Published on: 21.6.2021

Information produced by Finnish Environment Institute

Surface water varies in temperature

Surface water temperature in the sea and lakes varies following an annual cycle. Not all years are the same, however, and weather conditions have a large impact on water temperature.

When solar radiation increases in spring, waters begin to warm from the surface. In calm and sunny conditions, the very top layer of the water may be warm enough for swimming already in late spring, whereas a little deeper down the water remains perishingly cold. If it gets windy, the top layers get mixed up and surface water cools down for a time.

Later in the summer, the water warms up more permanently and to a greater depth. A warm top layer of water (‘epilimnion’) of up to 15 metres sits over a cold bottom layer (‘hypolimnion’) and stays warm even in windy weather.

In the sea, cold water may still catch you by surprise. If the conditions are right, water from the cold bottom layer rises up to the surface along the coast, and the surface water temperature may drop by up to ten degrees within 24 hours.

In the autumn the surface water starts to cool down. When its temperature drops to +4 °C, the entire water column gets mixed up all the way down to the bottom in a phenomenon known as lake turnover. A similar turnover occurs in the spring soon after the ice has melted and the surface water reaches the temperature of four degrees.

Four degrees is an important limit value for water at which water density is at its highest and the water consequently at its heaviest. In lakes and the sea, the bottom layer stays at four degrees throughout the year. The lighter layer of on top of the bottom layer is warmer in the summer and colder in the winter.

Image: © Sampo Kiviniemi, Vastavalo