wind is backed about 10 to 15 degrees from the isobars, towards the low. Over land the angle is greater, around 30 degrees.
• The geostrophic wind is read from the scale on the chart, which depends on latitude and isobar spacing. It overestimates the surface wind: allow about two-thirds of it over the sea and a half or less over land.
• Curvature matters. Around a tight low the surface wind is lighter than geostrophic, and around a high slightly stronger, but the strongest gradients occur around deep lows.

Estimating wind from isobar spacing

At 50°N, with isobars drawn every 4 hPa, the geostrophic wind is approximately as follows. Use the geostrophic wind scale printed on the chart for your latitude when you can.

Worked example

On a 4 hPa chart at 50°N, isobars over the western Channel are 100 nm apart. The geostrophic scale gives about 30 knots. Allowing two-thirds over the sea, expect about 20 knots at the surface, Force 5, with gusts to 28 knots in showers. The isobars run south-west to north-east with low pressure to the north-west, so the geostrophic wind is from the south-west (225 degrees); allowing a back of 10 to 15 degrees, the surface wind will be from about 210 to 215 degrees.

Air Masses

The character of the weather depends on the air mass over you as much as on the fronts crossing it. The five that matter for British waters are:

A polar maritime air mass that has been over the Atlantic for a while and is returning from the south-west (returning polar maritime) is warmer and moister, with less showery weather. The warm sector of a depression is Tm air. The air behind a cold front is Pm air. Knowing the air mass tells you whether to expect showers (unstable air heated from below) or layer cloud and fog (stable air cooled from below).

Depressions and Fronts

A typical Atlantic depression approaching the UK brings a warm front, a warm sector and a cold front. Knowing the sequence allows you to predict what will happen next from what you can see.

Warm front

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1. Cirrus appears high in the sky, 300 to 500 nm ahead of the front, perhaps 12 to 24 hours before it arrives. A halo around the sun or moon (cirrostratus) is a classic sign.
2. Altostratus thickens and lowers; the sun becomes "watery" and then disappears.
3. Nimbostratus brings continuous moderate rain for several hours; visibility drops.
4. Pressure falls steadily; the wind backs and freshens ahead of the front.
5. At the front the wind veers, the pressure fall slows or stops, rain eases to drizzle and temperature rises.

The warm sector behind has a steady barometer, a south-westerly wind, low stratus, drizzle and often poor visibility or fog over cold sea.

Cold front

The cold front is narrower and more violent:

• A line of heavy cumulonimbus, often visible as a dark band approaching from the west.
• Squalls, heavy rain or hail, possible thunder; gusts can be 50 per cent above the mean wind.
• The wind veers sharply, often 45 to 90 degrees within minutes (for example south-west to north-west).
• Pressure rises sharply after passage; visibility becomes very good; showers follow in the cold air.

Cold fronts typically move at 25 to 35 knots. The cold front is the moment to have reefed in advance: the squall line often arrives faster than the forecast suggested.

Occluded front

When the faster cold front catches the warm front, the warm air is lifted off the surface, forming an occlusion.

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Occlusions combine features of both fronts: prolonged rain, then a wind shift, sometimes with embedded heavy convective rain. They mark the mature stage of a depression and can bring severe and unpredictable weather, particularly near the triple point where warm, cold and occluded fronts meet. Treat them with the same caution as a cold front.

Summary of frontal passage

Other features

• Troughs are elongated areas of low pressure, often with a wind shift and a band of rain or showers. Treat a trough line like a weak cold front: the wind veers across it.
• Ridges are wedges of high pressure between lows; the weather is settled for a day or so but the next low is usually close behind.
• Secondary depressions often form on the cold front of a main low, particularly south-west of Ireland, and can bring the strongest winds of the system. A chart showing a small closed low developing on a trailing front is a warning to look at the next forecast carefully.
• Polar lows are small, intense lows that form in cold polar air over the sea north of the UK in winter, with rapidly rising winds and heavy showers.

Plotting the Movement of Weather Systems

The RYA expects you to be able to take a series of synoptic charts, or a set of coastal reports, and work out where a depression will be. The practical rules are:

• A frontal depression moves in the direction of the isobars in its warm sector and at about the speed of the geostrophic wind in the warm sector.
• A cold front usually moves faster than a warm front; a depression is mature when the cold front has caught the warm front and the occlusion has formed, and it then slows and fills.
• A depression that is deepening quickly (a fall of 24 hPa or more in 24 hours is called explosive deepening, or a weather bomb) brings much stronger winds than the earlier charts suggested.
• A high is usually slower-moving; look for the ridge axis to estimate when the next system will arrive.

