Collection: Field Notes — Old Fashioned Seamanship

Series Hub: Coastal and Offshore Passage Planning

Subject: The Bay of Biscay passage — the geography of the bay, its swell regime, the Atlantic depression track and the Azores high, the Iberian heat low and the Portuguese northerly, crossing strategies, Ushant and the Raz, when to go and when to wait

How to Plan a Bay of Biscay Passage — Atlantic Swell, the Storm Track, the Iberian Heat Low, and Timing the Crossing

A Bay of Biscay passage commits the boat to a body of water that receives Atlantic storm swell with no shelter, runs over a four-thousand-metre abyssal plain, and shallows hard onto the Breton, Vendée, and Basque coasts. The synoptic picture, the swell period, the season, and the route choice are the inputs. The Galveston story is what to remember about why the swell is the warning system rather than the chartplotter.

On September 8, 1900, residents of Galveston, Texas, remarked on the extraordinary swell rolling into the harbour on an otherwise clear and calm day. They did not know what it meant. The following day a hurricane killed more than six thousand people. The swell had outrun the storm that created it, arriving in time to serve as a warning if anyone had known how to read it. Tristan Gooley tells the story in How to Read Water for exactly the reason it still matters now. The mechanism has not changed.

The Bay of Biscay receives swell from the North Atlantic storm track with nothing between it and its source but open ocean. The fetch from the typical low-pressure formation area west of Ireland or south of Iceland to the northern corner of the bay is roughly a thousand miles over deep water. By the time it reaches Ushant and begins to compress on the shallowing continental shelf, that swell has had enough fetch and duration to develop period and height that bear no relation to the current wind at the bay’s surface.

Reading the swell before and during a Biscay passage is not a secondary skill. It is the primary warning system for what is coming from the southwest when the synoptic chart is several hours old and the next forecast has not yet arrived.

The geography that makes Biscay what it is

The bay is roughly triangular. The northern shore runs from Ushant down the Brittany and Vendée coast to the Loire. The eastern shore continues from the Loire down the Basque and Cantabrian coast to Cape Finisterre. The open southwestern corner faces the Atlantic. The bay is deep. The abyssal plain reaches four thousand metres in the centre, which means swell travels across it largely unmodified until it hits the shallowing edges. On the northern and eastern shores the depth changes rapidly as the continental shelf rises, compressing the swell and increasing its steepness just as a sailor attempting to enter a Breton harbour or a Basque port is dealing with it.

The direct distance from Ushant to Finisterre is roughly 440 nautical miles. Across open ocean this is a two-to-four-day passage for a yacht, depending on wind. The French shore route, hugging the Brittany and Vendée coast before crossing the southern portion of the bay, adds perhaps 200 miles but keeps the boat in shallower and more sheltered water for longer and offers harbour options if the weather deteriorates.

The choice between routes is not purely about distance. It is a risk management decision about which phase of the synoptic cycle the passage will encounter, and what the consequences of being caught in bad weather are in each position.

The synoptic pattern over the bay

The Atlantic depression track in winter and early spring passes close enough to Biscay’s northern approaches that the bay regularly sits in the southwest quadrant of a developing low. The zone of tightest isobar spacing, strongest gradient wind, and most active frontal weather. The warm sector southwesterly that precedes a cold front produces the long Biscay swell from the southwest at its most developed. The cold front itself brings the wind veer and the post-frontal northwest that sets up a confused cross-sea as cold air arrives over the previous swell pattern.

In summer the picture shifts. The Azores high expands northward and eastward, dominating the central North Atlantic and pushing the storm track further north toward Iceland. June, July, and August typically offer extended windows of light to moderate southwesterly or northwesterly wind across the bay with settled barometer and manageable swell. These are the months when most yacht passages take place, and most yacht passages in summer go smoothly precisely because the mean synoptic state is more benign.

The transitional months are the most demanding. May and September. In May the summer pattern has not yet established and deep Atlantic lows can still track close to the bay. In September the summer pattern is breaking down. Depressions are deepening again, the storm track is moving south, and the swell is rebuilding. The classic Biscay horror stories, the passage that left in settled conditions and encountered a vigorous depression the forecast had not adequately predicted, concentrate in these months.

The Iberian heat low and the Portuguese northerly

Simon Rowell’s description in Weather at Sea of the Iberian heat low is specific and practically important for any southbound passage. Intense solar heating over the Iberian plateau creates a persistent thermal low in summer that, when it presses against the Azores high to the west, generates a strong pressure gradient down the Portuguese coast. The result is a reliable northerly. The Portuguese Trade. It can reach twenty to thirty knots between Finisterre and Lisbon even when conditions elsewhere are benign.

The practical consequence is worth absorbing before the passage rather than after. A yacht that crosses the bay without difficulty and rounds Finisterre on a settled forecast may encounter a Force 5 to 6 northerly on the Portuguese coast that has nothing to do with the Atlantic frontal pattern and everything to do with the land mass heating. Rowell notes it is common in summer to have light winds across Biscay followed by twenty knots or more from the north as soon as Finisterre is rounded and the Portuguese coast begins. A southbound passage plan that does not account for this transition will be surprised by it.

The northerly is a reliable, persistent wind that a properly trimmed boat can work with all day. The problem is the contrast with what came before it. On a boat that has been reaching across Biscay on a fair southwesterly, the sudden hard beat into a Force 6 northerly in short steep Atlantic swell on a lee shore is a significant gear change that requires preparation rather than surprise.

Swell as the warning system

Gooley’s description in How to Read Water of the relationship between swell and approaching storms is the most useful single piece of meteorological knowledge for a Biscay passage. Storm systems generate swell that travels faster than the storm itself. The longer-period swell from a developing Atlantic depression arrives at the bay before the depression’s weather does. The approaching storm signals itself through the sea before it signals itself through the sky.

The specific diagnostic Gooley identifies is wave period. As the storm approaches, the period of the swell decreases. Successive crests arrive more frequently. Long-period swell, waves with a long, slow, regular rhythm widely spaced, indicates a distant storm. Shorter period swell indicates the storm is closer. A swell period that has been decreasing over several hours is the most reliable at-sea indicator that conditions will deteriorate regardless of what the last forecast said.

The complementary sign is height. Swell height increasing on a day with no significant local wind indicates that energy arriving from distant weather is growing. On a clear day in Biscay with light winds and a building southwest swell of increasing height and decreasing period, conditions are deteriorating somewhere to the southwest and will reach the bay within twenty-four to forty-eight hours. This is the system that Galveston’s fishermen saw but could not read. On a yacht mid-Biscay it is the most important information available.

The observation requires only attention. No instruments. Count the seconds between wave crests past a fixed point on the hull. Record the count at regular intervals through the watch. If it is decreasing, take note. If it is decreasing rapidly, make a decision.

Ushant and the Raz du Sein

Ushant, the Ile d’Ouessant, is the extreme western point of Brittany and the turning mark for the north Biscay crossing. The passage between Ushant and the mainland runs through the Fromveur, a tidal race that reaches five or six knots on springs. The alternative south of Ushant, through the Chenal du Four between Ushant and the mainland rocks, is also tidal. Neither passage is taken casually. Both require careful timing against the tidal stream and attention to the sea state in the acceleration zones around the headlands.

The Raz du Sein, the passage between the Pointe du Raz and the Ile de Sein, is the alternative southern exit from the Channel approaches into Biscay for a boat coming from the east. It carries a tidal stream of up to four and a half knots on springs and produces severe overfalls in any combination of strong wind and adverse stream. Gooley’s description of tidal currents in How to Read Water, accelerating from slack to peak flow at the mid-point between tides with half the total flow occurring in the central two hours, applies here in its most dramatic form. Timing the Raz correctly means passing within an hour either side of slack water. Missing it means waiting or committing to conditions that have damaged more vessels than all but a handful of Biscay storms.

Both Ushant and the Raz have been worked by traditional methods for centuries. Compass course. Lead line. Tidal timing. Visual transits. The chart of the Iroise Sea and the approaches to the Raz is one of the most detailed small-area charts published by the Service Hydrographique, and the traditional marks (the transit alignments that permit passage through the off-lying rocks without electronic assistance) are documented in the Admiralty pilots and the SHOM publications. Attempting either passage in fog without these resources is not advisable and not covered here.

The direct crossing versus the French shore

The two main strategies for a southbound passage are the direct offshore crossing, departing from the Ushant vicinity or Lorient and heading southwest for Finisterre or a Spanish port, and the French shore route, keeping within reasonable distance of the Breton and Vendée coast before cutting across the southern bay.

The direct crossing commits the vessel to open water for two to four days with no harbour option. In deteriorating conditions mid-bay, the nearest shelter may be two hundred miles away in any direction. The sea state in the central bay in Force 7 swell running onto a southwest-to-northeast wind against tide combination is not the same as Force 7 anywhere else in European waters. The deep water amplifies the swell to its full development. There is no shallowing or land shelter to dampen it.

The French shore route keeps harbour options available for longer. Lorient, La Trinité, La Turballe, St Nazaire, Les Sables d’Olonne, La Rochelle. Each is a possible stopping point if the weather deteriorates before the direct crossing has been committed. The route adds distance but reduces the consequence of a forecast error, which in the transitional seasons is a reasonable trade.

In settled summer conditions the direct crossing is the efficient choice. The Azores high is reliable, the swell is manageable, and the additional distance of the shore route is hard to justify. In late May, early June, or September, the shore route’s harbour access may be the most useful feature of the passage plan.

Timing the window

The synoptic pattern that offers the best Biscay crossing window is described throughout the Weather Forecasting series. A depression passes to the north, the cold front clears, the post-frontal Polar Maritime air moves in from the northwest, pressure rises, and a high-pressure ridge or a full high builds from the southwest. The passage window opens as the cold front clears and closes as the next depression approaches from the west.

How long the window lasts depends on the jet stream’s position and the speed of the systems. Rowell’s identification in Weather at Sea of the 500 hPa 5,640-metre contour as a proxy for the jet stream’s location applies directly here. If the contour shows the jet tracking well north of the passage area for the next five days, the window will be extended. If the contour is close to the latitude of Biscay, another system will arrive within forty-eight to seventy-two hours.

For a direct crossing of four days, the minimum useful window is five days, to give some margin. A forecast showing a five-day stable window at the time of departure should be verified at twenty-four-hour intervals by comparing the analysis chart against the forecast for that time, exactly as described in How to Read a Synoptic Chart. A forecast running ahead of the actual development, with conditions deteriorating earlier than predicted, is the signal to close the margin and if necessary seek the nearest shelter on the French shore.

Secondary depressions and the post-frontal trap

As described in The Anatomy of an Atlantic Depression, a secondary depression forming on the trailing cold front of a primary system can develop with great speed and arrive at Biscay with little warning. The primary low passes, conditions clear post-frontally, pressure rises, and the barometer begins to look promising. Then a secondary develops on the trailing front, or the primary recurves unexpectedly, and conditions deteriorate with a speed that belies the settled appearance of the post-frontal sky.

The specific sign to watch for mid-crossing is the barometer stalling or reversing. A rising barometer that stops rising and begins to fall is not a minor fluctuation to be noted and ignored. It is the primary instrument signal that the window is closing. Combined with a backing wind in what should be the post-frontal sector, and a south or southwest swell rebuilding after the post-frontal northwest normalises, it is a direct instruction to assess harbour options and distance to shelter.

The northbound passage

A northbound Biscay passage, from Spain or Portugal toward the Channel, has the reverse synoptic problem. The Portuguese northerly that makes the southbound passage uncomfortable in summer is typically a fair wind for the northbound passage. Broad reaching with the northerly until Finisterre is rounded, then running before or fetching across the bay in the predominant westerly. The hazard is the same system timing in reverse. The passage that looks set to arrive in a post-frontal window may instead be committed to the bay as a depression approaches from the southwest.

The swell reading skill matters particularly northbound because the approaching system is coming from astern and ahead of the boat at the same time. Swell building from the southwest on a northbound passage means the system is approaching from the quarter. The direction of travel and the direction of the threat are both southwest. The swell period shortening astern is the warning. The decision point is how far north the boat has progressed and whether the northern French harbours or the Spanish coast aft is the nearer option.

Navigation in the deep water

The open bay in settled conditions offers little for the traditional lead line navigator. The central bay is too deep and featureless for depth to carry positional information. The DR plot, the compass, the chip log, and the barometric trend are the primary instruments. (If the DR habit has gone rusty, the VAKA Dead Reckoning training tool is a way to practise it without needing the boat in the water.)

The French and Spanish coasts on approach are where the traditional skills apply fully. The approach to the bay’s northern shore uses the same lead-line technique described for the East Anglian coast in The Lead Line — Depth Sounding. The depth profile across the continental shelf as the boat closes the coast is distinctive enough to confirm position and correct DR error before the boat is in the traffic zones of the Ushant TSS.

The swell itself, as described in The Ocean’s Long Memory — Reading Swell, carries directional information. The dominant Atlantic swell in the bay sets from the west-southwest. A boat that has maintained a consistent DR course can cross-check its track against the swell direction as one input to the navigation picture.

Simon Rowell’s Weather at Sea (Fernhurst Books) is where the synoptic framework here comes from. The depression track over Biscay. The Iberian heat low mechanism and the Portuguese coast northerly. The Azores high expansion and its seasonal effect on the storm track. The advection fog regime. Tristan Gooley’s How to Read Water (Sceptre) is where the swell analysis lives. Fetch, period, the relationship between decreasing period and approaching storms, and the Galveston example. The companion VAKA posts working the same material are The Ocean’s Long Memory — Reading Swell and What Waves Know. The passage planning hub is here.

At VAKA I design and build boats that don’t destroy the environment. Find the plans as they are finalised at VAKA Plans and the full field notes here. VAKA. Traditional craft and natural materials. Nottingham. 2026.