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Shallow coastal areas warm up and cool down earlier than in open sea areas. Conversely, on the coastline, the phenomenon known as upwelling can cause surface temperatures to drop rapidly in summer, sometimes by more than 10°C. In winter, the water temperature often falls below zero. This is because brackish water has a lower freezing point than freshwater.

Satellites can be used to produce temperature maps of surface waters in areas free of cloud cover. At sea, the temperature is measured using both wave and surface temperature buoys. These buoys are only deployed in the sea during ice-free periods and they measure water temperatures from a depth of a few tens of centimetres. Year-round observations of sea levels are obtained from 14 mareographs, i.e. water level measuring stations. Mareographs can give different results than buoys because they have a temperature sensor mounted permanently at a depth of two to three metres above the mean water level.

Take a look at the latest satellite-generated surface temperature map SYKE TARKKA service