Weather and strategy

Lesson 1 of 7

Pressure and wind: reading the map and understanding the fleet's choices

Why start with pressure and wind?

For a sailing boat, the wind is the engine: almost every routing choice comes down to where the wind is, how much of it there is and which direction it blows from. On the cartography, the Wind layer is the place to start: gusts, waves and clouds will make more sense later on.

Wind is defined as a horizontal or vertical movement of air masses in the Earth's atmosphere, caused by air flowing from areas of high pressure (anticyclonic) to areas of low pressure (cyclonic).

It is therefore born from pressure differences between one area and another: understanding where the highs and lows are tells you not only where the wind is today, but also where it will be tomorrow.

In this lesson we learn to understand and read pressure and wind; technical terms are explained in the glossary.

The basics

What you see on the map

When you switch on Forecasts → Wind, the map fills with colour: the colour tells you how much wind there is, the animated lines tell you where it comes from and where it goes.

• Colour is wind strength. The scale runs from dark and light blue (light wind) through green and orange (moderate wind) to red and purple (strong wind).
• The unit is the knot (kn). One knot is one nautical mile per hour, or 1.852 km/h. So 20 knots is about 37 km/h.
• Particles show the flow. The Particles layer animates the wind like "smoke carried by the air": at a glance you can see where the flow is steady, where it bends and where it is stronger or weaker.
• Direction is given as where the wind comes from. A "northerly wind" blows from north to south. In degrees, 0° is north, 90° east, 180° south, 270° west. This is the opposite of a boat's heading, which tells you where the boat is going.
• Boat data. Click on a boat or a skipper and the panel shows the forecast wind at its position, in knots and degrees.
• Time matters. The forecast changes over time: clicking the panel at the top right shows the weather forecast for the coming hours, while moving the time slider at the bottom shows where the boats were and what the conditions were on previous days.

The average wind in the forecasts is measured 10 metres above the surface, the standard measuring height. The value shown is the true wind, not the apparent wind felt on board.

On board, the wind you feel (the one that decides which sail to hoist and how to trim it) is the combination of the true wind and the wind created by the boat's own speed.

Pressure and isobars

Atmospheric pressure is the weight of the column of air in the atmosphere pressing on a given area of the Earth's surface. It is measured in hectopascals (hPa), equivalent to the old millibars.

Pressure is closely linked to the influence of solar radiation: the Sun heats the Earth's surface unevenly, creating temperature differences. Warm, moist air is lighter and tends to rise, producing lower pressure; cold, dry air, on the other hand, is denser and tends to stay "heavy", producing higher pressure.

In general, high pressure (anticyclones) is associated with stable, clear and dry weather; low pressure (depressions) is associated with instability, cloud, wind and rain.

• The reference value. Average sea level pressure is about 1013 hPa. There is no fixed threshold between high and low pressure: what matters is the comparison with nearby areas.
• Typical values. An anticyclone is usually 1020 hPa or more. A shallow depression stays above 1000 hPa, a moderate one sits between 980 and 1000, a deep one drops below 980.
• Isobars are lines joining points of equal pressure, like the contour lines on a mountain map. They are usually drawn every 4 hPa, and some are labelled with their value.
• The centres. The centre of an anticyclone is marked H (A on French and Italian charts), the centre of a depression L (D on French charts, B on Italian ones). The central pressure is often shown alongside.
• The shapes. A ridge is a U shaped tongue of high pressure extending from an anticyclone, with rounded isobars, which favours stable conditions. A trough is a tongue of low pressure extending from a depression, with isobars often shaped like a U or a V. As it passes, a trough usually brings instability and a wind shift.
• Fronts. On classic weather charts, fronts are coloured lines: with triangles for a cold front, with semicircles for a warm front. A warm front is where warm air catches up with cold air, a cold front where cold air catches up with warm air. A cold front usually matches a V shaped kink in the isobars, in other words a trough.

How to read them, in three steps. First: find the H and L and their values. Second: look at the spacing between isobars, because the closer they are, the stronger the wind. Third: scroll through the forecasts over time, because the isobars move with the weather systems, and the wind moves with them.

Where wind comes from

Wind is born from the pressure difference between two areas: air moves from high pressure towards low pressure, like water running down a hill.

• Anticyclone (high pressure). Sinking air, usually stable weather, light wind at the centre and widely spaced isobars, pressure often above 1020 hPa.
• Depression (low pressure). Rising air, cloud and rain, tightly packed isobars and often strong wind around the centre, pressure often below 1000 hPa.
• Tight or wide isobars. Where the isobars are close together, the wind map shows strong wind colours; where they are far apart, light wind colours.

In our hemisphere, the wind turns clockwise around an anticyclone and anticlockwise around a depression. By watching the particles you can therefore recognise an anticyclone (a large clockwise swirl with little wind at the centre) or a depression (an anticlockwise swirl with strong wind around it).

What it means at sea

A number in knots becomes concrete once you translate it into sea state. The Beaufort scale, often used in marine forecasts, does exactly that: it is an empirical scale describing wind strength from 0 to 12 according to its visible effects at sea and on land. The Douglas scale, on the other hand, describes the sea state according to wave height.

The knot and wave values come from the Beaufort and Douglas scales published by the Met Office. Wave heights apply to a fully "developed" sea: when the wind builds, the sea takes time to grow, and when it drops, the sea stays rough for a while.

Going further

Why the wind doesn't blow straight towards low pressure

If only the pressure difference mattered, the wind would blow straight from high to low pressure. Yet out at sea it flows almost parallel to the isobars: this is the effect of the Earth's rotation.

1. The pressure gradient force pushes air at right angles to the isobars, towards lower pressure. The tighter the isobars, the stronger the push and the stronger the wind.
2. The Coriolis force, caused by the Earth's rotation, deflects moving air to the right in the northern hemisphere and to the left in the southern hemisphere.
3. Geostrophic wind. When the two forces balance, the wind blows parallel to straight isobars, with high pressure on its right.
4. Gradient wind. Around centres of high and low pressure the isobars are curved, and centrifugal force comes into play too. The result is anticlockwise rotation around depressions and clockwise rotation around anticyclones.
5. Friction with the surface slows the wind slightly near the sea. So the wind crosses the isobars a little: it flows out of the anticyclone and converges towards the centre of the depression.

A rule of thumb that follows. In the northern hemisphere, with the wind at your back, low pressure is on your left and high pressure on your right. By watching the particles on the map, you can work out where the pressure centres are even without seeing the isobars.

Wind and strategy in the Route du Rhum

The Route du Rhum course is almost free: between Saint-Malo and Guadeloupe each skipper chooses their route according to the weather and their own strategy. That is why pressure and wind are the key to reading every phase of the race.

The four big wind phases along the course

1. Leaving Brittany and Atlantic depressions

The first two or three days are often the toughest of the race. After the start, the fleet leaves the English Channel, rounds the tip of Brittany while staying outside the Ushant exclusion zone, and crosses the Bay of Biscay to Cape Finisterre, the north western corner of Spain.

In early November the North Atlantic is crossed by a "train of depressions" moving from west to east. In most cases a westerly or south westerly wind dominates at the start of the race: Guadeloupe lies to the south west, so the direct route is dead upwind.

West of Cape Finisterre there is a TSS (traffic separation scheme) for commercial shipping, which racing boats are not allowed to enter.

