Lesson 38 of 40 · Module 6 — Yachtmaster-level navigation

Weather for navigators

Forecasts, fronts, sea state and how weather shapes the plan.

Yachtmaster30 minVideo · Notes · Interactive · Exercises

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0:00 / 4:44Chapter 1 of 9: Weather shapes the plan

Transcript

Every passage plan in lesson thirty-two began with the forecast. The weather decides whether you go, when you go, which way round, and where you shelter if it changes. In this lesson you will read a weather chart, follow a depression past the Isle of Kerran, and see how wind against tide and fog change the plan. Then you will turn a forecast into a decision: go, wait, or change the plan.
A synoptic chart shows the pressure at sea level. Its lines are isobars, joining places where the pressure is the same, labelled in hectopascals. In the northern hemisphere the wind blows anticlockwise round a low and clockwise round a high, roughly along the isobars but crossing them slightly towards lower pressure. Where the isobars are close together the wind is strong, and where they are far apart it is light.
Buys Ballot’s law puts this in your pocket. Stand with your back to the wind, and in the northern hemisphere the low pressure is on your left. When the wind shifts clockwise, say from south to south-west, it has veered. When it shifts anticlockwise, from west to south-west, it has backed.
Here a depression passes north of the Isle of Kerran, and its fronts arrive in the typical order. Ahead of the warm front the cloud thickens and lowers, rain sets in, and the visibility drops. In the warm sector behind it, the wind has veered south-west, and the air is mild and damp, often with drizzle and poor visibility. The cold front brings heavy rain and squalls, then a veer and clearing skies. Where the cold front catches up with the warm front, the two form an occlusion, with its own band of rain.
Forecasts give the wind as a Beaufort force, which stands for a band of mean wind speeds. Force four is eleven to sixteen knots, force five seventeen to twenty-one, and force six twenty-two to twenty-seven. Force eight, a gale, is thirty-four to forty knots. Gusts blow harder than the mean, and the sea builds with every step up the scale.
When the tidal stream runs against the wind, the waves become shorter and steeper. Off a headland, where the stream runs hardest, that can build into overfalls: breaking, confused water. On the practice chart the strongest streams run off Gull Point and Kerran Head, at diamonds B and C. With a south-westerly on twelve June, the flood off Gull Point runs with the wind until about fourteen twenty-two, and from about fifteen twenty-two the ebb sets south-west, straight into it, at up to one point eight knots.
On a sunny day with little wind on the weather chart, the land warms faster than the sea, and a sea breeze blows onshore by the afternoon, dying away around dusk. At night the land cools, and a lighter land breeze can blow off the shore. Radiation fog forms over land on clear, calm nights and drifts out over harbours, but it usually clears during the morning. Sea fog forms when warm, moist air moves over colder sea, and it can last for days, so don’t sail expecting it to lift.
Forecasts come from the national met service’s shipping and inshore forecasts, coastguard radio broadcasts and NAVTEX. The timing words are exact: imminent means within six hours of the time of issue, soon six to twelve hours, and later more than twelve. Visibility is good above five miles, moderate from two to five, poor from a thousand metres to two miles, and very poor, which is fog, below a thousand metres. Weather-model GRIB files are smoothed and under-show the gusts, so use them as a guide only.
Put it together with a fictional forecast for the Isle of Kerran, issued at zero six hundred. South-westerly four or five, increasing six soon, veering north-westerly six later. Soon means force six, twenty-two to twenty-seven knots, between twelve hundred and eighteen hundred, while you are on passage, and if you are running late, the ebb off Gull Point runs straight into it from about fifteen twenty-two. Later, after eighteen hundred, the north-westerly blows into Kerran Roads, which is open to the north. So change the plan: stay in Port Kerran, or sail in the morning and be back before the wind gets up.

In this lesson you will learn

Read wind direction and relative strength from a synoptic chart’s isobars, using Buys Ballot’s law, veer and back
Describe the typical sequence as a depression’s fronts pass, and what each stage means for visibility, wind shifts and the plan
Convert between Beaufort forces and knots, and find when wind against tide will raise steep seas off Gull Point and Kerran Head
Recognise sea and land breezes, radiation fog and sea fog, and what each means for leaving or entering harbour
Decode a forecast’s timing and visibility words, use GRIB files sensibly, and turn a forecast into a go, wait or change-the-plan decision

Weather in the plan

Every passage plan in Lesson 32 started from the forecast, and the weather sits behind most of its decisions: whether to go at all, when to leave, which way round, where to shelter if things change. Lesson 34 showed what to do once the visibility closes in. This lesson is about reading the weather the way a navigator needs to — not the meteorology for its own sake (the separate Marine Weather course goes deeper), but what it means for the route, the timing and the crew.

