Sailors in the Arctic will encounter varying ice conditions and many types of ice. A discussion of ice types deserves an article in itself. In this second interview Nick Hughes, head of the Norwegian Ice Service, shares his knowledge on this topic.
“There are two main types of ice: sea ice, that is ice that has formed by the freezing of sea water, and icebergs, which are ice that has formed on land as glaciers and then broken off to float out to sea.
“For sea ice there are three main categories: new ice, first-year ice and multi-year ice. Again there are several different stages of new ice, which depend on the environmental conditions in which it is forming. On calm water you first get a very thin sheet, called grease ice, which thickens into nilas, and then grey-white ice. In water with waves the ice cannot form a sheet, so you get lots of ice crystals suspended in the water, called frazil ice, which thickens the water to a soupy consistency. The frazil tends to clump together into balls, which eventually become glued together as “pancake” ice. This is a circular disk of glued-together frazil crystals, which, as it gets shunted about by the waves, causes the edges to be broken up and the ice crystals built-up to form a rim.
“Eventually enough ice forms to freeze together into a solid stable and flat sheet, onto which snow can fall and remain. This is first-year ice. The ice growth then continues from the underside and through the course of a winter and spring can grow up to two metres thick. The ice still has traces of sea water, brine, enclosed as little pockets within its structure. This makes first-year ice relatively soft, and so ice drift causing the floes to push together can break it up to form ridges and its is easier for icebreakers to break through it. The ridges are made up of angular ice block that are not strongly bonded together, and the structure of the ice is quite plainly visible on the edges of these,” explains Nick Hughes.
“Multi-year ice is ice that has survived at least one summer melt. However, this definition could also include ice that has formed in the spring and survived, in other words ice that is less than six months old. Therefore some people also use the term second-year ice to cover this. The summer causes melting on the surface of the ice, allowing water to collect into melt ponds. The water in these is fresh, but sometimes the ice melts to the extent that it extends through to the sea below. Ridge blocks are melted and fused together so that the ridge becomes a solid mass. Refreezing in the autumn then gives the ice a bumpy top surface, with many small hills and valleys, for which the term hummocky is used. The melt process flushes most of the brine pockets out of the ice and on refreezing the ice is much more solid than first-year ice. This makes it more resilient to breaking-up either by natural forcing or by icebreakers,” says the ice expert.
“First-year ice can grow up to about two metres thick.
“Normally the circulation of ice in the Arctic Ocean can allow multi-year ice to survive for as much as four to five years. In this time the ice can accumulate up to an average thickness of six–seven metres. Of course where you find compressed ridges the ice can be much thicker, particularly in the region north of Greenland and Ellesmere Island. The largest ridge I have seen had a keel – the portion below sea level – that extended down to a depth of 38 metres. This is not as much as an iceberg, which can actually extend hundreds of metres below the surface.
“Icebergs are generally found in waters off Greenland and around the various Russian Arctic islands. Because these generally have more ice below the water level, they tend to move with the ocean and tidal currents rather than the wind. This means that sometimes they can be clearing their own path through a field of sea ice. Icebergs are also a serious problem for ships navigating in the Arctic. Because glaciers can grow to great thicknesses, the ice at the bottom of these is strongly compressed and is much tougher than sea ice. When an iceberg is broken up at sea the fragments of ice, called growlers and bergy bits, only have a small portion visible above the sea level, making them very difficult to spot. Most incidents involving ice damage to ships occur because of hitting these,” says Nick Hughes and adds:
“If you would like to know more about sea ice, I recommend reading The Mariner’s Handbook, as well as the Canadian Ice Service’s Manual of Ice.”