In summer, the surface water in open sea areas is usually 15 to 20°C, and up to five degrees colder in the Bay of Bothnia. During long warm periods, coastal waters can warm to well above 20°C. On the other hand, coastal waters can also cool rapidly if the weather conditions are such that cold off-bottom water wells up to the surface along the shoreline. In mid-winter, the surface water temperature drops slightly below 0°C.

At most observation stations, the average summer surface temperature has risen over the past 40 years. This is probably related to the warming of summer temperatures within this period.

Learn more about temperature variations in the Baltic Sea: long-term trends and both seasonal and vertical depth variations!