ARCHIVED - CHAPTER 3: Ice and Weather Environment
This information has been archived because it is outdated and no longer relevant.
Information identified as archived on the Web is for reference, research or recordkeeping purposes. It has not been altered or updated after the date of archiving. Web pages that are archived on the Web are not subject to the Government of Canada Web Standards. As per the Communications Policy of the Government of Canada, you can request alternate formats by contacting us.
3.2 Ice Physics
This section describes some key elements of the physical properties of ice. The intent is to provide information that will help in the interpretation of both regional ice conditions and ice charts, and that will be useful in subsequent discussions of ice navigation practices.
The terminology used in this manual is that used by mariners and scientists who deal with ice regularly. A list of Ice Terminology is provided in Annex A. These definitions have been developed and approved by the World Meteorological Organization. For more complete information on ice terminology, refer to the Manual of Standard Procedures for Observing and Reporting Ice Condition (MANICE), produced by Canadian Ice Service, Environment Canada. Copies of MANICE are available through Environment Canada, Ottawa.
Drift ice/ pack ice: Term used in a wide sense to include any area of ice, other than fast ice, no matter what form it takes, or how it is disposed. When concentrations are high, 7/10 or more, drift ice may be replaced by the term pack ice.
Fast Ice: Ice which forms and remains fast along the coast, and it is attached to the shore, to an ice wall, to an ice front, between shoals, or grounded icebergs. If thicker than 2 m above sea-level, it is called an ice shelf.
Floe: Any relatively flat piece of ice 20 m or more across.
Ice island: A large piece of floating ice protruding about 5 m above sea-level, which has broken away from an Arctic ice shelf. Has a thickness of 30 to 50 m and an area of from a few thousand square metres to 500 sq. km or more. It is usually characterized by a regularly undulating surface giving it a ribbed appearance from the air.
Ice shelf: A floating ice sheet of considerable thickness showing 2 - 50 m or more above sea-level, attached to the coast. Usually, an ice shelf is of great horizontal extent and has a level, or gently undulating, surface. It is nourished by annual snow accumulation and also by the seaward extension of land glaciers. Limited areas may be aground. The seaward edge is termed an ice front.
Iceberg: A massive piece of ice of greatly varying shape, protruding 5m or more above sea-level, which has broken away from a glacier, and which may be afloat or aground. It may be described as tabular, domed, pinnacled, wedged, drydocked, or blocky. Sizes of icebergs are small, medium, large, and very large.
Nilas: A thin elastic crust of ice, easily bending on waves, and swell, and under pressure, growing in a pattern of interlocking fingers (finger rafting). It has a matt surface and is up to 10 cm in thickness. May be subdivided into dark nilas and light nilas.
Different forms of ice can be distinguished on the basis of their place of origin and stage of development. The principal kinds of floating ice are:
- lake and river ice, formed from the freezing of fresh water;
- sea ice, formed from the freezing of sea-water; and
- glacier ice, formed on land or as an ice shelf from the accumulation and recrystallization of snow.
Types of lake ice are identified as being new, thin, medium, thick, or very thick, on the basis of their stage of development. New lake ice is recently formed and is less than 5 cm thick. Thin, medium, and thick lake ice range in thickness from 5-15 cm, 15-30 cm, and 30-70 cm, respectively, whereas very thick lake ice is greater than 70 cm in thickness.
Sea ice is categorized as new ice, young ice, first-year ice, and old ice. Within each of these categories there are terms referring to more specific types of ice. Details concerning more specific ice types can be found in Annex A. New ice is recently formed and composed of ice crystals which are only weakly frozen together and as the ice develops it forms a thin elastic crust over the ocean surface (nilas). Young ice represents a transition stage between nilas and first-year ice. Young ice ranges in thickness from 10-30 cm and, as it thickens, grows progressively lighter in colour from grey to grey-white. First-year ice is ice of not more than one winter's growth, ranging from 30 cm to over 2 m thick. Old ice is sea ice which has survived at least one summer's melt. It is thicker and less dense than first-year ice and generally has smoother or rounder surface features. It can be divided into second-year or multi-year ice if the history of the ice is known.