On the map: look for the large anticlockwise swirls to the north west, with strong colours around them. The line where the particles turn sharply from south west to north west is the cold front. Whoever crosses it first starts heading south.

• Ahead of the warm front of a depression the wind is south easterly, then veers south westerly after the warm front passes. In the warm sector, boats are pushed upwind towards the west, away from the shortest route and towards the cold front.
• The cold front is the trickiest moment: the wind swings sharply from south west to north west; gusts can reach 50 knots; the sea is confused, with the tail of the south westerly waves meeting new north westerly ones, on top of the long Atlantic swell.
• Behind the cold front the north westerly wind finally lets the boats head south west downwind, on course for Guadeloupe.

2. The Azores High

It takes its name from the Portuguese archipelago in the middle of the Atlantic, but it doesn't stay put over the Azores. The anticyclone shifts with the seasons and with passing depressions: it can move up into the Bay of Biscay, down towards Madeira, or out towards Bermuda. In November it is usually weaker than in summer.

At its centre the wind is extremely light, so nobody wants to cross it. Boats look for its edge, preferably on the southern side, where the wind comes from behind. On the northern side the wind is stronger, but you sail upwind in a more demanding sea. The strategy is always to find the right compromise to sail as few miles as possible without ending up in the windless zone.

On the map: it is the large clockwise swirl with a pale blue, almost windless centre. Watch how it moves as you scroll through the forecast: the gap to get round it to the north or south can open or close within a few hours.

3. Trade winds

Below the anticyclone blow the trade winds, the steady, constant winds flowing from the subtropical high pressure areas (around 30° latitude) towards the equatorial low pressure belt and the Caribbean. They are considered stable and moderate, averaging 15 to 20 knots, but they can drop or reach 40 knots under squalls. Their direction can vary by around a hundred degrees, from north north east to south east.

The position of the anticyclone affects where the trade winds begin. If it sits close to the Azores, the trades are found around the 30th parallel, between Madeira and the Canaries. If it shifts south, boats have to head down towards Cape Verde to pick them up. Once in the flow, boats try to stay on the most direct route to Guadeloupe, gybing several times.

Approaching the Caribbean, squalls become more and more frequent. Squalls are sudden, short lived weather events marked by a sharp increase in wind and often by heavy rain. Within minutes the wind can go from 10–15 to 35–40 knots and shift by tens of degrees, then collapse once the squall has passed. The most experienced skippers use the extra wind to accelerate, while avoiding getting stuck in the lull that follows.

On the map: the trade winds appear as a broad green band south of the anticyclone, with particles flowing from east to west. Individual squalls, however, are too small to show up: we will cover them in the lesson on clouds.

4. Rounding Guadeloupe

After more than 3,500 miles, it is often the last 60 that decide the result. Boats head for Tête à l'Anglais, a rocky islet north of the island, and leave it to port. They then sail down the west coast, leave the Basse-Terre buoy to starboard, round the Vieux-Fort lighthouse and finally head back up to the finish off Pointe-à-Pitre.

Here the wind no longer depends on the large Atlantic systems but on the island's terrain, dominated by the Soufrière volcano. The wind accelerates around the headlands, while in the lee of the island there are windless zones, with a more chaotic sea close to the coast. Passing Basse-Terre can take several hours, and the full rounding at least six.

On the map: look at the colours around the island, but with caution. A model with a grid of several kilometres only partly captures the effects of the terrain: here experience and local knowledge count most. In 2018 François Gabart reached Guadeloupe more than 35 miles ahead. Francis Joyon caught and passed him during the island rounding, winning by just over 7 minutes.

How to read the fleet's choices

When a class splits, ask yourself:

1. Who is heading for stronger wind? A boat moving away from the theoretical route is aiming to be better placed to catch better wind in the following days.
2. Who risks ending up in light wind? Staying close to the direct route can mean sailing into the anticyclone or into windless zones.
3. Who has the best VMC now, and who will have it in 24 to 48 hours? Scroll through the forecast: the boat that looks behind today could be the first to reach the new breeze.
4. How big is the bet? The more spread out the fleet, the more the weather updates over the following days will decide who is best placed.

An exercise to understand

Open the cartography, switch on Wind and Particles, and follow these steps.

1. Find the big swirls. Look for a clockwise swirl with little wind at its centre (the Azores High) and anticlockwise swirls with strong colours further north (the depressions).
2. Find the wind corridors. Where are the green and yellow areas leading towards Guadeloupe? Usually there is one to the north, linked to the depressions, and one to the south, in the trade winds.
3. Look at where the boats are. Is each one in a corridor, on the edge of the anticyclone or in light wind? Compare Speed and VMC in the panel.
4. Move time forward. Shift the forecast by 24 to 48 hours. Does the corridor move, widen, close? A boat that looks far off the route today could be in the best wind tomorrow.

The limits of the map

The map shows a forecast, in other words an estimate calculated by a model, not the wind measured on board.

• Models work on a grid. For example, the European ECMWF model has a grid spacing of about 9 km. Smaller phenomena, such as a single tropical squall, either don't appear or appear smoothed out.
• The wind shown is an average wind. Gusts can be much stronger: we will look at them in the next lesson.
• Boats use several sources. Skippers, and in classes where it is allowed their routers, compare several models and the data from their own sensors. A boat going "against" the map may have information the map doesn't show.
• The forecast gets updated. A situation that looks clear today can change with the next update, and the further ahead a forecast looks, the more uncertain it is.

Lesson 2 of 7

Gusts: how much stronger the wind can get

Why gusts matter

The Wind layer shows an average wind, but a boat takes much more strain in the gusts than in the average wind. For a solo skipper it is essential to consider the gust forecasts too when deciding which sail to hoist and how much sail to carry.

In this lesson we learn to read the Gusts layer together with the Wind layer: the difference between the two tells you how stable the wind is, and whether it will be steady or gusty. We assume you have done the lesson on pressure and wind; technical terms are explained in the glossary.

The basics

What you see on the map

When you switch on Forecasts → Gusts, the map shows the strongest gust forecast at each point: it is always equal to or higher than the average wind at the same point.

• Colour is gust strength, in knots, on the same colour scale as the Wind layer.
• The comparison is the key information. Switching between Wind and Gusts at the same point and the same time lets you start to estimate how stable the wind is: if the colour barely changes, the wind will be steady; if it "lights up" a lot, the wind will be irregular.
• Time matters here too. Gusts change as weather systems pass: scrolling through the forecasts shows you when the toughest phase will end.

What is a gust

A gust is a sudden, brief increase in wind speed compared with its average.

The average wind is the wind at 10 metres above the surface averaged over 10 minutes: it is the wind used in marine forecasts and in the Beaufort scale. The gust is the maximum wind speed averaged over 3 seconds during the observation period.

According to Météo-France, gusts are generally about 40% stronger than the average wind. With 20 knots average, gusts of around 28 knots are therefore normal. Under thunderstorms, however, they can be much stronger and change direction unpredictably.

What it means at sea

A boat set up for the average wind finds itself overpowered in a gust: it heels, becomes hard to steer and puts mast, sails and rigging under strain. That is why skippers choose their sails with the gusts in mind, not just the average wind, accepting compromises between the boat's safety and performance when they have to.

