Ice has many forms in the Baltic Sea
Sea ice can be extremely strong and clear or full of bubbles and whitish in appearance. It may remain in one place or drift. In addition, the duration of ice cover varies in the Baltic Sea.
Sea ice vertically – columnar ice, snow-ice, and brine columns
If sea ice is viewed from top to bottom, many different layers can be distinguished within. From these layers we can see how the ice was formed.
In the autumn, as the air cools, the water chills and eventually freezes. This type of ice, frozen directly from the water, is called columnar ice and it is very strong and clear.
Water and snow falling on the ice form slush which eventually freezes. This type of ice is known as snow-ice. It is light in colour, formed from small crystals and contains many air bubbles. Compared to columnar ice, snow ice is much weaker.
Brine channels are typical features of sea ice. They are created because ice is formed of pure freshwater. Thus, growing sea ice tends to repel salt. This leaves salt within the ice, which forms cylindrical brine channels.
Sea ice increases in thickness by first freezing from the surface downwards from seawater to columnar ice. When snow has fallen on the ice, its thickness also increases upwards to form snow-ice. In springtime the ice thickness remains fairly constant, but the heat makes it brittle. Eventually it will melt completely.
A sample sawn from the ice. The upper part is light-coloured snow-ice and the lower part columnar ice.
Sea ice horizontally – fast-, drift- and pack ice
Sea ice can be divided into three types based on its mobility: fast ice, drift ice and pack ice.
Fast ice is stationary ice that is attached to islands, skerries, and shallows. It occurs along the coast and in the archipelago. Solid ice forms in early winter and remains in place until it melts.
Drift ice moves according to wind and water currents. Sea ice on the open sea can be regarded as drift ice and its coverage can range from 1 to 100 %. If the sea is completely covered by ice floes and there is no open water, the ice coverage is 100 %. If only individual floes float in open water, then the coverage percentage is low.
Pack ice is formed as accumulated banks of drift ice.
Satellite image of the eastern parts of the Gulf of Finland in winter 2021. There is solid ice on the coast, while the ice on the surface is loose drift ice.
Ice seasons in the Baltic Sea
While the Baltic Sea has sea ice every winter, its extent varies. During the mildest winters, the ice is mainly in the Bay of Bothnia, while in the harshest winters, almost the entire Baltic Sea is frozen.
In winter, the timing of the most extensive ice cover varies. In some years the greatest ice cover has been in January, while in others it has occurred in March.
The severity of an ice winter is classified based on the greatest surface area coverage of sea ice:
- Mild: less than 115,000 km²
- Average: 115,000–230,000 km²
- Severe: 230,000–345,000 km²
- Extremely severe: greater than 345,000 km²
A mild ice season can also be challenging for shipping. For example, even if there is relatively little ice, it can become very ridged and difficult to force.
Winter of 2007-2008 was mild and 2010-2011 was severe.
The largest ice area varies. Climate change decreases the ice covered area.
Ice seasons have become shorter. The figure shows the average length of ice seasons in different locations over three different 30-year periods.