The weather charts and forecasts here are all fictional, for the Isle of Kerran practice area. Real forecasts come from your national met service; always get the latest before you sail.

Reading a synoptic chart

A synoptic chart is a map of the pressure at sea level at one time, either as observed (an analysis) or as forecast for a time ahead. Its lines are isobars, joining places of equal pressure and labelled in hectopascals (hPa, the same as millibars). Closed rings of low pressure are depressions or lows (L); high pressure areas are anticyclones or highs (H). Lows usually bring fronts, stronger winds and changeable weather; highs often bring lighter winds and settled weather.

In the northern hemisphere the isobars tell you the wind:

Direction. The wind blows anticlockwise round a low and clockwise round a high, roughly along the isobars but crossing them slightly towards lower pressure (in towards a low, out from a high) — a little more over land than over the sea.
Strength. The closer together the isobars, the stronger the wind; where they are far apart, it is light.
Buys Ballot’s law. Stand with your back to the wind and the low pressure is on your left. It works the other way too: knowing where the low is tells you roughly which way the wind blows.

A wind shift has its own words. The wind veers when it shifts clockwise (south to south-west, say) and backs when it shifts anticlockwise (west to south-west). Directions are where the wind comes from, so a veer of 40° from 200° gives 240°, and a back of 40° gives 160°.

InteractiveRead the wind from the isobars

Tap the chart to place the yacht (or focus it and use the arrow keys). Read the wind from the isobars first, then check the panel. Step through the chart times to watch the depression pass the Isle of Kerran.

Fictional chart for teaching. Isobars every 4 hPa; red semicircles warm front, blue triangles cold front, purple occlusion.

At the yacht · Now

Wind southerly

Isobars far apart: light winds.

Back to the wind, lower pressure is on your left: to the west. Pressure here about 1026 hPa.

Ahead of the warm front: cloud thickening and lowering, pressure falling, rain on the way.

Wind log at this spot

Chart	Wind	Change
Now	southerly	—
+6 h	southerly	steady
+12 h	south-westerly	veered
+18 h	westerly	veered
+24 h	westerly	steady

Back to the wind, low on your left: the wind runs anticlockwise round a low, stronger where the isobars are close.

When a depression passes

A depression usually carries fronts, the boundaries between air masses. The warm front (drawn with semicircles, red on colour charts) leads; behind it lies the warm sector of mild, moist air; then the cold front (triangles, blue). The symbols sit on the side the front is moving towards. As the cold front catches up with the warm front it lifts the warm air off the surface, forming an occlusion (alternating symbols, purple). When the centre passes to the north of you, the stages typically arrive in this order:

Stage	What you typically see	Wind and visibility	For the plan
Ahead of the warm front	High cloud thickening and lowering; pressure falling; then steady rain	Often backing and freshening; visibility falling in the rain	Reef early; check pilotage plans for poor visibility
Warm sector	Mild, damp, low cloud and drizzle; pressure steadier	Veered at the warm front; visibility moderate or poor, sea fog possible	Marks and landfalls harder to see; keep the plot going
Cold front	Heavy rain and squalls	A veer, often sharp, and gusty; poor visibility in the rain	Be reefed and clear of hazards before it arrives
Behind the cold front	Clearing skies and showers; pressure rising	Gusty, squally in showers; good visibility outside them	Check your anchorage against the new wind direction
Occlusion (nearer the centre)	A band of cloud and rain	A wind shift, typically a veer, as it passes	Treat it like a front

On the chart above the island sits south of the depression’s track. Its wind through the day: Now southerly (1026 hPa), +6 h southerly (1022 hPa), +12 h south-westerly (1018 hPa), +18 h westerly (1017 hPa), +24 h westerly (1022 hPa). The pressure falls as the fronts approach and is rising again by +24 h, behind the cold front, while the wind veers from southerly to westerly. For a navigator, the shifts matter most. Kerran Roads is sheltered from southerly winds but open to the north (Lesson 32), so an anchorage that suits a southerly can become exposed once the wind veers into the north-west.

Wind strength: the Beaufort scale

Forecasts give the wind as a Beaufort force: a band of mean wind speeds. Gusts can be much stronger than the mean, especially in showers and squalls, so reef for the gusts. Learn the bands you will sail in.