Worked example

A low is plotted at 1200 UT at 54°N 20°W, central pressure 996 hPa. The warm sector isobars (4 hPa) are 100 nm apart at 50°N, so the geostrophic wind is about 30 knots. The warm sector isobars run from the south-west to the north-east (bearing 060°). The low will move at about 30 knots on 060°: in 12 hours it moves about 360 nm, reaching a position at about 57°N 11°W, some 150 nm west of the Hebrides. If the chart for 0000 UT shows it at 984 hPa, it has deepened 12 hPa in 12 hours, which is 24 hPa in 24 hours if continued: an explosively deepening low, with the winds around it likely to be stronger than the isobars on the earlier chart suggested.

To plot a front's arrival at your position, measure the distance from the front to you along the direction of motion, divide by the front's speed, and add the time. A cold front 180 nm to the west of you, moving at 30 knots, will arrive in 6 hours. If your passage will take 8 hours, you will be at sea when the squall line arrives, which is why the start time matters.

Reading the Sky: Clouds as Forecasters

The sky shows what is coming before the chart does. Learn the ten main types and what they tell you.

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

Other warnings:

• A halo around the sun or moon (ice crystals in cirrostratus) signals a warm front, usually with rain within about 12 hours.
• A red sky in the morning with cloud thickening to the west indicates an approaching system; a red sky at night usually means a clear horizon to the west and a drier outlook.
• Cumulonimbus build up rapidly in the cold air behind a cold front. If you see the cauliflower tower grow an anvil, expect a squall within 10 to 30 minutes and reef before it arrives.
• A line of cumulus along the coast on a summer afternoon marks the sea breeze front.

The Barometer as a Decision Tool

Your barometer is the only weather instrument that tells you about the system you are actually in. Record the pressure in the log every hour on passage (at least every three hours), and plot it.

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The rate of change matters more than the absolute value. The pressure tendency terms used in the shipping forecast and coastal station reports describe the change in the last three hours:

Old rules of thumb: "long foretold, long last; short notice, soon past" (a slow fall heralds a prolonged blow; a sudden fall, a short sharp one), and "first rise after low, foretells a stronger blow" (the wind after a cold front can be stronger than before it).

A common prediction guide when the wind is steady: a fall of 6 hPa or more in 3 hours indicates a gale within hours; a fall of 1 to 2 hPa in 3 hours with a backing wind indicates a deepening low approaching. A rise of 3 to 5 hPa in 3 hours after a front often brings a squally north-westerly with clearing skies.

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If the barometer is falling faster than the forecast implies, believe the barometer: the system is deepening or moving faster. Reef, prepare for heavy weather and reconsider the plan now. A rising barometer against a gloomy forecast is encouraging, but stay alert.

Barometer practice

• Check the barometer against a known reading (a coastal weather station or the forecast pressure) before a season. An error of 2 hPa is common and is acceptable so long as you know it.
• Tap it gently before reading; mechanical (aneroid) instruments stick.
• Use the barograph or a log and plot the trace. The tendency, not the single reading, is what tells you about the weather.
• A diurnal variation of about 1 hPa (higher at 1000 and 2200, lower at 0400 and 1600 local) exists in many areas. Do not mistake it for a front.

Wind Forecast Terminology and Scales

You must be able to read a forecast exactly, since each term has a defined meaning.

The Beaufort scale gives the wind force, mean speed and typical open-sea wave height. Use it to link a forecast to the sea state you will meet.

The shipping forecast is issued four times a day by the Met Office and broadcast on BBC Radio 4 (long wave and FM), at around 0048, 0520, 1201 and 1754 local time. The inshore waters forecast covers the coast out to 12 nm. Coastguard MSI broadcasts on VHF are announced on Channel 16 and read on a working channel; NAVTEX (518 kHz) prints the forecast and gale warnings automatically. Know where each source gives its information and how recent it is.

Comparing Forecast Sources

No single forecast is enough. The Coastal Skipper should compare at least three.