Rule of thumb: never read the Wind layer on its own; it is the ratio between gusts and average wind that tells you how "nervous" the wind will be. An example: 25 knots average with gusts to 30 is demanding but manageable; the same 25 knots with gusts to 45 is a completely different scenario.

Going further

Why the wind is sometimes steady and sometimes gusty

Gusts can arise in several ways. Two worth mentioning are friction with the sea surface and vertical air movements. The second mechanism produces the strongest gusts.

1. Friction and turbulence. The sea slows the wind in the lowest layers, while a little higher up the air moves faster. This difference creates eddies and turbulence: small, short, random gusts that are always present offshore.
2. Stable air. When the air is stable, meaning there is no significant vertical movement, surface air doesn't mix with the air above and gusts remain weak and random. This typically happens under a large anticyclone, in the warm sector of a depression and at night.
3. Unstable air and convection. When the air near the sea is warmer than the air above, it rises and forms cumulus clouds, while other air sinks to replace it. The sinking air comes from higher up, where the wind is stronger, and brings that speed down to the surface. The more vigorous the convection, the higher the air comes from and the stronger the gusts.
4. Showers and cumulonimbus. Under large thunderclouds, rain drags air downwards at high speed. When it reaches the sea, this air spreads out in all directions and produces very strong gusts, from unpredictable directions, over an area a few kilometres across.
5. Behind the cold front. After a cold front passes, cold and unstable air arrives. With the same isobar spacing, the wind here is stronger and much gustier than in the warm sector ahead of the front.

Rule of thumb: cold air flowing over a warmer sea becomes unstable, and unstable air is gusty. This is one of the reasons why, in the lesson on temperatures, we will compare air temperature with sea temperature.

Gusts and strategy in the Route du Rhum

Gusts matter above all at three points on the course: behind the cold fronts at the start of the race, under the trade wind squalls and during the rounding of Guadeloupe. In the Azores High, on the other hand, the problem is the lack of wind, not the gusts.

The phases where gusts matter

1. Leaving Brittany and cold fronts

The passage of an active cold front is the trickiest scenario of the first phase: skippers can expect gusts of up to 50 knots and a sharp wind shift from south west to north west. Behind the front the air is cold and unstable, so the wind stays gusty even when the average drops.

On the map: compare Wind and Gusts along the frontal line and just behind it. Where the gusts "light up" much more than the average, boats will probably be sailing with reduced sail and conservatively. In the warm sector, ahead of the front, the difference between the two layers is usually smaller.

3. Trade winds and squalls

The trade winds average 15 to 20 knots, but under squalls they can reach 40. Approaching the Caribbean, squalls become more frequent: within minutes the wind can go from 10–15 to 35–40 knots and shift by tens of degrees. After the squall, the wind typically collapses.

The most experienced skippers use the gusts to accelerate, but they always have to find the compromise between two situations: an overpowered boat that gets knocked flat by the strongest gusts, and an underpowered boat in the lighter wind after the squall.

On the map: individual squalls are too small for the model to see. The gusts forecast in the trade winds should therefore be read as a sign of unstable air, not as the exact position of the squalls. We will cover this in the lesson on clouds.

4. Rounding Guadeloupe

The island's terrain, dominated by the Soufrière, strongly disrupts the trade winds: the wind accelerates around the headlands and collapses in the lee of the island. When it does manage to cross the high ground, the wind can create violent gusts in the areas below, alternating with calm patches.

On the map: here the model is unreliable, because its grid is too coarse to represent the terrain properly. Use the forecast gusts only as a general indication.

How to read the fleet's choices

1. Is a boat going slowly in strong conditions making a mistake? Not necessarily: it may have reduced sail in anticipation of the gusts. In a two week race, breaking something costs far more than slowing down for a few hours.
2. Who is avoiding an area of strong wind? A route with the same average wind but lower gusts can be faster in practice, because it lets the boat carry more sail with less risk.
3. Are all boats the same? No: older or more fragile hulls, like those in the Vintage classes, have different margins from an IMOCA or an Ultim.

An exercise to understand

Open the cartography and pick a boat leading its class.

1. Read the average wind. With the Wind layer, note the value where the boat is.
2. Read the gust. Switch to the Gusts layer at the same point and the same time, and note the value.
3. Work out the difference. If the gust is about 40% above the average wind, conditions are normal. If it is much higher, the air is probably unstable: look for a cold front or an area of convective cloud.
4. Look ahead. Scroll the forecast 12 to 24 hours ahead along the boat's route: will it enter a more or less gusty area? Which sail would you expect it to carry?

The limits of the map

The gust on the map is a model estimate, not a measurement: the model doesn't simulate individual gusts, it calculates how strong they could be from the conditions it forecasts.

• Squalls are invisible. Phenomena a few kilometres across, such as a single squall or thunderstorm, are smaller than the model grid. The most violent gusts of the race may therefore not show up on the map.
• The gust is given at 10 metres. The masthead of an IMOCA is 29 metres above the sea, where the wind is stronger.
• Close to the coast it is less reliable. High ground, headlands and islands create local turbulence and gusts that the model only partly represents.

Lesson 3 of 7

Clouds and squalls: reading the sky on the map

Why clouds matter

Clouds are the visible sign of how the air is behaving in the atmosphere, and the type of cloud tells you what kind of wind to expect. Where air rises, clouds form: a uniform grey layer usually means steady wind and rain; isolated, towering cumulus clouds mean gusts and squalls.

In this lesson we learn to read the Clouds layer and connect it to what we saw about wind and gusts in the previous lessons; technical terms are explained in the glossary.

The basics

What you see on the map

When you switch on Forecasts → Clouds, the map shows how much of the sky will be covered by cloud at each point.

• Cloud cover is the weather parameter giving the fraction of the sky (the visible half sphere above you) covered by cloud at a given moment. It is measured as a fraction of the sky covered; meteorologists express it in eighths (oktas): 0 is a clear sky, 8 a completely overcast sky. Model maps often show it as a percentage.
• The shape of the cloudy areas matters more than the colour. A long continuous band of cloud usually indicates a front, while scattered patches indicate convective clouds, in other words cumulus.
• Clouds move with the wind. As you scroll through the forecasts, watching how the cloud bands move is the simplest way to follow an approaching front.

Which clouds you meet at sea

To read the map correctly, you need to tell apart two big families.

• Layer clouds. They cover the sky evenly, over large distances. They form when a large mass of air rises slowly, as ahead of a warm front. They bring widespread rain but usually steady wind.
• Heap clouds. These are clouds with vertical development, isolated or in lines. They come from convection, when the air near the sea is warmer than the air above. The biggest grow into cumulonimbus, the clouds of thunderstorms and the most violent squalls.

What it means at sea

Under layer clouds, sailing is grey and wet, but the wind is predictable. Under cumulus, on the other hand, the wind keeps changing and is unstable.

A squall is a convective cloud under which conditions change within minutes: the wind can go from 10–15 to 35–40 knots, shift by tens of degrees and arrive with heavy rain, then collapse once the squall has passed.

Rule of thumb: a uniform band of cloud signals a slow change; scattered, towering clouds signal rapid, local changes.

Going further

How a squall is born and dies

A single storm cell usually lives for 30 to 60 minutes and goes through three stages.

