Why Frozen Danish Lakes May Still Be Deadly

Opuere Odu

Writer

February 3, 2026
4:15 pm

Despite Denmark’s frozen start to the year with temperatures plummeting to minus 19 degrees Celsius in January, many lakes across the country remain unsafe for skating and ice activities due to highly variable ice thickness influenced by water depth, underground currents, and solar radiation.

Why Ice Thickness Varies So Much Between Lakes

The cold has certainly made its mark on Denmark this winter. In January alone, the thermometer dropped to around minus 19 degrees, and the cold has affected not just people but also the lakes scattered throughout the country. However, appearances can be deceiving when it comes to ice safety.

Even though several lakes have transformed into beautiful ice landscapes, far from all of them are safe to venture onto, according to climatologist Mikael Scharling. In fact, conditions can vary wildly even between two lakes located in the same area.

A reader example illustrates this point well. In one region, locals say that when Tystrup Lake can bear a crow, Sorø Lake can bear a horse and wagon. While unscientific, this folk wisdom highlights genuine differences in ice formation between nearby bodies of water.

The Science Behind Ice Formation

The general rule of thumb states that ice grows by about one-tenth of a millimeter per degree below freezing per hour. Generally speaking, the smaller the body of water that needs cooling, the faster the lake freezes to ice, Scharling explains.

A shallow lake has a relatively large surface area where heat can escape. In contrast, a very deep lake will have a smaller surface area relative to the water volume. This is pure physics at work.

However, Scharling warns against being fooled by surface appearances. You cannot simply assume that because a lake is shallow, it is safe to walk on.

Hidden Dangers Beneath the Surface

People must be very careful with generalizations. Beneath the surface of a lake, there can be factors invisible to the naked eye that affect ice safety.

There can be currents flowing into the lake that influence the temperature. Groundwater is generally quite warm, around 8 degrees Celsius, and if it seeps in, it keeps the temperature elevated. This varies from lake to lake, so you cannot simply say that if the temperature is low enough, all lakes in the area can be accessed safely.

Solar Radiation Adds Another Variable

As the sun gradually appears more frequently, it can also affect ice conditions, Scharling notes. While we are still in early February with low sun angles, as we move further into the year and into March, the sun begins to stand higher in the sky, and this also plays a role.

Even on a day with frost in the air, if the sun stands high in the sky in certain parts of the country, it can actually send warmth down through the ice and heat the water, causing the ice to melt.

In Aarhus, for example, employees measured the ice thickness on one of the stadium lakes and found it to be just around 13 centimeters thick, well short of the 16 centimeters required by Aarhus Municipality before permission is granted to venture onto the ice.

Official Recommendations for Ice Safety

For anyone considering pulling their skates out of the closet, the recommendation is clear. The best thing you can do is simply look at the signage at the specific lake and stick to the recommendations. Otherwise, it can end badly.

Meanwhile, several municipalities have given the green light for skating on certain approved lakes where ice thickness meets safety standards. However, authorities continue to emphasize caution and adherence to local guidelines.

Context: Denmark’s Largest Lakes Present Unique Challenges

Denmark’s largest lakes continue to attract ice fishers despite a cold and unstable winter start. Many of these lakes remain unsafe due to varying ice thickness, eutrophication, and local weather conditions.

Arresø, Denmark’s largest lake with an area of 39.87 square kilometers, is more than twice as large as the second-largest Esrum Lake at 17.29 square kilometers. Its enormous size contributes to slower and uneven ice formation, which increases uncertainty.

Esrum Lake, with an average depth of 13.5 meters reaching up to 22 meters, is Denmark’s most water-rich lake. Located in North Zealand near the Kings’ North Zealand National Park, its depth can delay full ice coverage even after frost periods, making it attractive for experienced ice fishers but potentially dangerous.

Other large lakes like Mossø in eastern Jutland at 16.90 square kilometers, Tissø at 12.33 square kilometers, and Furesø in North Zealand at 9.41 square kilometers attract thousands of visitors annually for activities like fishing and swimming. These lakes have rich biodiversity, but eutrophication, which affects most Danish lakes, can impact water quality and ice stability by increasing algae blooms during thaws.

Denmark’s Lake Landscape and Recreation

Denmark has approximately 120,000 lakes, many of which are artificially created for hunting and duck release. Wetlands have shrunk dramatically over the past century. For example, in Vesthimmerland, wetlands decreased from 1,000 hectares to 67 hectares between 1880 and 1960, underscoring the role of lakes as rare recreational zones.

Recreational infrastructure supports outdoor activities like ice fishing, with recreational routes growing from 34,000 kilometers in 2021 to 63,725 kilometers in 2025. This increases access to lake areas but also exposure to risks during unsafe periods.

Looking Ahead

The cold weather does not appear to be ending anytime soon. Therefore, there may be good news if you want a trip on the ice. Scharling suggests that the number of lakes being cleared for public access is likely to increase as sustained cold temperatures allow ice to thicken to safe levels.

Nevertheless, visitors must continue to exercise caution and check local signage before venturing onto any frozen lake. Ice conditions remain highly variable across Denmark, and what appears safe on the surface may hide dangerous weak spots underneath.