Kieler Woche: When a high-pressure system dominates the world’s largest regatta

YACHT: On Windy, Windfinder and other weather services, a distinct high-pressure system can be seen over Northern Europe. Where has this system come from, and why has it settled so stubbornly over Kiel Week of all times?

Sebastian Wache: About a week ago, warm air began to flow from Spain into France – that is typically what triggers it. When warm or very hot air makes its way northwards, a high-pressure system can build up within that air mass. The warmer the air, the more pronounced and stable the system becomes.

During the summer months, when temperatures are extremely high, we refer to this as a ‘heat dome’ – a dome-shaped layer of air that remains stationary, continues to heat up due to solar radiation, and prevents fresh air from entering from outside, which could break up the system. It can only dissipate by drifting slowly eastwards with the upper-level winds – which is what will happen over the course of the week, albeit very gradually.

The temperatures show just how severe this heatwave is: today, temperatures of 40 degrees were already recorded in parts of France at 12 noon, whilst yesterday’s highs reached 43 degrees. As the situation is getting slightly worse every day, temperatures there are set to rise further.

Is the high-pressure system moving?

Yes, it will initially move towards England as the week progresses and then towards Germany as the weekend approaches. Fortunately, Kiel has so far been on the periphery – which is why it hasn’t got quite so hot here yet. Although we’ve had temperatures of just under 30 degrees at times, it’s still fairly mild at the moment. Towards the weekend, however, it will get significantly warmer again. Once Kiel Week is over, the high-pressure system is likely to have moved on from Germany and Central Europe – and we’ll then see a spell of more moderate conditions.

Essentially, it is a summer high-pressure system which, as a result of climate change, is fuelled by extreme heat and is correspondingly stable.

The Azores High or the Continental High?

Is the current high-pressure system more of an offshoot of the classic Azores High, or is it an independent continental high originating in Eastern Europe?

The starting point was, in fact, a combination of both. On the one hand, there was an advance of the Azores High, which also brought some warm air with it. On the other hand, there was initially a link between the Azores High and a high-pressure system over Eastern Europe – a so-called high-pressure bridge. During this phase, the weather here was still changeable and somewhat cooler than usual for early June.

Then a decisive change took place: the atmosphere shifted from a zonal flow – that is, a west-east Atlantic flow – to a meridional flow. This wave-like flow directs air masses first from south to north and then from north to south. Kiel found itself in the south-north branch of this wave, which, due to its large amplitude, brought extremely hot air from North Africa.

In summary: The system developed as a result of the interaction between an Eastern European continental high and the Azores High, and was extended so far north by the meridional wave pattern in the atmosphere that the warm air was able to flow in.

Wind forecast for Kiel Week

Windfinder currently shows wind speeds of two to three. Should we expect higher speeds – perhaps even a force four?

That’s tricky. The days are gruelling for everyone involved, because, of course, everyone wants wind – and we’re seeing extremely varied conditions on the different courses.

The inner courses have a clear advantage: they benefit from a reliable sea breeze and are recording a good ten to eleven knots – which is perfectly acceptable for this time of year. The outer courses, on the other hand, are struggling to catch even the slightest breeze. As a result, some have already been moved inwards so that races can still take place at all.

This is the typical pattern for a sea breeze: further out to sea, a calm zone forms, from which the wind then builds up and reaches its highest speeds along the coastline – that is, where the water meets the land.

This is unlikely to change much over the next few days. Land temperatures will rise slightly further during the day – which is generally a good thing, because the greater the temperature difference between land and water, the stronger the sea breeze can become and the further out it can reach. However, the water is warming up too. We’re measuring surface temperatures that are about half a degree to one degree higher every day. This keeps the temperature difference between land and water limited – and with it, the potential for wind. If we stick to wind force 3 over the next few days, we should actually count ourselves lucky.

Sea breeze: Formation and mechanism

Could you briefly explain how thermal and sea breezes form?

The principle is based on temperature differences between land and water. In early summer, the water is usually still cooler than the land. The sun warms the land – and it is not so much the air temperatures we are familiar with from weather apps that are decisive, but above all the ground temperatures: the darker the ground, the more it heats up, and the more hot air rises.

This rising air over the land is replenished by cooler air from the sea. The whole process works like a box circulation: the air rises over the land, flows out over the water and sinks again there – further out to sea or further inland, depending on the strength of the sea breeze. Where the air sinks, a thermal high forms, resulting in complete calm. From this point, the wind flows back towards the coast and reaches its highest speed there – at the boundary between water and land.

The further you glide towards this thermal high, the more likely you are to run into a lull. And this cycle has to develop over the course of the day – it takes time and is extremely susceptible to disruption.

Possible factors that may cause disruption:

1. Cirrus clouds, as we see them above us today: even a slight amount of shade can lower ground temperatures by one or two degrees – and that’s enough for the sea breeze to die down or fail to blow properly.
2. Slight low-pressure outflow, which bring their own wind field with them and penetrate the fragile system.
3. High-pressure branch, such as those over Denmark, which can also disrupt circulation.