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GRIB files are raw output from numerical weather models (such as GFS, ECMWF, UKMO and high-resolution regional models like AROME and the UKV). They give a detailed grid of wind and pressure and are excellent for spotting trends and timing. But they are unedited: they typically show mean wind (not gusts), smooth out local effects, and coarse models miss sea breezes, headland acceleration and katabatic winds.

Text forecasts (shipping forecast, inshore waters forecast, Met Office coastal products) are written by forecasters who combine several models with local knowledge and climatology. They give gusts, warnings and uncertainty ("occasionally 7 later") but have coarse geography and fixed issue times.

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Commercial and app forecasts are convenient and may blend several models, but their methods and quality are often unknown.

When sources disagree, treat the disagreement itself as information: the atmosphere is uncertain. If one source says Force 4 and another Force 7, plan for 7 or do not go. Look at several runs of the same model too; if successive runs swing wildly, confidence is low.

Remember also that the forecast is for the mean wind in open sea. Gusts are typically 30 to 50 per cent stronger, more in showers and in the lee of high land. Always compare the forecast with what you observe on the day, particularly the barometer trend and the cloud sequence.

Local Effects

Sea breeze

On a sunny day with light gradient wind, land heats faster than the sea. Air rises over the land and cooler sea air flows in to replace it. The sea breeze typically sets in by late morning, strengthens to 10 to 15 knots (occasionally more) by mid-afternoon, veers through the day in the Northern Hemisphere under the Coriolis effect and dies at sunset. It extends perhaps 10 to 20 nm offshore.

An offshore gradient wind can delay it, producing a calm zone where the two meet; an onshore gradient wind adds to it. A line of cumulus a few miles inland marks the sea breeze front. A land breeze, weaker, may blow offshore at night as the land cools faster than the sea.

Sea breeze is strongest where the land-sea temperature difference is greatest, so on a sunny day in June it can reach Force 5 on some coasts. Reckon on it for afternoon sailing, and note that the wind may veer 30 to 60 degrees between the morning gradient wind and the late afternoon sea breeze.

Katabatic and anabatic winds

At night, air cooled on high ground drains downhill as a katabatic wind. In steep-sided lochs, fjords and mountainous coasts it can arrive suddenly and violently, catching anchored boats. By day, warmed air flows up the slopes as a gentler anabatic wind. Choose anchorages with this in mind, and allow for a wind that reverses between day and night.

Acceleration zones

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Wind compresses around headlands and funnels through straits and gaps, often 20 to 50 per cent stronger than in open water. A forecast Force 5 can be Force 7 off a big headland. Add this to the wind-against-tide effect at the same headlands and you have the reason most yacht trouble happens there. Reef before rounding, not halfway round.

Wind against tide

When the wind blows against a strong tidal stream, the waves become steeper and shorter. A stream of 2 knots against a Force 5 wind can produce a sea like Force 6 or 7, with breaking crests; over a rock ledge or off a headland the stream can form overfalls or a race. Rules of thumb:

• Wind with tide gives a smoother sea; wind against tide steepens it.
• The effect is greatest at the strongest part of the stream, usually halfway through the tide, and decreases at slack water.
• Where possible, plan to pass the dangerous headland at slack water or with the wind and tide together.

Check the chart and the pilot book for named races, and the tidal stream atlas for the time of the strongest stream.

Squalls and thunderstorms

Squalls are sudden, short-lived increases in the wind, with a rise in wind speed of at least 16 knots to 22 knots or more, lasting at least a minute or so. They come with cumulonimbus, along a cold front and in showers in unstable air. Warning signs are a dark cloud base with rain below it, a roll or arcus cloud at its leading edge, and a sudden cold gust front.

• Reef early. If you see a squall line approaching, reef the mainsail and put a reef in the headsail before it hits.
• Lightning can strike a yacht, which is the tallest object for miles. Stay away from the mast, rigging, backstay and metal fittings, and disconnect or switch off electronics if possible. A spare handheld VHF and GPS in a metal oven or biscuit tin acts as a Faraday cage. Lightning is a main cause of electronics failure, and a good reason to be able to navigate without it.
• Hail and heavy rain can reduce visibility to well under a mile. Slow down, turn on the navigation lights and radar, and keep a good lookout.
• Waterspouts and tornadoes form in unstable air and are rare in UK waters, but they can be violent: sail away at right angles to their track if you see one.

Fog

Fog is the main visibility hazard in UK waters.

