Guide about ice for the Sollentuna Winter Marathon
When winter settles over Sollentuna and Edsviken grows still, nature begins its miracle – the ice starts to form. For us, as organizers of the Sollentuna Winter Marathon, everything comes down to one thing: the ice must be thick and safe enough. For the race to take place, we need 12 centimeters of solid black ice – clear, strong ice that can carry hundreds of runners. If the ice isn’t thick enough or of good quality, we will never compromise your safety. In that case, the event will turn into a winter trail run around Edsviken instead.
From the First Frost to Runable Ice – How Does It Happen?
When temperatures drop below zero, the surface water of Edsviken begins to cool. Tiny ice crystals start to form and merge into a thin layer, and then the ice slowly thickens downward. It takes time, and the weather is the decisive factor — not just the cold itself, but a few clear, freezing nights can work wonders.
The main factors that determine how fast the ice grows are:
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Cold is essential — the colder it is, the faster the ice forms. It can hardly ever be too cold, but there must be at least a few degrees below zero.
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Snow acts as insulation and slows down the growth. It might be -15°C, yet no new ice forms because of the snow cover.
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Clear nights allow heat to escape and cool the surface. Heat rises, but when it’s cloudy, it gets trapped, and barely any ice forms at all.
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Wind can help, but only in moderation. A light breeze helps to cool the water and keep the surface clear, but too much wind prevents freezing and can even break up the forming ice.
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Water movement is never good for ice formation. Strong currents can completely stop the freezing process or create weak spots and cracks.
How Fast the Ice Grows
Once the cold really sets in, the ice can grow surprisingly quickly. At –10°C, it can thicken by 1–1.5 centimeters per night if the weather is clear and snow-free. In other words, we can reach 12 centimeters of solid ice in about 7–8 days if everything goes our way.
Before SWM25, the ice on Norrviken was only about 6–8 cm thick just ten days before the race — but by race day, it had grown to 12–14 cm thanks to five or six really cold, clear nights. Meanwhile, Edsviken was still completely open, without any ice at all.
When the Ice Is Safe
Not all centimeters of ice are created equal. The clear, bluish black ice is the strongest — that’s the kind we need for safe running. White snow ice may look beautiful, but it’s porous and only about half as strong.
Over the years, we’ve seen it all — from solid black ice (2025), to slushy double layers (2019), and stunning mixed ice that looked like the sky itself (2022). On Edsviken, there are often several “generations” of ice, a blend of all these types, which places extra demands on us as organizers.