1. Development. Warm, moist air rises from the sea, cools and condenses: a cumulus forms and grows upwards like a tower. At this stage there is little or no rain.
2. Maturity. The drops become too heavy and fall, dragging air with them: a downdraught forms next to the updraught. When it reaches the sea, the cold air spreads out and forms a gust front. This is the most intense stage: heavy rain, strong wind and sometimes lightning.
3. Dissipation. The downdraught takes over from the updraught, which no longer feeds the cloud. Light rain and light wind remain until the cloud dissolves.

Why some squalls last and others don't

Without wind, a squall dies on its own: its own rain chokes off the rising air and stops it from feeding itself. When a steady wind is blowing, as in the trade winds, the wind increases with height and tilts the cloud. The rain and cold air then fall away from the updraught, and the squall keeps feeding itself: the cold air spreading out at the surface lifts more warm air ahead of it, like a small front. These squalls can keep going even at night.

Squalls are more active where the water is warmer. In the North Atlantic trade wind zone, the most developed convective clouds are also more frequent at night.

How the wind changes around a squall

A squall moves with the existing wind: in the trade winds it travels from east to west, so it always arrives from windward of the boats. Under the cloud, the sinking cold air reaches the sea and spreads out in all directions. The wind felt on board is the sum of two things: the gradient wind already blowing and the air spreading out. That is why it is important to think about where the boat is in relation to the squall, using the gradient wind direction as the reference.

• Boat ahead of the squall, as it approaches from windward. The cold air spreads in the same direction as the general wind: the two add up and the wind increases, often suddenly.
• Boat behind the squall, after it has passed. The cold air spreads in the opposite direction to the gradient wind: the two cancel out, the wind drops and sometimes reverses.
• On the sides of the squall. The cold air spreads across the general wind: the wind changes direction, turning as if to move away from the cloud.
• Under the squall, if it forms right above the boat. The boat can be left almost without wind until the cloud moves away.

Rule of thumb: ahead of a squall the wind increases, behind it the wind drops. That is why skippers try to use the gust ahead of the cloud and get out of the lull behind it quickly.

Clouds and squalls in Route du Rhum strategy

Clouds help you read the sky in every phase of the course.

The phases where clouds matter

1. Leaving Brittany and fronts

An Atlantic depression brings different bands of cloud with it. Ahead of the warm front the air rises slowly and layer clouds form, with widespread rain and a steady south westerly wind. Along the cold front, on the other hand, the cold air lifts the warm air abruptly: cumulonimbus and squalls form, with the strongest gusts and the wind shift to the north west. Behind the cold front, the unstable air produces scattered showers.

On the map: look for the long cloud band that comes with the front and follow it over time. When it passes over a boat, expect the wind shift. The scattered patches behind the band indicate unstable, gusty air.

3. Trade winds and squalls

Approaching the Caribbean, squalls become more and more frequent and can open up big gaps between boats. They are often organised in lines or isolated cells.

For a solo skipper, night is the hardest time: squalls arrive unseen, and you have to rely on radar or on the stars disappearing.

On the map: in the trade winds, look for areas of scattered cloud and compare them with the Gusts layer. Where the two match, the air is unstable and squalls are likely. But remember that a single squall is too small for the model: the map shows where squalls are likely, not where they are.

4. Rounding Guadeloupe

The island's terrain strongly disrupts the trade winds and can encourage clouds to form over the high ground. In the final miles, squalls and windless zones in the lee of the island combine: this is one of the reasons why the island rounding can turn the rankings upside down.

On the map: the model's resolution is too coarse to show the clouds forming over the terrain. Here the map is of little help.

How to read the fleet's choices

1. Two boats close together with very different speeds? One of them may be under a squall, or in the lull that follows it. In a squally area, gaps can change within hours without anyone making a routing mistake.
2. A boat suddenly changes course? It may be avoiding a squall or, on the contrary, trying to catch one to use its gust.
3. Who is about to meet the front? By following the cloud band over time, you can predict which boat will get the wind shift to the north west first.

An exercise to understand

Open the cartography and switch on the Clouds layer.

1. Find the bands and patches. Is there a long continuous band of cloud? It is probably a front. Are there areas of scattered cloud? There, the air is convective.
2. Compare with the wind. Switch on Wind and Particles: does the wind change direction along the cloud band? If so, you have found the front.
3. Compare with the gusts. In the areas of scattered cloud, does the Gusts layer light up more than the average wind? If so, expect squalls.
4. Follow time. Scroll through the forecasts: which boat will meet the cloud band first? Which boats will enter a squally area at night?

The limits of the map

The Clouds layer shows a model estimate, which describes large systems well but not individual phenomena.

• A single squall is invisible. A squall is often less than a mile across, far smaller than the grid of a global model. The map shows where the air favours squalls, not where they are.
• Cover doesn't tell you thickness. A sky covered by layer clouds and one covered by cumulonimbus can have the same percentage of cover, with opposite consequences for the wind.
• Terrain effects are poorly represented. The clouds that form over the mountains of Guadeloupe are too small for the model.

Lesson 4 of 7

Waves: reading the sea state

Why waves matter

With the same wind, the sea can be an ally or an enemy. A long, regular swell from behind lets the boat slide along fast; a short, steep or confused sea slows it down, multiplies the impacts on the hull and takes its toll on both the boat and its performance.

That is why skippers don't just look for the best wind, but for the best compromise between wind strength and sea state. In this lesson we learn to read the Waves layer and connect it to the wind from the previous lessons; technical terms are explained in the glossary.

The basics

What you see on the map

When you switch on Forecasts → Waves, the map shows the forecast wave height at each point, in metres, and their direction in degrees.

• Colour is wave height. The scale runs from dark blue (calm sea) to red (rough sea), up to white (high sea).
• Boat data. Click on a boat and the panel shows the forecast wave height at its position.
• Time matters. Waves build and drop with the wind, but with a delay: scrolling through the forecasts you can see the sea building after the wind arrives and staying rough after the wind has already eased.

Wind sea and swell

At sea there are almost always two types of waves at the same time.

• Wind sea is made up of waves created by the local wind, meaning the wind blowing at that spot at that moment. These waves are shorter and irregular, and they die down when the wind stops.
• Swell (houle in French, onda lunga in Italian) is made up of waves born elsewhere, sometimes in a storm hundreds of miles away, which have travelled beyond the area where they formed. They have more regular crests and longer periods, usually over 8 seconds.

When the two types of waves come from different directions, this is called a confused sea: the sea becomes chaotic and the boat struggles to find a steady rhythm.

Height and period

The wave height in forecasts is the significant wave height: the average of the highest third of the waves. So it is not the highest wave you will meet: on average one wave in 10 is 1.25 times the significant height, and one in 1,000 reaches double.

The period is the time between two crests. It matters as much as height: for the same height, a short period means steeper, harder waves, while a long period means gentler ones.

What it means at sea

With 2 metres of long, regular swell from behind, a fast boat can "surf" and pick up speed. With 2 metres of short sea on the nose, the same boat slams, slows down and risks damage. The same number on the map can therefore mean two completely different situations.

Rule of thumb: don't just look at how high the waves are, but at where they come from in relation to the boat's route and whether they combine with waves from another direction.