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Advection fog is the most dangerous for coastal sailors because wind does not clear it. Fog needs air that is saturated, so watch the dew point: when the air temperature on deck falls to within about 2 degrees of the dew point and is still falling, fog is imminent. A wet-and-dry bulb thermometer (psychrometer) or a hygrometer gives the humidity; if the sea temperature is below the dew point of air moving over it, fog forms.

Example: air temperature 14 degrees C, dew point 13 degrees C, sea temperature 11 degrees C, wind south-west 12 knots. The air is nearly saturated and the sea is colder than the dew point, so advection fog is likely over the water. Forecast: persists until the wind shifts to a drier air mass, perhaps after a cold front.

Procedures in fog

1. Reduce speed to a safe speed (Rule 6) and sound the correct fog signals (Rule 35): a power-driven vessel making way sounds one prolonged blast every two minutes; a sailing vessel sounds one prolonged and two short blasts every two minutes.
2. Post a lookout, listen, and use radar and AIS if fitted. Use the radar reflector.
3. Fix your position, keep a careful EP, and avoid shipping lanes, narrow channels and harbour entrances.
4. Consider anchoring in shallow water out of the channel, or heading for safe water, if you do not have radar.
5. Wear a lifejacket, clip on, and have the dinghy and grab bag ready.

Making the Go or No-Go Decision

1. Gather at least three forecasts and the latest synoptic charts (now, plus 24 and 48 hours).
2. Locate the systems: where are the lows and fronts, which way are they moving, how fast, and when will they reach you?
3. Identify the worst plausible case along the route and add local effects (headlands, wind against tide).
4. Check the barometer trend and the sky against the forecast.
5. Compare with the boat and crew: can this crew safely handle that worst case on this boat, at night, with seasickness?
6. Decide, and set weather-based abort criteria for the passage, with a plan B port.

Treat the decision as a risk assessment, not a forecast contest. A crew that is tired, a boat that is poorly equipped or a harbour of refuge that is hard to enter at night each lower the weather limit at which you should go.

Worked Example: A Weekend Decision

Friday evening. You plan a 40 nm passage west along the coast on Saturday. The evening shipping forecast for your area: "South-west 4 or 5, increasing 6 at times later. Rain later. Good, becoming moderate or poor." The synoptic chart shows a low of 990 hPa west of Ireland moving east, with its warm front 400 nm to the west. GRIB data shows 15 to 18 knots from the south-west during the day, rising to 25 knots after 1800. The inshore waters forecast says "occasionally 6 later" and mentions fog patches.

Analysis:

• The warm front is roughly 400 nm away. At a typical 25 knots of frontal speed, it will arrive around 16 hours after the chart time, so about mid-afternoon Saturday: increasing cloud, rain, backing wind, falling visibility.
• The passage is westward, into the south-westerly. Beating 40 nm at around 4 knots VMG takes 10 hours; you would meet the strengthening wind and poor visibility in the second half.
• Your route rounds a headland with a known 3 knot stream; a south-westerly 6 against the ebb would be ugly, because wind against tide would steepen the sea to a Force 7 equivalent.
• Fog patches mean you need to check the dew point and dew point spread on the morning of departure.

Decision: either leave at 0500 to be round the headland on the fair tide by 1100 and in harbour before 1500, with plan B a port halfway; or postpone until after the cold front passes on Sunday, when the wind will veer north-westerly and give a reach, though with gusty showers. Saturday 0500: the barometer has fallen 2 hPa in three hours (no qualifier) and cirrus is thickening. This is consistent with the forecast. You choose to sail early with the halfway port as a firm contingency, and set an abort criterion: if the barometer falls by more than 3.5 hPa in any three hours, or the visibility drops below 2 nm, go into the halfway port.

Common Mistakes

• Using one source. GRIB alone misses gusts and local effects; text alone lacks spatial detail.
• Forgetting gusts. The forecast is a mean wind; plan for 30 to 50 per cent more in gusts.
• Ignoring the barometer. Not logging it means you cannot see the tendency.
• Misapplying Buys Ballot's law. Back to the wind, low on the left in the Northern Hemisphere.
• Using the geostrophic wind as the surface wind. Allow two-thirds over the sea and more for local effects.
• Misreading forecast terms. "Soon" means 6 to 12 hours, not "in a moment", and "poor" visibility means 1,000 m to 2 nm.
• Assuming fog will clear in wind. Advection fog does not.
• Leaving reefing too late before a cold front. The squall line arrives abruptly.
• Ignoring headland and funnelling acceleration. Add 20 to 50 per cent off major headlands.
• Ignoring wind against tide. A moderate wind against a 3-knot stream can produce a dangerous sea.
• Forgetting the weather in the evening when you plan arrival. Many fronts arrive at night; plan for the whole passage window.

