Sea ice is a very important element in Viking travels. Not only do floating pieces of ice present a hazard to sailing vessels, but when large areas of water freeze over, they make it impossible to travel by ship. This fact along with the more intense weather which affects the Norwegian region during the winter, Vikings usually did not travel by sea during this time.
The major current flow in the Atlantic is clockwise, with the North Atlantic Current (warm) flowing across the northern Atlantic and often picking up large pieces of ice (icebergs) as they fall from the glaciers of Greenland. The current then continues on to Europe where the current splits and a portion turns northeastward and the rest turns southward and flows along the coast of Portugal and is known as the Canaries current. The northeast branch often brings up icebergs along the Norwegian coast. It is important to realize that only 1/3 of an iceberg is above water and that the uneven jagged edges under the water often extend a great distance.
In general, the ice season begins in late October and ends in early June. Surface waters are then around 10°C to 11°C (50°F to 52°F) when ice begins to form in the Northern Baltic. During March, increasing solar radiation or warmer days due to more sunlight heats up the surface waters and the ice begins to melt and break up. The entire Baltic can freeze over during a severe winter except for a small area in the extreme south. Also, ice may form in the Oslofjord and along the Skagerrak coast. Large sections of the Baltic remain ice-free during a mild winter, with an average temperature of 4°C (39°F) in the Southern Baltic Sea.
In areas of the Baltic Sea and within its gulfs, where the salinity, or the salt content, is low (this is due to fresh water runoff from the numerous rivers that flow into the area), and the air temperature is comparatively low in the winter, ice forms every winter. In the middle of the winter the whole gulf is usually covered by “continuous ice“. Near the turn of the year continuous and “fast ice” forms also at Helsinki and at Stockholm and far into the Bothnian Sea, although the central area of this sea is normally covered by “drift ice” or “pack ice” only. It is not until the beginning of February that ice forms in the southern most harbors of Sweden and in the ports of the Danish Islands of the Baltic Sea.
Just a reminder that seasonal temperatures during each winter vary greatly and during a warm winter, sea ice will not form as extensively as during a colder one.
Another factor to consider is that during an abnormally cold winter with persistent easterly wind, ice in the Baltic Sea is pushed out through the Danish Sounds into the Kattegat and the Skagerrak. This ice can congest and block or impede shipping traffic.
During such winters, even the Oslofjord and the ports along the coasts of Kattegat and Skagerrak may be blocked by ice for a longer period of time.
Ice can also accumulate on the surface of vessels. This makes the vessel heavier and slows it down or cause it to sit lower in the water increasing the chance of water coming over the bow and flooding. Ice accumulation may occur from the following:
- If a vessel is in fog during freezing conditions. Fog is basically water suspended in air around particles of dirt or salt. When the fog comes into contact with a ship’s surface under freezing conditions, it will freeze to that surface.
- During periods of rain or freezing rain. As the cool water comes into contact with the vessel, it freezes on contact. This type of ice accumulation is a very hazardous as ice may build up rapidly on the surface adding weight on rapidly to the ship.
- Sea spray or sea water breaking over the ship when the air temperature is below the freezing point of seawater (28.6°F). This type is the most serious of all as intense storms with high winds and seas can that last several days.
Superstructure icing potential is highest in the Gulfs of Bothnia and Finland, this is due to their northern setting (cooler temperature compared to the southern Baltic) and the addition of the fresh water run-off which allows the water to freeze at a higher temperature. Conditions for moderate icing are present from about October through April. The potential for moderate icing from the Skagerrak to the Baltic is a possibility from November through April.