Water temperature 23.4.2025

Surface water temperature measurements started in lakes in the southern and central parts of Finland

Due to the early spring and ice break-off, the water surface temperature measurement season began in April in many lakes in the southern and central parts of the country. On Wednesday 23 April, temperatures of 4–9 degrees Celsius were recorded. They were close to or slightly warmer than the long-term averages. The temperatures were higher last week, but the cooler weather has caused them to drop. In the May Day week, the waters are expected to start getting warmer again. The water warms up the fastest in small and shallow lakes and in calm weather when the wind does not mix water.

The surface water temperatures are measured at the depth of 20 cm at 8 in the morning, during open water season. The measurement points are usually close to the shore. Temperatures are representative of these locations and cannot be applied to larger areas. You can view the latest observations on the water temperature map (in Finnish). Everyone can also record their own observations, making them appear on the map. The Finnish Environment Institute does not verify these observations. You can observe the general development of surface water temperatures by the colours on the map.

Further information

Estimate of water temperature on a map (in Finnish)
Measuring the temperature of water systems (in Finnish)

The information on this page was produced by the Finnish Environment Institute (Syke).

How is surface temperature of water measured?

The surface temperature of water varies significantly depending on the season and weather. Temperatures are measured daily in lakes, rivers and the sea during the ice-free season, and the results are available without delay.

Lakes and rivers

The surface water temperatures of lakes and rivers are measured at 34 monitoring stations, and temperature data are available from 80 sites in total. The measurements come from automated instruments placed at the depth of 20 centimetres close to the shore, and the results are available in almost real time. The official reference value is the reading recorded every day at 8:00 a.m.

Marine areas

At sea, surface water temperatures are measured from a depth of 20 to 30 centimetres using buoys. Water temperature data are also obtained from sea water level monitoring stations which operate round the year. Their measurements are taken at a depth of two to three metres, however, and the results cannot be compared to the readings from the buoys.

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Fun facts about water temperature

Water is heaviest at four degrees

The water of lakes and the sea is stratified according to temperature. In summer, surface water is warmer than the water mass below it, while in winter surface water is colder. This is because the density of water is greatest at four degrees centigrade. In suitable temperature, flow and wind conditions, the stratification is broken and the top and bottom water layers are mixed. This phenomenon, called lake turnover, happens usually twice a year, in spring and autumn.

Cold water can be enjoyable

Bathing in cold water is believed to have positive wellbeing effects: winter swimmers find that a dip in freezing cold water refreshes the body and the mind. Winter swimming has been found helpful in stress management, and many also feel that exposure to cold relieves pain. Winter swimming is presumed to improve circulation, speed up metabolism and lower blood pressure. It is suitable for almost everyone, but it is a good idea to talk to your physician before taking the first plunge.

Climate change is warming up lakes

Lake waters have warmed up in recent decades faster than the oceans or climate in general. The pace of warming is fastest in northern latitudes, including in Finland, where the surface water temperature of lakes is estimated to increase by one or two degrees by 2050. Such a change has a significant impact on lake ecosystems and water quality. Animal and plant species that have adjusted to previous temperatures may even be lost, and eutrophication and algal blooms will be boosted.

Lake temperature profile

In seven Finnish lakes, water temperatures are measured at different depths. The measurement interval is one metre to a depth of 20 metres from the surface, and two metres to a depth of 50 metres. Any deeper than this, the measurement interval is five metres. These results can be used to create lake temperature profiles. The measurements are taken using a temperature probe in lakes Kallavesi, Pielinen, Päijänne, Konnevesi, Säkylän Pyhäjärvi, Pesiöjärvi and Inarijärvi three times a month.

Water may cool down or warm up rapidly

The temperature of bathing water may change rapidly; water that was nice and warm yesterday may be surprisingly cool today. This happens when water from the cold bottom layer rises to the surface, which occurs most frequently in coastal areas but also in large lakes especially in early summer when the warm surface layer is still quite thin. On the other hand, water on the shores of large lakes may warm up quickly if onshore winds push surface water warmed by the sun to the shore.

Water situation snapshots

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Water temperature situation