Going further

What wave height depends on

The height of the sea depends on three factors:

1. Wind strength. The wind pushes on the back of each wave, a bit like a hand pushing a swing. As long as it is faster than the wave, it keeps pushing it and giving it energy, and the wave grows. As they grow, waves also become longer and faster; when they travel almost as fast as the wind, the wind can no longer push them and they stop growing. Wave height increases much faster than wind speed: according to the Beaufort scale, with about 10 knots the probable wave is 0.6 metres, with about 20 knots 2 metres, with about 40 knots over 5 metres. The wind doubles twice, while the wave becomes almost ten times higher.
2. Duration. Waves need time to grow: a squall bringing 40 knots for ten minutes churns up the surface and creates short, steep waves, but doesn't have time to build five metre waves; a gale blowing for a whole day does.
3. Fetch. This is the stretch of sea over which the wind blows unobstructed, with similar strength and direction. In a puddle, for example, even a very strong wind only creates ripples, because the wind has just a few metres of water to work on; in the ocean it has thousands of kilometres. In Brittany, with a strong westerly wind, the waves arrive after crossing the whole Atlantic and are very large; with the same wind from the east (that is, off the land), the fetch near the coast is very short and the sea stays relatively small.

Of the three factors, the one in shortest supply always wins. Strong but brief wind: small waves. Strong, long lasting wind but little space: small waves. Only when all three come together do you get the "fully developed" sea described by the Beaufort scale.

When they leave the area where they formed, waves become swell: the period lengthens and the height slowly drops, but they can travel thousands of miles.

Why the sea builds late

When a depression arrives the wind increases within a few hours, but the sea needs time and space to build. When the wind drops, on the other hand, the waves stay up for longer, often turning into swell. That is why, in the hours after a gale, a boat can find itself with little wind and a sea that is still big: the worst situation for sailing fast.

Waves against the current

When waves run against the current, they become shorter, steeper and higher, and break more easily. When they run in the same direction as the current, they lengthen and the sea calms down. Near the Breton coast, where tidal currents are strong, this effect noticeably changes the wave height.

Waves and the seabed: the Channel versus the Atlantic

In open water, waves don't "feel" the seabed. A wave starts to feel it when the depth drops below about half its length. In deep water, length depends on period: in metres, it is roughly 1.56 times the period squared. A 10 second wave, for example, is about 156 metres long and starts to feel the bottom in less than 75 to 80 metres of water.

Once it feels the bottom, the wave slows down and shortens, but keeps its energy: so it grows taller and steeper. When its height exceeds about one seventh of its length, the wave becomes unstable and breaks.

Comparing two situations on the Route du Rhum course:

The Channel is part of the continental shelf: the submerged edge of the continent, which extends down to about 200 metres deep. Beyond the shelf, the seabed drops quickly to the great ocean depths. For the Route du Rhum fleet, the first hours of the race are therefore sailed exactly where the sea tends to be shortest and steepest: shallow water, strong tidal currents and, often, Atlantic swell rolling in from the west.

Rule of thumb: a "confused" sea usually comes from one of these four things: a wind that has just changed direction, two wave systems crossing, waves running against the current, or shallow water.

Waves in Route du Rhum strategy

The sea state matters in every phase of the course, but in different ways: at the start it can stop the fleet or damage the boats, in the trade winds it decides how fast they go, and at the finish it can complicate the last few miles.

The phases where waves matter

1. Leaving Brittany and Atlantic depressions

Ahead of a depression's front, the swell coming in from offshore, from the west and north west, crosses the sea built up by the south westerly wind. The result is a very rough and disorganised sea, right in the first days of the race.

A confused sea, or one created by wind against current, slows the boat down a lot and increases the loads and impacts on the hull. Skippers therefore look for the best compromise between wind strength and sea state: sometimes a slightly longer route with a more manageable sea is both the fastest and the safest.

Then there is the seabed. As long as the fleet sails in the Channel and over the continental shelf, with depths of a few tens of metres and strong tidal currents, the same wind produces a shorter, steeper sea than in the open ocean. Once past the edge of the shelf, the bottom drops to thousands of metres and the waves depend only on wind and swell.

On the map: compare Waves and Wind. Where the waves are high but the wind has already dropped, the depression has passed and the sea is still up. Where the waves grow more than the wind, swell is arriving from a more distant storm.

2. The Azores High

Near the anticyclone the wind is light, but the sea isn't necessarily calm: swell from the depressions further north can reach this far. With little wind and swell, the sails slat and the boat struggles to make headway.

On the map: if you find waves of a few metres in an area of light wind, it is almost certainly swell from far away.

3. Trade winds

In the trade winds the wind sea comes from the north east, but it often combines with a north westerly swell generated by depressions passing much further north. In that case the sea becomes confused and sailing more uncomfortable. Without swell, on the other hand, the trade winds allow long surfs with wind and waves from behind.

On the map: look at the waves along the route in the trade winds. If they are higher than you would expect from the local wind, there is probably a north westerly swell.

4. Rounding Guadeloupe

Close to the island's coast the sea becomes more chaotic, just as the wind accelerates around the headlands and collapses in the lee. It is one more variable that makes the final miles unpredictable.

On the map: the model represents coastal effects poorly. Here, experience is what counts.

How to read the fleet's choices

1. A boat slows down in strong wind? It may not be the wind holding it back, but the sea: look at the wave height at its position.
2. A boat chooses a route further south or further from the depression? It may be looking for a more manageable sea, even at the cost of less wind.
3. Different boats, same sea, different speeds? Large, fast hulls like Ultims and IMOCAs react to the sea very differently from a Class40 or a Vintage boat.

An exercise to understand

Open the cartography and pick two boats in the same class on different routes.

1. Read the waves. With the Waves layer, note the height at each boat's position.
2. Compare with the wind. With the Wind layer, check whether the waves "fit" the local wind: with about 20 knots, in a fully developed sea, the probable wave is around 2 metres. Much more? There is probably swell from far away.
3. Look at the directions. Where is the wind coming from relative to the boat's route? Are the waves coming from behind, from the side or head on?
4. Move time forward. Over the next 24 hours, which of the two boats will have the more manageable sea? That may explain their choice of route.

The limits of the map

The waves on the map come from a wave model, which calculates them from the forecast wind. If the forecast wind is wrong, so are the waves.

• It is a statistical height. Significant wave height is not the highest wave: in a developed sea, waves almost twice as high can occur.
• One number doesn't tell the whole story. If the layer only shows total height, you can't tell wind sea from swell, nor see the period and the directions that make the sea confused.
• Currents and coasts are difficult. The effect of currents and seabed near the coast, as in Brittany or around Guadeloupe, is only partly represented.

Lesson 5 of 7

Currents: the sea moving beneath the boat

Why currents matter

The sea the boats sail on is not still: the water moves, and the boats move with it.

A favourable current of just one knot, kept for 24 hours, gives you 24 miles; sail against the current and those 24 miles are lost. In a race of more than 3,500 miles, even weak currents can make the difference.

Currents also matter indirectly: when they run against the wind, they make the sea shorter and steeper, as we saw in the lesson on waves. In this lesson we learn to read the Currents layer; technical terms are explained in the glossary.

The basics

What you see on the map

When you switch on Forecasts → Currents, the map shows the direction and speed of the surface current at each point.