Ice of land origin includes icebergs and ice islands. Icebergs are further typed by size and shape, with growlers (length less than 5 m) and bergy bits (length 5 to 15 m) representing the smallest iceberg pieces. Larger icebergs range from small (5 to 15 m above sea level and 15 to 60 m in length) to very large (higher than 75 m and longer than 200 m). According to shape, icebergs are frequently described as being tabular, domed, pinnacled, wedged, drydocked, or blocky.
Brine: Water containing salt(s).
First-year ice: Ice of not more than one winter's growth, ranging from 30 cm to 2 m thick.
Old ice: Ice which has survived at least one summer's melt. It is thicker and less dense than first-year ice and generally has smoother or rounder surface features.
Salinity: Amount of salt(s) in solution in water, usually given as parts per thousand (ppt).
The structure of an initial ice cover is dependent on weather and sea-state conditions at the time of ice formation. Under calm conditions, large ice crystals form at the surface which gradually interlock. This layer may be as little as 1 to 2 cm in thickness. In more turbulent conditions, ice crystals in the surface layer will tend to be smaller, and may form quite a deep layer, for instance, up to 3 m thick off the Alaskan Coast.
Once an initial layer of ice has formed on the surface, ice growth continues downward. Beneath a transition zone the ice is composed primarily of long columnar ice crystals. Figure 12 illustrates the characteristic crystal structure within young sea ice.
As the ice grows downward, brine is frozen into the ice crystals, but through the winter the brine solution gradually drains downward with the result that, at a given level in the ice, the salinity will change as the ice cover thickens. During the summer season, surface melt-water drains through the ice, helping to flush out additional brine from the ice. Ice which survives more than one year takes on a layered structure and horizontal layers represent ice growth during successive years.
In addition to the fact that old ice tends to be thicker than first-year ice, its lower salinity is an important consideration for ice navigation, as ice strength is closely related to brine volume. With lower salinities, old ice is much stronger than first-year ice.
WARNING: OLD ICE IS HARDER, STRONGER, AND USUALLY THICKER THAN FIRST-YEAR ICE. CONTACT WITH OLD ICE SHOULD BE AVOIDED WHENEVER POSSIBLE.
Frazil Ice: Fine spicules or plates of ice suspended in water.
Grease Ice: A later stage of freezing than frazil ice, where the crystals have coagulated to form a soupy layer on the surface. Grease ice reflects little light, giving the water a matte appearance.
Grey Ice: Young ice is usually 10-15 cm thick. It is less elastic than nilas, and breaks on swell, and usually rafts under pressure.
Pancake Ice: Predominantly circular pieces of ice 30 cm to 3 m in diameter, up to 10 cm in thickness, with raised rims due to the pieces striking against one another. It sometimes forms at some depth where water bodies of different physical characteristics meet, then it floats to the surface. The growth of pancake ice may rapidly cover wide areas of water.
Several forms of ice may be encountered: sea ice, lake ice, river ice, icebergs, and ice islands. The freezing of fresh- and salt-water does not occur in the same manner and the following brief explanation is limited to the formation of sea ice from salt-water.
When considering the freezing process, dissolved salts are important not only because they lower the water's freezing temperature (typically around -1.8°C for sea water of 35 parts per thousand salt), but also because they effect the density of water. The loss of heat from a body of water takes place principally from its surface to the surrounding air or water. As the surface water cools, it becomes more dense and sinks, to be replaced by warmer, less dense water from below. The cycle repeats until the water temperature reaches its freezing point. This process takes longer as the amount of salt in the water increases. As a result, the onset of ice formation will be delayed.
The first visual indication of ice formation is the appearance of spicules or plates of ice in the top few centimetres of water. These spicules are also known as frazil ice and give the sea surface an oily appearance. As cooling continues, the ice crystals grow together to form grease ice, which gives the sea surface a matt or dull appearance. Eventually, sheets of ice rind or nilas are formed, depending on the rate of cooling and on the salinity of the water. Wind and waves frequently break the ice into smaller pieces which soon become rounded as they collide with each other. The resultant ice is termed pancake ice. Individual pancakes may later freeze together, gradually thickening from below as additional sea-water cools and freezes.
