Sea Ice

ice that forms in the sea as a result of the freezing of seawater. It differs substantially in physical properties from river and glacier ice.

The characteristic feature of sea ice is its salinity. When sea ice forms, small droplets of seawater (brine) are held between the ice crystals (which are pure water), making the ice saline. With the passage of the brine flows downward, the ice becomes desalted, and air bubbles appear in it, making it porous. The salinity and porosity of sea ice determine its density, which ranges from 0.85 to 0.93-0.94 g/cm3. As a result of its low density the ice is raised by one-seventh to one-tenth of its thickness above the water surface. Sea ice is not as strong as freshwater ice; its strength increases with decreasing salinity, porosity, and temperature. Unlike freshwater ice, sea ice begins to thaw given any increase in its temperature at temperatures higher than -23°C.

The following stages of development and types of sea ice are distinguished according to age: the initial forms (ice needles, slush, snow slush, floating slush, and bottom ice) and newly formed ice (pancake ice, brackish ice crust, young ice, gray and white ice). Sea ice is divided into three types depending on location and mobility: shore ice, or immobile ice that is frozen to the shore; floating (drifting) ice; and pack ice, or perennial ice, 3–5 m thick.

A distinction is made between sea ice that accumulates naturally (level ice) and ice that piles up (block ice and ice hummocks). According to dimensions, sea ice is divided into large ice fields (more than 2 km long), fragments of fields (2 km to 200 m), large broken ice (200-20 m), and small broken ice (less than 20 m). According to age, a distinction is made among spring ice, which formed in the spring preceding a given summer and is the least stable and most saline; yearly or annual ice, which formed in the fall of the preceding year and has great strength and thickness but lower salinity; and perennial ice, which has lasted through a winter, summer, and the following winter and has great stability and low salinity. The density of sea-ice cover is given in points, from 0 (open water) to 10 (complete ice cover). The area of a sea that is covered by ice changes from month to month and from year to year, depending on the reserves of heat in the sea and the duration of cooling of the sea and thawing of the ice.

The condition of sea ice is very important for navigation as a major obstacle—and, on the Northern Sea Route, a decisive obstacle—to shipping. Navigation in ice is usually possible only with a correct ice forecast, ship and air ice reconnaissance, and thorough study of ice conditions.