Swedish Ice Service

The ice characteristics change as the salinity of water changes. In lakes and along the northern coasts of Sweden, the ice will become solid and clear. In the more saline waters of Skagerrak, the structure of the ice is more granular and porous.
In calm weather conditions the new ice will form a thin film. In particular this type of ice can be found in the archipelagoes and on small bays in calm weather conditions. When the wind becomes moderate or fresh, this ice will break up and warmer water from deeper layers will reach the surface. Instead, if the weather remains favourable for ice growth, the ice film will transform into thin level of continous new ice within 1-2 days.

Fotos illustrating icetypes (Click to enlarge picture)

New ice or dark nilas
The new ice (or dark nilas) is only a few centimeters thick and transparent.

Fast ice
Next stage in the development is the opaque fast ice. The fast ice not necessarily has to be connected to land but is always stationary when once formed and consists of clear ice (forming as water freezing on its under side) and an opaque layer of frozen wet snow on the surface of the ice. Generally a more or less coherent snow cover is on top of the fast ice.

Shuga
In open sea the water is usually at motion which means that an ice film can not form as wavelets in the surface circulates the water, resulting in a uniform cooling. Instead, at cold temperatures the freezing water forms so called grease ice or frazil ice, a viscous floating mass form which reduce the impact of waves and thus increasing the possibility of further ice formation and ice growth.

Pancake ice
Finally the shuga accumulate and freeze into circular pieces with raised, white rims as the pieces striking against one another.
This type of rims can also be found in close drift ice, originating from an area of consolidated ice. If such an area breaks up into irregular polygons, the ice floes gradually takes a round shape. One always can see the difference between this type of ice and the ’real’ pancake ice.

Pancake ice on Swedish west coast
A special type of pancake ice can be found on the Swedish west coast, sometimes also in southwestern Baltic.
Here, the freezing process takes place below the surface of the water between two layers; one low-salinity upper layer (brackish water) and one more saline lower layer, the latter with a temperature below zero degrees (however above the freezing point of saline water).
As the brackish water (top layer with less density) is overriding the saline water, large areas can be covered by ice in a few hours as it spontaneously rise to the surface with a splashing sound.

Jammed brash ice barrier
The wind can compact an area of broken new ice, pancake ice or slush against land or against a fast ice edge. Hereby a jammed brash ice barrier forms consisting of shuga. The brash ice barrier can extend several meters below the surface, extending horizontally up to a few nautical miles.
It may be very hard for ships to force due to the sticky and thick consistence.
As the prevailing wind direction changes, the ice barrier dissolves. It can also become very compact in connection with cold weather – becoming much thicker than the surrounding ice.

Rafted ice
The ice at sea obviously is very exposed to wind and currents. Depending on its thickness, it will more or less easily break up as the wind speed increases. In cases of thin and level ice, floes or vast ice fields often slide on top of each other, known as rafting. The edges usually form a zig-zag pattern with interlocking ”fingers” that alternatively push on top of or under each other. The ice thickness hereby rapidly may become doubled or tripled.

Drift ice
Drift ice at sea is a constant threat to the shipping during winter time. It is formed of broken, land fast ice from coastal areas or from ice frozen together at sea in calm conditions.
The drift ice is never at rest – even if the changes could be very small, they are always dangerous. Leads and cracks are not as persistent as in the archipelagoes and often close rapidly. Ridges then form, which can be difficult to pass.

Ice pressure and ridging
Ice under pressure is deformed against land, against land-fast ice or slower drifting ice. Ridges form continously as the floes are stacked on top of each other.
The visible part (so called ’sail’) is located up to a few meters above the sea surface while the underwater part (’keel’) may extend 10-30 meters deep. The floes are usually not frozen together.
If such a ridge is covered by snow, it may appear less dangerous. In spite of this, it is a significant hindrance to winter navigation. The ridge often drift aground. If the ice pressure then continues, the floes will build large and high stacks of ice.

Floebits
At the break-up of ice, single heavy drift ice floes (originally old ridges) exist, so called floebits.
The top side located at the surface of the water may be dark and rotten (porous) while the submerged part still may be very hard and of great dimensions. Floebits are a potential threat to shipping, particularly during darkness of in situations with poor visibility.