The system is so sensitive that favourable sea-breeze conditions on paper are no guarantee of favourable sea breezes in reality.

A southerly wind, hot weather and the weekend

How are the conditions looking as Kiel Week draws to a close?

Over the next two days, we expect very light winds – even in the mornings, which is generally not a bad thing for the build-up of the sea breeze. From Thursday onwards, the wind will tend to veer south as it rotates clockwise around the high-pressure system. However, a southerly wind is not ideal for the build-up of the sea breeze.

The weekend is set to be a challenging one weather-wise: the approaching high-pressure system is bringing not just warmth, but heat to Kiel. We’re expecting temperatures well above 30 degrees, with Saturday likely to be the peak. Some models are even forecasting 36 degrees for Kiel – that would be close to the historic all-time record of 36.5 degrees, which has stood since 1881. In any case, it’s clear which way the weather is heading.

As temperatures rise, the risk of thunderstorms also increases.

Risk of thunderstorms in the second half

We’ve already had a few thunderstorms over the last few days. Are there more on the way?

That is certainly a possibility. Once the high-pressure system moves away, we will once again be under the influence of low pressure. However, the air in this high-pressure system is extremely dry, which is why the models still rate the risk of thunderstorms on Saturday as low. Sunday will be a little more critical – although most of the races will have been completed by then anyway.

The models are currently forecasting a major change in the weather for Monday. This would be good news for visitors, sailors and athletes – but bad news for the dismantling crews. We experienced just how powerful such a change in the weather can be during the night from Friday to Saturday: suddenly, at 3 am, there were gusts of force 6 to 8 in Kiel, and on the North Sea coast there were even gale-force gusts of 124 km/h in Hörnum on Sylt. It was fortunate that the wind had weakened on its way to Kiel – a force 12 gale over the festival site would have had disastrous consequences. Similar squalls cannot be ruled out for Monday.

Looking beyond Kiel Week

What can sailors expect next week, and is it worth heading out to the North Sea instead?

Once the high-pressure system has moved on, the situation is expected to return to normal: back to a zonal flow with westerly winds from the Atlantic, changeable weather and the influence of a low-pressure system in the north. However, I cannot rule out the possibility that the next high-pressure system will follow soon – in recent decades, we have observed increasingly frequent periods of high pressure over Europe, accompanied by intense heat and dry conditions. In July, temperatures could even be higher than they are now in June.

Is the North Sea a good alternative, then?

With the high-pressure system currently in place, I wouldn’t recommend any long trips there. On the North Sea, too, sea breezes are the dominant factor, with the tide adding to the mix, and the high-pressure system is also situated over the German Bight. On the Baltic Sea, the situation is as follows: there is wind along the coast, whilst further out the calm zone begins – a crossing from Kiel to Marschtal, for example, would be difficult. Conditions on both sides are currently similar. It is virtually impossible to escape the high-pressure system.

With the change in the weather next week, westerly winds will return to the North Sea – at times strong and changeable, meaning you’ll need to look for windows of good weather for your trips.

Climate change and the future of Kiel Week

Are these the usual conditions for Kiel Week – or can you already see any noticeable changes?

The changes are clearly evident. The classic image of Kiel Week – 14 degrees, drizzle, a brisk breeze – is becoming less and less a reality. For several years now, there have been an increasing number of days with plenty of sunshine, high temperatures and little wind. Last year, there were hardly any races in the first half of the event due to a persistent lack of wind.

What the raw weather data show particularly clearly: Hot days – that is, days when the temperature exceeds 30 degrees – are increasing significantly. I have analysed the Kieler Woche data for the past 40 years: up until 2019, there was virtually not a single hot day – except in the year 2000, when three such days occurred in a row. That was the absolute exception.

Since 2019, however, having at least one hot day during Kieler Woche has almost become the norm. On the first weekend, we already had 27 degrees on Friday and just under 30 degrees on Saturday – just short of an official hot day, but close. We could well exceed the 30-degree mark again over the final weekend.

At the same time, a trend towards drier weather is emerging: it is not out of the question that we won’t see a single drop of rain until Sunday. All of this is a clear indication: More high-pressure systems, more hot days, more dry spells – and with that, the growing question of where the wind for sailors is going to come from. Kiel Week has changed over the last five to ten years – driven by global climate change and its impact on weather systems.

Implications for the race committee

What conclusions should the race organisers draw from these changed conditions? Are coastal courses the way forward?

That’s a valid question, but I’d urge caution: sea breezes are extremely variable. If conditions change even slightly – say, the high-pressure system shifts by 50 to 100 kilometres, or the water is two degrees colder following a cool May – good wind can suddenly blow on all courses.

The conditions at this year’s Kiel Week are an extreme example. I wouldn’t fundamentally overhaul the regatta format just because it was particularly difficult this year. There will be years when everything falls into place again. The adjustments required are minor – and that should be a cause for calm.

Yes, it’s becoming more challenging. But for the time being, the circumstances are still manageable. Kiel Week is alive and well – even if it’s taking place under different circumstances than in the past.