Fluctuating sea levels

People living by oceans are accustomed to tides: the sea goes in and out regularly. While there are little or no tides in the Baltic Sea, its water levels fluctuate for other reasons.

The water surface in a bucket is always level, and the thickness of water is the same everywhere. In the sea the situation is more complex, and due to the effects of winds and atmospheric pressure, the water level can be up to several metres higher on one coast than on the other.

A high pressure over Finland usually means sunny weather. As its name indicates, high pressure means that the air mass over a certain area is slightly heavier than in other areas, whereas in conditions of low pressure it is lighter. This pressure difference affects the sea level, pushing the water away from the part under high pressure into an area where the atmospheric pressure is lower.

Wind also drives the water. The winds in the Baltic Sea frequently come from the southwest or west, pushing more water onto the Finnish coasts, whereas the opposite is true when the winds are easterly or north-easterly.

The effects of the wind are the most visible in the Gulf of Finland and Gulf of Bothnia. A wind direction parallel to the gulf pushes water towards its far end. Once the water comes across the shore it cannot go any further, and the sea level rises dramatically. In the northernmost corner of the Bay of Bothnia the water level can fluctuate by up to 3.5 metres within a few days.

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