• Colour is speed, in knots. The scale runs from dark blue and green (little current) through yellow and red (moderate current) to light blue and white (strong current).
• Direction is given the opposite way to wind. For wind we say where it comes from; for current we say where it goes. There is no such thing as a "westerly current": we say the current "sets west".
• The values are small. In open water, currents are almost always below one knot. So differences in colour need to be read carefully: even half a knot, over several days, counts.

Types of current

Currents have three main causes.

• Wind, which pushes the surface layer of the water. In deep water, after at least 12 hours of steady wind, the drift current is about 2% of the wind speed: with 20 knots, less than half a knot. On an ocean scale, however, steady winds create large permanent currents.
• Tide. The Sun and the Moon make the sea rise and fall, creating currents that change direction regularly and can be predicted. They are strongest near the coast, in narrow passages and in bays: in some places they exceed 8 knots.
• Differences in temperature and salinity of the water. They drive the deep waters and are much slower: for the race they matter little.

What it means at sea

The boat's speed through the water and its speed over the ground are two different things: the current makes the difference. With 10 knots through the water and 2 knots of foul current, the boat moves over the ground at 8 knots; with the current in its favour, at 12 knots.

The current also pushes the boat sideways: if you don't allow for it, the actual track drifts away from the intended one.

Rule of thumb: in open water the current adds up slowly, day after day; near the coast it can change everything within a few hours.

Going further

The great North Atlantic loop

The ocean's steady winds create huge rings of currents called gyres. In the North Atlantic the ring turns clockwise, like the wind around the Azores High, and the Route du Rhum course crosses its eastern and southern sides.

1. To the west, the Gulf Stream flows north along the American coast: it is narrow and fast, 2 to 4 knots off Florida. It then widens and slows as it crosses the Atlantic eastwards.
2. To the east, the current flows south. It passes off the Bay of Biscay and along Europe down to the Canaries, where it becomes the Canary Current. Like other eastern boundary currents, it is wide, shallow and slow.
3. To the south, the North Equatorial Current starts north of the Cape Verde islands and flows almost due west, at about 0.7 knots on average, in the trade wind zone. Part of it reaches the Lesser Antilles and enters the Caribbean.
4. In the centre, the Sargasso Sea has no well defined currents.

Why the current doesn't follow the wind exactly

As with the wind, the Earth's rotation matters for water too. The wind sets the surface layer moving, but the Coriolis force deflects it to the right in our hemisphere: at the surface the current flows up to 45° to the right of the wind in deep water, and the deflection increases with depth.

Tides and eddies

• Tidal currents change direction roughly every six hours, following the tidal cycle. They are strongest at spring tides and lose 20 to 30% of their speed at neap tides.
• Eddies are rings of current 50 to 150 km across that break away from the large currents, with maximum speeds of around 2 knots. They can even reverse the surface current. They are more frequent near strong currents like the Gulf Stream.

Rule of thumb: on the Route du Rhum course, the general current in the south flows in the same direction as the boats, towards the west: a small but constant helping hand.

Currents in Route du Rhum strategy

Currents matter above all at the start and finish of the race, near the coast, where the tide is strong. In open water they are weak but constant, and in the trade wind zone they work in the fleet's favour. In the Azores High, at the centre of the great Atlantic loop, they matter little.

The phases where currents matter

1. Leaving Brittany and tides

The start takes place in an area of strong tidal currents. Around the tip of Brittany, tidal currents are concentrated in narrow passages: in the Fromveur passage, between Ushant and the Molène archipelago, they can reach 9 knots. The strongest currents occur at mid tide and at spring tides.

That is why getting round the tip of Brittany is the first big tactical choice. With wind against current, the sea in the passages also becomes very steep.

On the map: watch how the direction and colour of the currents change around the Breton tip as you scroll through the forecasts hour by hour. Reaching a passage with the current with you or against you can open up big gaps in the very first hours.

3. Trade winds

In the trade winds, the Canary Current and then the North Equatorial Current set south west and west, towards Guadeloupe. They are weak, the North Equatorial Current for example averages about 0.7 knots, but they carry the fleet along for days: a small but constant help.

On the map: here the differences in colour are minimal. Rather than looking at individual boats, check whether there are eddies or areas of foul current along the route: they can explain small gains or losses between boats close to each other.

4. Rounding Guadeloupe

On the island rounding, between Tête à l'Anglais, the Basse-Terre buoy and the Saintes channel, skippers have to deal with local currents as well as windless zones. When the wind drops in the lee of the island, even a weak current weighs more heavily on the boat's progress.

On the map: the model is not very accurate near the coast. Use the colours only as a general indication.

How to read the fleet's choices

1. A boat is going faster than another in the same wind? It may have a favourable current: compare the colour of the currents at the two positions.
2. A boat chooses a longer passage around Brittany? It may want to avoid foul current, or a passage where wind against current makes the sea too steep.
3. Is the speed shown through the water or over the ground? If the tracker calculates speeds from the boats' successive positions, it includes the effect of the current.

An exercise to understand

Open the cartography and switch on the Currents layer.

1. Find the strongest currents. Where are the most intense colours? Probably near the coast, especially around Brittany.
2. Follow the tide. Scroll through the forecasts hour by hour around the tip of Brittany: can you see the current change direction roughly every six hours?
3. Look at the open ocean. In the trade wind zone, which way does the current set? Is it with or against the boats heading for Guadeloupe?
4. Compare with the wind. Switch on Wind: where wind and current run in opposite directions, expect a shorter, steeper sea. Check with the Waves layer.

The limits of the map

The currents on the map come from an ocean model, which combines satellite observations and measurements at sea. The likely candidate is the European Copernicus Marine system, which works on a 1/12 degree grid, about 8 km, and forecasts the ocean 10 days ahead with daily updates.

• Narrow passages slip through. A passage like the Fromveur is only a few kilometres wide: its strongest tidal currents may be underestimated or not show up at all.
• Not all models include the tide. Some products only show the general current; others add the tide and the drift caused by waves.
• It is the surface current. The boat, with its keel, also feels the water a few metres further down, where the current can be different.

Lesson 6 of 7

Sea and air temperature: the hidden engine of the weather

Why temperatures matter

Air and sea temperatures have no direct effect on the boat, but they influence many of the phenomena we saw in the previous lessons. Comparing the two tells you whether the air will be stable or unstable, and therefore whether the wind will be steady or gusty. Air temperature also changes the "weight" of the wind on the sails. Sea temperature decides how active squalls will be and where tropical cyclones could form.

Then there are living conditions on board: within a few days, skippers go from the damp cold of Brittany in November to the heat of the tropics. In this lesson we learn to read the Air T° and Water T° layers; technical terms are explained in the glossary.

The basics

What you see on the map

The cartography has two separate layers: Forecasts → Air T° and Forecasts → Water T°. Both are in degrees Celsius.

• Air T° is the air temperature close to the sea surface.
• Water T° is the sea surface temperature.
• Boat data. Click on a boat and the panel shows both the air and the water temperature at its position.
• Colour is temperature, in degrees Celsius. The scale runs from purple (very cold) through blue (0 °C) to red (very hot).

Temperatures along the course

The Route du Rhum crosses very different environments in just a few days. According to November climate averages:

In between, the sea temperature rises as the fleet heads south and enters the trade winds.