Summary

• Closer isobars mean stronger wind; reduce geostrophic wind by about a third over the sea and allow for backing towards the low.
• Air mass identifies the weather type: polar maritime gives showers and visibility; tropical maritime gives cloud, drizzle and fog.
• Warm fronts give hours of warning (cirrus, falling pressure, backing wind); cold fronts arrive abruptly with squalls and a sharp veer; occlusions combine both.
• A depression moves along the warm sector isobars at about the warm sector geostrophic wind speed.
• Log the barometer and judge the rate of change; trust a fast fall over an optimistic forecast.
• Know the forecast terms exactly: imminent, soon, later, gale warning criteria, and the visibility scale (good over 5 nm, moderate 2 to 5 nm, poor 1,000 m to 2 nm, fog under 1,000 m).
• Compare GRIB, text and other sources; disagreement means uncertainty, so plan for the worst case.
• Sea breezes, katabatic winds, acceleration zones, wind against tide and squalls modify the forecast near the coast.
• Advection fog is the main maritime fog hazard and persists in wind; watch the dew point.
• Combine forecast, synoptic analysis, barometer, local effects and crew capability into a clear go or no-go decision with weather abort criteria.

Check Your Understanding

1. In the Northern Hemisphere, you stand with your back to the wind. Where is the low pressure?

Answer: On your left (and slightly behind you), according to Buys Ballot's law.

1. Describe the wind and pressure changes as a warm front passes.

Answer: Ahead of the front the pressure falls steadily and the wind backs and freshens. As the front passes, the wind veers, the pressure stops falling and the rain becomes lighter or turns to drizzle.

1. How does a cold front differ from a warm front for a sailor?

Answer: It arrives abruptly with heavy squalls and a sharp veer, gusts well above the mean wind, then pressure rises sharply and visibility improves. Warm fronts are gradual with hours of warning.

1. The barometer falls 5 hPa in three hours although the forecast was for moderate winds. What should you do?

Answer: A fall of "quickly" (3.6 to 6 hPa in 3 hours) suggests strong winds are coming. Trust the barometer: reef early, prepare for heavy weather and reassess the passage, possibly diverting.

1. A GRIB file shows 15 knots but the inshore waters forecast says Force 6 occasionally 7. How do you plan?

Answer: Treat the disagreement as uncertainty and plan for the worse: Force 7. Remember GRIBs show mean wind from a model and miss gusts and local effects.

1. Why does advection fog pose a particular risk?

Answer: It forms when warm moist air moves over colder sea, can form in moderate winds, covers large areas and can persist for days until the air mass changes, so wind does not clear it.

1. You plan to anchor overnight in a steep-sided loch. What local wind should you consider?

Answer: Katabatic wind: cold air draining down the slopes at night can arrive suddenly in strong gusts. Choose a spot and scope to cope with it.

1. Why might you meet Force 7 off a headland when the forecast is Force 5?

Answer: Wind accelerates as it is compressed around headlands, often by 20 to 50 per cent, and gusts add further. Wind against tide can make the sea state worse still.

1. When does a sea breeze typically develop and how strong does it get?

Answer: It develops by late morning on sunny days with light gradient winds, peaks at around 10 to 15 knots in mid-afternoon, veers during the day and dies around sunset.

1. A shipping forecast issued at 0600 says "Gale 8 imminent". By when should you expect the gale, and what mean wind does it mean?

Answer: "Imminent" means within 6 hours of issue, so by 1200. Gale force 8 means a mean wind of 34 to 40 knots, or gusts of 43 to 51 knots.

1. On a 4 hPa chart at 50°N, the isobars are 60 nm apart. What is the geostrophic wind, and what surface wind do you expect over the sea?

Answer: About 50 knots geostrophic. The surface wind over the sea is about two-thirds, or about 33 knots (Force 7). Expect higher gusts and more near headlands.

1. A low has a warm sector geostrophic wind of 40 knots from the south-west. How fast and in what direction will it move?

Answer: About 40 knots (960 nm in 24 hours), moving along the warm sector isobars, which run from the south-west towards the north-east.