Ice thickness

Jäätilannekartta esittää pistemäisten havaintojen lisäksi ennustemallin laskemat arviot. Jäälle ei pidä mennä pelkän kartan arvioiden perusteella, sillä paikallinen vaihtelu voi olla suurta! Etenkin heti jäätymisen jälkeen jää voi olla paljon heikompaa virtapaikoissa ja selkävesillä kuin vaikkapa ranta-alueilla. Keväällä, kun lumi on sulanut, aurinko voi haurastuttaa paksunkin jään jo parissa tunnissa petollisen heikoksi.

Ice situation 11.5.2023

Ice has melted in central Finland, the season for ice thickness measurement continues in Northern Lapland

The ice in central Finland melted around May Day, but the season for ice thickness measurement continues in May in Northern Lapland and in Lake Pesiöjärvi. The shores of North Ostrobothnia and South Lapland are already unfrozen at many observation sites and measurements can no longer be carried out for safety reasons. When the spring comes, the sun begins to warm and brittle the ice cover. After this, the ice thickness will no longer an indication of its bearing strength. The bearing strength of ice can change rapidly in sunlight. The ice cover may be deceptively brittle, especially in places with a current, straits and near ditches and pipe discharges.

In the measurements carried out on 10 May, the total ice thickness varied between 30 cm and 55 cm in the observation sites of Lake Inarijärvi, Lake Ounasjärvi and Lake Kevojärvi, and the ice cover had already started to become thinner. At the Lake Kilpisjärvi observation site, the ice thickness was 90 cm, and compared to the measurement at the beginning of May, the ice cover thickness remained the same.

The estimated ice situation is based on observations

The Finnish Environment Institute updates the estimate of the ice situation on this page three times a month. The estimate is based on ice thickness measurements at 46 observation sites in different parts of Finland. The ice thickness is measured approximately on the 10th, 20th and 30th day of every month.

You can view the latest observations on the ice situation map (available in Finnish and Swedish). Everyone can also record their own ice observations, making them appear on the map. The Finnish Environment Institute does not verify these observations. You should not go on the ice solely based on the estimates and observations on the map because ice quality and thickness vary greatly even within the same lake and between different waterbodies.

You can observe the general development of the ice situation by a computational estimate of ice thickness, i.e., the colours on the map. The estimate is made by modelling, and it utilises ice thickness observations as well as weather data and forecasts, for instance. It is not possible to determine the actual ice thickness of any waterbody based on the ice situation map. The ice thickness observations are only representative of the specific site on the date of the measurement and are therefore indicative and cannot be extrapolated to large areas.

Further information

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Stay safe on ice from autumn till spring

If you intend to go out on ice, you should always make sure that it is strong enough on the route you intend to take. Get to know the properties of ice in different seasons and pay attention to possible hazards.

Autumn ice

The water starts freezing in autumn when its surface temperature drops below zero. Shores and bays are the first to freeze over, while any deep areas and places with a current get their ice cover last. It is typical for ice thickness to vary in different areas of a lake or the sea in autumn. Measure the thickness of the ice on your route to be sure that it will carry you. Transparent clear ice must be at least five to ten centimetres thick before it carries a person walking on it.

Spring ice

As the air grows warmer and the sun’s radiation becomes stronger in spring, the snow and ice cover starts to melt. Once the snow is gone and solar radiation starts melting the ice, its structure becomes more fragile and cavities appear in the ice layer, and you can no longer conclude the bearing strength of the ice by measuring its thickness. Even if it were 20 to 30 centimetres thick, the ice can be too fragile to carry the weight of a person.

Hazardous areas

Particularly hazardous areas are straights, deep areas and places with a current, including at river mouths and near springs, sewers and stormwater drains, where the ice may remain thin throughout the winter. You should also monitor the ice thickness near large stones, docks and other structures as well as aquatic plants. Shipping lanes, and cracks and holes cut in the ice, are dangerous even in mid-winter. Snow cover makes it more difficult to spot areas of weak ice.

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Fun facts about ice

Clear ice and slush ice

The ice cover on the sea or a lake may consist of clear ice, slush ice and water layers inside the ice, and the quality of the ice may vary in different areas. On still waters, clear ice is formed, which is transparent and looks dark when a hole is drilled through it. When water rises on top of the ice, the snow becomes slushy and then freezes into slush ice. Slush ice is opaque and often milky in colour, and it only has around one half of the bearing strength of clear ice.

Ice measurements at monitoring stations

Ice thickness is measured regularly around Finland at 47 monitoring stations, which usually are located at least 50 metres away from the shore. The stations are visited three times a month, and each time the total thickness of not only clear ice but also slush ice and any water layers are measured. Other measurements include the water level from the lower surface of the ice and thickness of the snow layer on ice. The bearing strength of ice is always expressed for clear ice.

Freezing and melting

When surface water temperature drops below zero, the water starts to freeze. The ice cover may grow by one or two millimetres a day per degree of subzero temperature. When the average air temperature is –10 degrees centigrade, several centimetres of ice may form in 24 hours. Once the snow has melted in spring, solar radiation may penetrate the ice layer and also melt it from below. Cavities with rod-like crystals appear in the ice cover, and even if it is thick, it will no longer carry a person.

Ice cover and climate change

Data collected over the years on ice thickness and the length of the ice-covered period show how Finnish winters have changed. These datasets indicate that the ice cover forms a few weeks later and melts a few weeks earlier than hundred years ago. Consequently, the period of ice cover is now around one month shorter. The trend in ice thickness is less clear as it has decreased in Southern Finland but increased in Northern Finland as a result of higher precipitation and a larger share of slush ice.

Recreation on ice

Ice may be difficult to walk on, but strong ice also offers great possibilities for outdoor exercise and recreation. Many people enjoy tour skating, skiing on lake ice, winter swimming and fishing through ice. Ice-free winters and the shortening period of ice cover reduce these recreational opportunities. If you go out on ice, you should learn to check that it will carry you and observe your surroundings and potential hazardous places.

Water situation snapshots

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