The rate of freezing is controlled by the severity and duration of cold air temperatures. At -30° to -40°C, grey ice can form from open water in 24 hours. However, the thickening ice also acts as an insulator against the cold air, and the growth rate gradually diminishes. Even at these low temperatures, it would take a month for the ice to reach the thin first-year stage. Snow cover, which has approximately 10 times greater insulating value than sea ice, will also contribute to lower growth rates.
Sometimes the amount of snow cover may be so great that its weight depresses the underlying ice to the point that its surface is below the water level. The lowest layers of the snow cover may then become waterlogged and freeze, adding to the ice thickness. This happens often on the Great Lakes and the lower St. Lawrence River.
During the initial ice formation process, as ice crystals form and existing ones grow larger, brine becomes trapped in small cells within the ice matrix. The amount of brine trapped in the ice depends on the rate at which ice forms, with greater amounts of brine retained when ice formation is rapid. Slow ice growth allows a large portion of the brine to drain away. The amount of brine in the ice has an important bearing on its strength: the greater the brine content, the weaker the ice.
A second factor affecting the strength of ice is its age. As air temperatures warm and the ice approaches its melting point, entrapped brine begins to drain away, lowering the overall salinity of the ice cover. Should temperatures drop back below the freezing point before the ice melts entirely, it will re-freeze as purer and stronger ice. For this reason, ice more than one year old will be stronger than first-year ice for a given thickness and temperature, an important factor to consider when navigating in regions where old ice may be found.
Deform: To change the shape; for ice this usually involves ridging and rafting.
Floe: Any relatively flat piece of ice 20 m or more across.
Hummocked ice: Ice piled haphazardly one piece over another to form an uneven surface. When weathered has the appearance of smooth hillocks.
Ice edge: The demarcation at any given time between the open water and sea, lake, or river ice (whether fast or drifting). May be termed compacted or diffuse.
Lead: Any fracture or passage-way through ice which is navigable by surface vessels.
Polynya: Any non-linear shaped opening enclosed by ice. May contain brash ice and/or be covered with new ice, nilas, or young ice; submariners refer to these as skylights.
Rafted ice: Type of deformed ice formed by one piece of ice overriding another.
Ridged ice: Ice piled haphazardly one piece over another in the form of ridges or walls. Usually found in first-year ice.
Ice normally forms near coasts first and then develops seaward. A band of fairly level ice becomes fast to the coastline and is held immobile. The seaward extent of fast ice formation will be limited by factors which can contribute a stable anchor for the ice. As an example, more fast ice would be expected in shallow coastal areas, or ones with numerous islands, than in areas where water depths drop sharply from the coast. Beyond this fast ice lies the pack or drift ice, which is free to move in response to wind and water forcing.
An area of newly formed ice seldom remains unaltered for long. Winds, currents, tides, and thermal forces cause the ice to undergo various forms of deformation. Wind causes ice floes to move generally downwind at a rate which varies with wind speed, concentration of the pack ice, and the extent of ice ridging or other surface roughness. A rule of thumb which is often used to estimate pack ice motion is that the ice will move at 30° to the right of the wind direction at about 2 per cent of the wind speed.
One effect the wind has when it blows from the open sea onto floating ice is to compact the floes into higher concentrations along the ice edge, producing a relatively well-defined boundary between ice and open water. When winds blow off the ice toward the sea, the floes near the ice edge will be dispersed, resulting in lower ice concentrations and a diffuse ice/water boundary. As sea ice is partially submerged in the sea, it will also move in response to near surface currents and tides. As a result, the net movement of the ice is a complex product of both wind and water forces and consequently is difficult to forecast.