Force	Description	Mean wind	Open sea
0	Calm	less than 1 kn	Like a mirror
1	Light air	1–3 kn	Ripples
2	Light breeze	4–6 kn	Small wavelets; crests don’t break
3	Gentle breeze	7–10 kn	Large wavelets; crests begin to break; a few white horses
4	Moderate breeze	11–16 kn	Small waves becoming longer; fairly frequent white horses
5	Fresh breeze	17–21 kn	Moderate waves; many white horses; some spray
6	Strong breeze	22–27 kn	Large waves begin to form; extensive white foam crests; some spray
7	Near gale	28–33 kn	Sea heaps up; foam blown in streaks
8	Gale	34–40 kn	Moderately high waves; crests break into spindrift
9	Severe gale	41–47 kn	High waves; dense streaks of foam; spray may reduce visibility
10	Storm	48–55 kn	Very high waves with overhanging crests; the sea looks white
11	Violent storm	56–63 kn	Exceptionally high waves
12	Hurricane force	64 kn or more	Air full of foam and spray; sea completely white

The descriptive names are those used in UK forecasts; some versions of the scale call force 9 a strong gale.

InteractiveKnots and forces

Drag the slider (or use the arrow keys) to a mean wind speed, or jump to a force.

Mean wind speed: 18 kn

Force 5 · Fresh breeze

Mean wind 17–21 kn

Open sea: Moderate waves; many white horses; some spray.

The sea description is for open water. The sea gets rougher with every step up the scale, and in shallow water, near headlands and with the stream against the wind it can be much worse than the wind alone suggests.

Worked example

What does this forecast mean in knots, and when?

1Forecast issued 0600 BST: “South-westerly 4 or 5, increasing 6 soon, veering north-westerly 6 later.”
24 or 5: force 4 is 11–16 kn, force 5 17–21 kn. Plan for a mean of 11–21 kn this morning, with gusts above that.
3“Increasing 6 soon”: soon is 6 to 12 hours from the time of issue, so between 1200 and 1800. Force 6 is 22–27 kn.
4“Veering north-westerly 6 later”: later is more than 12 hours from issue, so after 1800. Veering is clockwise, south-west through west to north-west — the typical veer behind a cold front.

11–21 kn now, 22–27 kn between 1200 and 1800, then north-westerly after 1800: not a night for Kerran Roads.

Wind against tide

When a tidal stream runs against the wind, the waves meet the moving water head on: they become shorter and steeper, and break more readily. Where the stream runs hardest, off headlands, this can build into overfalls — breaking, confused water. On the practice chart the strongest streams are off Gull Point (diamond B) and Kerran Head (diamond C), so those are the places to time with care. With the stream running the same way as the wind, the sea is smoother, even in the same wind.

The stream table (Lesson 28) tells you when. A stream is against the wind when it sets roughly towards the direction the wind is coming from. Remember that directions in the table are where the stream goes, but wind directions are where the wind comes from.

Tidal streams referred to HW Port Kerran — direction °T, rates in knots at springs and neaps
Hours	B49°48.3'N 004°03.2'W	C49°46.4'N 004°20.4'W
Hours	DirSpNp	DirSpNp
6 before	2301.60.8	2501.40.7
5 before	2351.00.5	2550.90.4
4 before	3000.30.1	3400.30.1
3 before	0401.00.5	0650.90.4
2 before	0451.90.9	0701.70.8
1 before	0502.41.2	0752.11.0
HW	0502.21.1	0752.01.0
1 after	0501.80.9	0751.60.8
2 after	0551.10.5	0801.00.5
3 after	1200.30.1	1600.30.1
4 after	2201.10.5	2451.00.5
5 after	2252.01.0	2501.80.9
6 after	2302.31.1	2552.01.0

Worked example

Leaving Kerran Roads round Gull Point, Sat 12 Jun, north-easterly 4 or 5

1The north-easterly is now blowing into Kerran Roads, which is open to the north, so you decide to move round to Port Kerran. HW Port Kerran is 1152 BST. Find when the stream off Gull Point (B) runs against a wind from 045°.
2The flood at B sets 040°–055°T in the rows HW−3 to HW+2: straight into a north-easterly. Each row covers the hour centred on its time, so this runs from 0822 to 1422 BST.
3It is strongest in the HW−1 row, 1022–1122, at 1.8 kn today (interpolated for the day’s range, as in Lesson 28).
4Around 1422–1522 the stream is weak and turning; from 1522 the ebb sets south-west, with the wind (as it also did until 0722 this morning). Southbound, the ebb is also a fair stream.

Wind against tide off Gull Point 0822–1422 BST, worst 1022–1122. Round it on the ebb after about 1522, or before 0722.

Compare the Lesson 32 passage in its south-westerly: it rounds Gull Point about 1238–1307 on the north-east-going flood, with the wind. Leave it too late and from about 1522 the ebb runs straight into the south-westerly instead. The same headland, the same stream, a different wind: a different best time.

InteractiveWind against tide off the headlands

Choose a day and the forecast wind, and see when the stream off Gull Point or Kerran Head runs against it. Rates come from the day’s range.

Day (June 2027)Forecast wind

HW Port Kerran: 1152 BST (4.6 m). Stream at diamond B, off Gull Point, from the table rows (rates for the day’s range).

Against the windWith the windAcross the windWeak (near the turn)

Wind against tide off Gull Point: 0822–1422 BST (strongest 1.8 kn, 1022–1122); 2042–2200 BST (strongest 1.5 kn, 2142–2200). Expect shorter, steeper seas, and overfalls off the headland when the stream is strong.

Time (BST)	Stream at B	With a north-easterly
0501–0542 HW+6	230°T 1.6 kn	With the wind
0542–0622 HW−6	230°T 1.2 kn	With the wind
0622–0722 HW−5	235°T 0.7 kn	With the wind
0722–0822 HW−4	300°T 0.2 kn	Weak (near the turn)
0822–0922 HW−3	040°T 0.7 kn	Against the wind
0922–1022 HW−2	045°T 1.4 kn	Against the wind
1022–1122 HW−1	050°T 1.8 kn	Against the wind
1122–1222 HW	050°T 1.7 kn	Against the wind
1222–1322 HW+1	050°T 1.4 kn	Against the wind
1322–1422 HW+2	055°T 0.8 kn	Against the wind
1422–1522 HW+3	120°T 0.2 kn	Weak (near the turn)
1522–1622 HW+4	220°T 0.8 kn	With the wind
1622–1722 HW+5	225°T 1.5 kn	With the wind
1722–1802 HW+6	230°T 1.8 kn	With the wind
1802–1842 HW−6	230°T 1.3 kn	With the wind
1842–1942 HW−5	235°T 0.8 kn	With the wind
1942–2042 HW−4	300°T 0.2 kn	Weak (near the turn)
2042–2142 HW−3	040°T 0.8 kn	Against the wind
2142–2200 HW−2	045°T 1.5 kn	Against the wind

Sea breezes, land breezes and fog

Sea breeze. On a sunny day with little wind on the synoptic chart (a light gradient wind), the land warms faster than the sea. Air rises over the land and cooler air flows in from the sea to replace it: a sea breeze, blowing onshore by the afternoon and dying away around dusk. Near the coast it can be quite different from the forecast wind, so a calm morning may give you a lively afternoon beat — or a rough entrance on a sunny coast.

Land breeze. At night the land cools faster than the sea and the flow reverses: a usually lighter breeze blowing off the shore.

Fog matters more to a navigator than almost anything else, and the two common kinds behave very differently (Lesson 34 covers navigating in it):

	Radiation fog	Sea (advection) fog
How it forms	Over land, on clear, calm nights as the ground cools	Warm, moist air moving over colder sea
Where and when	Drifts out over harbours and estuaries; worst around dawn	At sea and on the coast; around the UK commonest in spring and early summer
How it clears	Usually during the morning	Can last for days; the sun doesn’t burn it off — it needs a change of wind or air
Leaving harbour	Wait a few hours; it is often clearer outside	Don’t set off expecting it to lift; wait for the forecast to change
Entering harbour	A dawn arrival may find the harbour fogbound: wait in safe water, or time arrival for later in the morning	It may cover the approach too: use your fog plan, or stay at sea in safe water away from shipping

Getting and reading the forecast

Get forecasts from more than one source, before you sail and during the passage:

The national met service’s shipping and inshore forecasts, broadcast on radio and published online — the inshore forecast is the one for coastal sailing.
Coastguard maritime safety broadcasts on VHF, which include forecasts and warnings.
NAVTEX, a receiver that automatically prints or displays forecasts and navigational warnings.
GRIB files: gridded output from computer weather models, shown on apps and plotters.

A forecast’s words have exact meanings. UK forecasts and gale warnings use three timing words, measured from the time the forecast was issued — not the time you hear it:

Timing word	Meaning	Issued 0600: expect it
Imminent	within 6 hours of the time of issue	before 1200
Soon	6 to 12 hours from the time of issue	between 1200 and 1800
Later	more than 12 hours from the time of issue	after 1800

Visibility	Meaning
Good	more than 5 M
Moderate	2–5 M
Poor	1000 m–2 M
Very poor (fog)	less than 1000 m

GRIB files are attractive — arrows for every hour, anywhere — but they come from a model that smooths the weather over each square of its grid. They under-show gusts and can miss local effects such as sea breezes. Use them as a guide to the pattern and its timing, alongside the official forecast and what you can see.

From forecast to decision

Read the whole forecast, then ask three questions: what is the worst it allows, when, and where will you be then?

Go when the forecast for the whole passage, with time in hand, is within what your crew and boat can manage, and the plan still works with it — tides, headlands and the night’s anchorage included.
Wait when the problem is short-lived and you are safe where you are: radiation fog that should clear, a front about to pass, a stream about to turn.
Change the plan when the forecast breaks part of it: a different departure time, route, destination or overnight stop — or not today.

InteractiveDecode the forecast, then decide

Each fictional forecast comes with a plan. Tap the underlined phrases to decode them, then choose: go, wait or change the plan.

Isle of Kerran waters (fictional)

Issued 0500 BST, Sat 5 Jun 2027 · next 24 hours

Wind
Sea state
Weather: otherwise fair.
Visibility

Tap an underlined phrase to decode it.

Your plan

It is 0630 in Port Kerran after a clear, still night. The harbour is in thick fog: from the pontoon you can’t see No 3 and No 4 buoys. You planned to leave at 0700 for a day sail.

What do you do?

The forecast tells you the worst you might meet and when; the plan decides where you will be when it comes.

Key terms

Synoptic chart: A map of sea-level pressure at one time, with isobars, highs, lows and fronts.
Isobar: A line joining places of equal pressure. Close together: strong wind; far apart: light wind.
Depression (low) / anticyclone (high): Northern hemisphere: wind blows anticlockwise round a low, clockwise round a high, crossing the isobars slightly towards lower pressure.
Buys Ballot’s law: Northern hemisphere: with your back to the wind, the low pressure is on your left.
Veer / back: A clockwise / anticlockwise shift in the wind direction.
Warm sector: The mild, moist air between a depression’s warm and cold fronts; often drizzle and poor visibility.
Occlusion: Where the cold front has caught up with the warm front: a band of cloud and rain.
Beaufort force: A band of mean wind speeds: force 4 is 11–16 kn, force 6 22–27 kn, force 8 (gale) 34–40 kn.
Wind against tide: A stream running against the wind: shorter, steeper seas and overfalls off headlands.
Radiation fog / sea fog: Fog over land after a clear, calm night, usually clearing in the morning / warm, moist air over colder sea, which can last for days.
Timing words: From the time of issue: imminent within 6 h, soon 6–12 h, later more than 12 h.
GRIB file: Gridded computer-model forecast data: smoothed, under-shows gusts — a guide only.

Exercises

Check your understanding. You get a second try at each question, then the worked answer. Score 70% or more to complete the lesson — your progress is saved in this browser.

On this fictional chart, which best describes the wind at X?

In the northern hemisphere you stand with your back to a south-westerly wind. Where is the low pressure?

As a cold front passes, the wind shifts from south-west to north-west. What has it done?

The wind is from 200°T and backs 40°. From what direction does it blow now?

A forecast gives force 6. What is the highest mean wind speed in force 6, in knots?

The mean wind over the last ten minutes is a steady 30 knots. What Beaufort force is that? (Give the number.)

Heavy rain and squalls, then the wind veers and the sky clears. What has most likely just passed?

After a clear, calm night, Port Kerran is in thick fog at dawn. The forecast says “fog patches at first, otherwise fair”. What is the best plan?

A forecast issued at 0600 BST says “increasing 6 soon”. When is force 6 expected?

Saturday 12 June, HW Port Kerran 1152 BST. The forecast is north-easterly 4 or 5, and you are leaving Kerran Roads for Port Kerran, round Gull Point. Which time is the worst to be off Gull Point?

0 of 10 answered · 0 correct

Weather drills

Beaufort forces, veer and back, Buys Ballot, forecast words, wind against tide and the synoptic chart: five fresh questions each round.