What it means on board

At the start, skippers sail in cold and damp conditions, often soaked by the waves: the cold adds to fatigue and makes manoeuvres harder. In the trade winds the problem flips: heat, sun and humidity make working on deck and sleeping below exhausting. For a solo skipper, managing these conditions is as much a part of the race as choosing the route.

Rule of thumb: think about what the number of degrees means, and what it implies for life on board.

Going further

The difference between air and sea

The sea exchanges heat with the air flowing over it. That is why comparing the two temperatures says a lot about how stable the air is:

It is the same mechanism we saw in the lessons on gusts and clouds: cold air over a warmer sea increases instability and gusts. When warm, moist air passes over colder water, on the other hand, it cools and the water vapour can condense into fog.

Rule of thumb: don't look at the two temperatures separately: it is the difference between air and sea that tells you most about the weather to come.

The "weight" of the wind

The force of the wind on the sails depends on the square of the wind speed, but also on air density: cold air is denser than warm air, so at the same speed it pushes harder. With air at 0 °C, the force is about 5% higher than with air at 15 °C. Between cold, dry air and warm, humid tropical air, the difference reaches about 10%.

For comparison, going from 14 to 15 knots increases the force on the sails by about 15%. So 20 knots in the Channel in November "weighs" a little more than the same 20 knots in the trade winds.

Warm water and tropical cyclones

Tropical cyclones, such as hurricanes, form over tropical waters warmer than about 26.5 °C. The Atlantic hurricane season tends to end around late November, so it partly overlaps with the Route du Rhum. Late in the season, formations are rarer and, when they do occur, are mostly concentrated in the Caribbean Sea.

Rule of thumb: the warmer the water compared with the air, the more the air tends to rise and the more you should expect convective clouds, squalls and gusts.

Temperatures in Route du Rhum strategy

Temperatures rarely explain a routing choice on their own, but they help you understand what kind of wind there could be and what living conditions are like on board. They matter most behind the cold fronts at the start of the race and in the trade winds, where warm water feeds the squalls.

The phases where temperatures matter

1. Leaving Brittany and cold fronts

Behind a cold front comes polar air, colder than the sea. As it warms from below it becomes unstable: that is why, after the front, the north westerly wind is gustier and comes with showers. And because it is cold air, at the same speed it pushes harder on the sails. In the warm sector ahead of the front, on the other hand, the subtropical air arriving from the south west is more stable and the wind steadier.

On the map: compare Air T° and Water T° before and after a front passes. When the air becomes clearly colder than the sea, expect gusts and showers: check with the Gusts and Clouds layers.

2. The Azores High

Under the anticyclone the air sinks and is stable: the wind is light, but steady. Heading south, air and water gradually warm up.

On the map: follow how the two temperatures change along the boats' route: it is a good indicator of when the fleet is moving from Atlantic air into tropical air.

3. Trade winds

In the trade winds the water is warm, and squalls are more active where the water is warmer. Approaching the Caribbean, with sea temperatures close to 28 °C, squalls become more frequent. At the end of the hurricane season, sea temperature is also one of the indicators to watch for possible tropical disturbances.

On the map: look at the warmest areas of the Water T° layer along the route and compare them with Clouds and Gusts: where all three match, squalls are more likely.

How to read the fleet's choices

1. Two boats with the same average wind are going at different speeds? One may be in cold, unstable air, with gusts forcing it to reduce sail; the other in more stable air.
2. Is the fleet avoiding a tropical area? If a tropical disturbance is forming, Race Direction and the skippers follow it closely: sea temperatures help show where it might develop.
3. Are skippers slowing down in the last few days? Heat, accumulated fatigue and frequent squalls also put physical endurance to the test.

An exercise to understand

Open the cartography and pick a boat.

1. Read both temperatures. In the boat's panel, note the air temperature and the water temperature.
2. Work out the difference. Is the air colder or warmer than the sea? By how much?
3. Draw the conclusions. Air colder than the sea: expect unstable air, gusts and showers. Warmer air: steadier wind, maybe haze. Check with the Gusts and Clouds layers to see whether the forecast is consistent.
4. Follow the journey. Pick a boat at the front and one at the back of the fleet: how many degrees apart are their water temperatures? It is a simple way to understand how far apart they are, not just in miles but in climate too.

The limits of the map

• Sea temperature changes little over time, but air temperature changes between day and night and as fronts pass: scroll through the forecasts to see how it evolves.
• It is the surface temperature. The ocean model describes the sea surface on a grid several kilometres wide: small local variations, near the coast or around islands, can slip through.
• The air and sea difference is only a clue. Air stability also depends on what is happening higher up, which the map doesn't always show.

Lesson 7 of 7

Putting it all together: reading the Route du Rhum map

From single layers to the whole map

In the previous lessons we learned to read one layer at a time: pressure and wind, gusts, clouds and squalls, waves, currents and temperatures. At sea, though, everything happens at once: skippers choose their strategy and route by weighing wind, sea, gusts and current at the same time.

In this lesson we put the pieces together: we start with a method for reading the map in a few minutes, then go back through the four phases of the race, looking at the choices and conditions of the last edition of the Route du Rhum, in 2022. Technical terms are explained in the glossary.

The method: six questions in front of the map

Every time you open the cartography, ask yourself these questions in order.

1. Where are the highs and lows? By looking at the isobars or the swirls of the wind particles, find the anticyclone and the depressions. This is the big picture that explains everything else.
2. Where is the wind, and where is it coming from relative to the route? With the Wind layer, find the corridors leading towards Guadeloupe and the light wind areas to avoid.
3. Will the wind be steady or gusty? Compare Gusts and Wind, then look at Clouds and the difference between Air T° and Water T°: cold air over a warm sea and scattered clouds mean gusts and squalls.
4. What sea will the boats find? With the Waves layer, check the wave height and ask yourself whether the sea will be confused or calm.
5. Are the currents helping or hindering? Near the coast, look at the Currents layer with the tides in mind; in open water, the general current.
6. How will conditions change tomorrow? Scroll the forecasts 24 to 48 hours ahead: a boat's choice makes sense when you look at where it wants to be for the conditions to come, not where it is today.

Rule of thumb: a boat that seems to be on the wrong route today is almost always betting on what will happen tomorrow. Question 6 is the one that explains most choices.

The four phases at a glance

The table sums up what to look for in each layer during each phase. An empty cell means the layer matters little in that phase.

Phase 1: leaving Brittany and Atlantic depressions

What to read on the map. This is the phase where all the layers matter together. Pressure and wind show the train of depressions; Gusts and Clouds flag the cold fronts; Waves tells you how demanding the sea will be, short in the Channel because of the shallow water and confused ahead of the fronts; Currents shows the strength of the tides around the Breton tip; temperatures confirm the arrival of cold, unstable air behind the fronts.

The key question: how much wind and how much sea can the boat take, and which route reconciles speed and safety?

What happened in 2022

The postponed start. For the first time in the race's history, the start scheduled for 6 November 2022 was postponed by three days. A very violent depression, with very high seas and gusts of up to 100 km/h at the tip of Brittany, left no safe, conservative route south. It was a perfect case of reading the layers together: the wind alone was not enough to explain the decision; it was the combination of waves and the lack of a safe exit corridor that forced the start to be postponed.