Thermal forces cause ice deformation: as temperatures drop, ice expands. For a drop in ice temperature from -2°to -3°C, ice with a salinity of 10 parts per thousand will expand 0.3 m for every 120 m of ice floe diameter. At the same temperatures, for ice with a salinity of 4 parts per thousand, the rate is about one third this amount. Below -18°C and -10°C respectively, 10 parts per thousand saline ice and 4 parts per thousand saline ice cease expanding and, as temperatures drop further, contraction occurs. Although the amounts of thermal expansion and contraction may seem small, they can result in pressure ridge development under some circumstances.
Atmospheric and oceanographic forces contribute additional energy to deform pack ice. As ice is subjected to pressure from winds or currents, it may fracture and buckle to produce a rough surface. In new and young ice, this results in rafting as one ice sheet overrides another. In thicker ice, pressure leads to the formation of ridges and hummocks, when large pieces of ice are piled up above the general ice surface and large quantities of ice are forced downward to support the additional weight. As a general rule, the below-water portion of ice is in the order of three to four times as deep as the above-water height.
NOTE: Total ice thickness below water is three to four times the ice height above the water-line.
Pressure arising from strong winds can be severe and usually persists until the wind subsides or changes direction. The extent of ridging caused by pressure depends on whether or not the leeward boundary of the ice field was against land or closely packed ice when onshore winds began. In such cases, the floes within the ice field may become pressed together, eventually increasing to 10/10 concentration, with pressure developing throughout.
Pressure within an ice field can also be caused by tides. Tidal pressure is usually of short duration, lasting from one to three hours and, although less heavy than pressure from winds of longer duration, it can at times bring shipping operations to a halt. Tidal pressure can be particularly significant in restricted channels where the tidal effect is enhanced and ice movement is restricted.
NOTE: Onshore winds and tidal currents may cause pressure within ice fields. Pressure may be so severe as to restrict a vessel from moving.
Cracks, leads, and polynyas may form as pressure within the ice is released or tension occurs. Offshore winds may drive the ice away from the coastline and open a shore lead or push pack ice away from fast ice. In some regions where offshore winds prevail during the ice season, local shipping and vessel movement may be possible throughout much of the winter season. However, brief periods of onshore wind may cut off any leads and entrap vessels.
WARNING: MARINERS NAVIGATING THROUGH OPEN WATER LEADS ARE URGED TO DO SO WITH EXTREME CAUTION. THE NAVIGATOR SHOULD TRY TO ANTICIPATE THE EFFECT OF WINDS AND CURRENTS ON POSSIBLE CHANGES IN LEAD CONDITIONS.
Ablation: Wasting or erosion of floating ice or iceberg, by melting, or water action, or evaporation.
Close ice: Floating ice in which the concentration is 7/10 to 8/10, composed of floes mostly in contact with one another.
Open ice: Floating ice in which the concentration is 4/10 to 6/10, with many leads and polynyas. Generally floes are not in contact with one another.
Open water: A large area of freely navigable water in which ice is present in concentrations less than 1/10. No ice of land origin is present.
Very close ice: Floating ice in which the concentration is 9/10 to less than 10/10.
Very open ice: Ice in which the concentration is 1/10 to 3/10 and water dominates over ice.
Ice may be cleared from an area by winds and/or currents, or it may melt in place. Where the ice field is well broken (open ice or lesser concentrations), wind plays a major part as resulting wave action will cause considerable melting. Where the ice is fast or in very large floes, the melting process is primarily dependent on incoming radiation. Air and water temperatures and some types of precipitation also have a significant effect on ice melt.
Snow cover on the ice acts initially to slow ice ablation, because it reflects almost 90 per cent of incoming radiation back to space. However, as temperatures rise above 0°C, and the snow begins to melt, puddles form on the ice surface. These puddles absorb about 60 per cent of incoming radiation, causing the water to warm and the puddle to enlarge rapidly. Heat from the melt-water is transferred to the ice below causing the ice to weaken. In this state, it offers little resistance to the decaying action of wind and waves. The puddling of melt-water on the ice, which usually occurs extensively in the Canadian Arctic, promotes accelerated ice decay and breakup.
- Date modified: