Water temperature 1.9.2023
Surface waters have cooled down slightly but remain warmer than the average of the season
During the past week, the temperature of surface waters decreased by a few degrees, but September begins warmer than average in terms of surface waters. Surface water temperatures were in many places 1–3 degrees Celsius warmer than the average of the season. On Thursday 31 August, the surface water temperature in southern and central Finland varied between 16 and 19 degrees. In North Ostrobothnia, the surface water temperature varied between 15 and 17 degrees and in Lapland between 13 and 16 degrees. The warmest surface water was still at the observation site of Lauritsala in Lake Saimaa, nearly 19 degrees. According to current medium-term water system forecasts, the temperature of surface waters will gradually cool down as the autumn arrives. Surface water temperature is measured at 8 in the morning at the depth of 20 cm.
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.
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.
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.