The start on 9 November. The fleet set off in 15 to 18 knots from the west south west. On the first night of the race the fleet sailed close to the coast, playing the wind shifts and tidal currents. Then came the first big choice: go outside Ushant, through the Fromveur, or through the Chenal du Four. All the Ultims chose the Chenal du Four, while Erwan Le Roux, who went on to win the Ocean Fifty class, went outside Ushant and took the lead in his class.

The Bay of Biscay. There were three options: a northern route, faster on paper but very demanding, with strong wind and a chaotic sea ahead of a front; a southern route, more manageable but upwind; and a middle way, close to the direct route, with the risk of getting stuck in a calm behind the first front. One router described the northern route as "video game" routing: going there would have meant seeking out exactly the conditions that the postponed start was meant to avoid. Most of the fleet chose westerly routes, more or less to the south, looking for a compromise between wind and sea state.

The Class40s and Ambrogio Beccaria. In Class40, the largest class with 55 boats, Italian Ambrogio Beccaria on Allagrande Pirelli, launched only a few months earlier, slotted straight into the leading group. His account of the first phase touches almost every layer: "playing" the currents in the first miles, then the choice of how to round Ushant, and finally the open sea. The first front caused damage across the fleet: he lost the masthead wind sensor, and from then on had to sail without wind data. The second front was the one that opened up the first big gaps: Ambrogio says nobody wanted to go into it, and his positioning at that moment later proved to be an advantage. In the same phase, the eventual class winner, Yoann Richomme, served a 4 hour penalty, choosing a moment when foul current and a dropping wind were slowing the fleet down anyway.

Phase 2: the Azores High

What to read on the map. Here the key layer is pressure and wind: find the centre of the anticyclone, where the wind is extremely light, and its ridges. The waves add an important detail: swell from the depressions to the north can make the sea rough even where the wind is light. Gusts, clouds and currents matter little.

The key question: where is the passage that lets you get round the anticyclone in as few miles as possible without running out of wind?

What happened in 2022

Getting round the anticyclone. After the Azores, the challenge was to let a major front pass to the north and get round the anticyclone to head west. The leading Ultim and Ocean Fifty groups sailed along the eastern side of the Azores, while Éric Péron in Ocean Fifty chose an option further south, about 220 miles from the archipelago, to pass under the anticyclone (it did not turn out to be particularly rewarding).

The IMOCA ridge. In the IMOCA class, Charlie Dalin had led since the start and was about 90 miles ahead as the fleet crossed a ridge of light wind after the Azores. Thomas Ruyant then chose to attack by taking a route further west. It is an example of how to read a strategic choice: moving away from the shortest route to look for better wind further on. Ruyant's choice paid off in the following days, and he went on to win the race in the IMOCA class.

Beccaria and the edge of the anticyclone. For Beccaria, the fastest moment of the race was precisely the entry into the anticyclone: sailing downwind under the big gennaker, he felt like he was flying over the water. And on the way out, with the wind on the beam, the boat was still going very fast. It proves that the edge of the high, if you get it right, is anything but a slow zone: the only problem is avoiding its centre. Meanwhile, on 14 November, Richomme had taken the class lead near the Azores. The Class40s arrived here days after the Ultims, and therefore in a different weather situation from the faster classes: in the same race, each class plays its own game with the weather.

On the map: when you see two boats in the same class splitting near the Azores, look at where the ridge and the centre of the anticyclone are, then scroll the forecast: who will reach the new breeze first?

Phase 3: trade winds

What to read on the map. The wind becomes steady and the race is decided by details. Clouds, Gusts and Water T° tell you where squalls are most likely, especially at night and where the water is warmest. Waves shows whether there is a north westerly swell and therefore a confused sea. The currents, weak but favourable, carry the fleet westwards.

The key question: how do you stay close to the direct route, gybing at the right moments, using the squalls without getting caught out?

What happened in 2022

The IMOCA overtake. On Friday 18 November, thanks to his more westerly route, Thomas Ruyant overtook Charlie Dalin and took the lead. Dalin closed to within five miles on the night before the finish, but Ruyant made better use of the local weather systems and pulled away again.

The Ultim race. Charles Caudrelier, who had been leading for days, approached Guadeloupe about 68 miles ahead of François Gabart, who had been chasing him for days.

The Beccaria and Douguet duel in the trade winds. In the trade winds, Beccaria and Corentin Douguet fought for second place over about 1,300 miles, all the way to the northern approach to Guadeloupe. At one point Douguet sailed a few hours higher on the wind to put himself ahead of the Italian and remove the lateral separation between the two boats, reducing Allagrande Pirelli's chances to attack. It is a classic covering move, which you can spot on the map when a boat changes course to line up with an opponent rather than to look for wind. Beccaria, without wind instruments, had to sail on instinct against opponents who still had their data. He too says that in the trade winds the race became more complicated.

On the map: in this phase the gaps change even without big routing choices. If two boats close together have very different speeds, look for areas of scattered cloud and gusts: one of the two is probably under a squall or in the lull behind it.

Phase 4: rounding Guadeloupe

What to read on the map. Here the map helps less than in any other phase: the island's terrain creates acceleration around the headlands, windless zones in the lee, clouds and local currents that a global model only partly represents. The map is mainly useful for understanding which direction the trade wind is coming from and how strong it is before it reaches the island.

The key question: which line along the west coast avoids the calm patches, down to the Basse-Terre buoy and then on to the finish?

What happened in 2022

The IMOCA finish. With less than 200 miles to go, just 10 miles separated Ruyant and Dalin. To reach the finish, about 53 miles had to be sailed around the west side of the island, through local calms and sudden changes in wind strength and direction. In the end Ruyant won by just over 2 hours, in 11 days 17 hours and 36 minutes: a new class record.

The Ultim record. Charles Caudrelier won the Ultim class and was the first to finish the race, in 6 days 19 hours and 47 minutes, setting a new Route du Rhum record.

Beccaria's finish. The duel with Douguet was decided in light wind along the west coast of Guadeloupe, with the two boats less than a mile apart. Beccaria describes the island rounding like this: windless patches, clouds everywhere and one squall after another. Douguet closed to within half a mile of Beccaria, but in the end Beccaria broke free and crossed the line almost ten miles ahead. He finished second in 14 days 7 hours and 23 minutes, about 4 hours behind Richomme, who won and broke the class record by two days. It was one of the most important results ever for Italian offshore sailing, and the start of Beccaria's success in Class40.

Same course, different weeks. Caudrelier finished in under 7 days, Ruyant in under 12, Beccaria in just over 14. The three boats therefore went through each phase of the race at different times, with different weather systems: that is why, to understand the fleet's choices on the map, you should always compare boats in the same class.

The 2018 precedent. Four years earlier, Francis Joyon had overtaken François Gabart precisely during the island rounding, winning by just over 7 minutes: a reminder that "it ain't over till it's over".

On the map: in the final miles, watch the boats' positions and speeds above all, more than the weather layers. When a boat suddenly slows down close to the coast, it has almost always sailed into a calm patch.

Final exercise: reading a complete scenario

Pick any moment of the race and two boats in the same class that have taken different paths. Then, for each boat, answer the six questions below:

Finally, ask yourself: which of the two made the better bet? Then come back to the map the next day and check whether you were right. It is the most effective way to learn, comparing the forecast with what actually happened every day, just like the skippers do.