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WMO concentration colour code: | Chart Item | Explanation |  | Ice free: no ice present |  | Open water: ice concentration less than 1/10. A large area of freely navigable water, in which sea ice is present in concentrations less than 1/10. |  | Very open ice: ice concentration 1-3/10. Floating ice in which the concentration is 1/10 to 3/10 and water preponderates over ice. |  | Open ice: ice concentration 4-6/10. Floating ice in which the concentration is 4/10 to 6/10, with many leads and polynyas, and the floes are generally not in contact with another. |  | Close ice: ice concentration 7-8/10. Floating ice in which the concentration is 7/10 to 8/10, composed of floes mostly in contact. |  | Very close ice: ice concentration 9/10 to 10/10. |  | Fast ice: Sea ice which forms and remains fast along the coast, where it is attached to the shore or between shoals. Vertical fluctuation may be observed during changes of sea-level. Fast ice may be formed in situ from sea water or by freezing of drift ice to the shore, and it may extend a few meters or several hundred kilometres from the coast. |  | Ice shelf. |  | New, close or very close ice with ice concentration 7/10 -10/10, but thickness less than 10 cm. |  | Nilas, ice concentration 9-10/10. Grey ice, mainly on leads. A thin elastic crust of ice, easily bending on waves and swell and under pressure, thrusting in pattern of interlocking “fingers” (finger rafting). Has a matt surface and is up to 10 cm in thickness. |
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Symbols: | Chart Item | Explanation |  | Ice-free: no ice present. |  | Open water: ice concentration less than 1/10. A large area of freely navigable water, in which sea ice is present in concentrations less than 1/10. |  | Very open ice: ice consentration 1-3/10. Floating ice in which the concentration is 1/10 to 3/10 and water preponderates over ice. |  | Open ice: ice concentration 4-6/10. Floating ice in which the concentration is 4/10 to 6/10, with many leads and polynyas, and the floes are generally not in contact with another. |  | Close ice: ice concentration 7-8/10. Floating ice in which the concentration is 7/10 to 8/10, composed of floes mostly in contact. |  | Very close ice: ice concentration 9/10 to less than 10/10. |  | Undefined ice |  | New ice: A general term for recently formed ice. These types of ice are composed of ice crystals, which are only weakly frozen together. |  | Level ice: Sea ice which has not been affected by deformation. |  | Fast ice: Sea ice which forms and remains fast along the coast, where it is attached to the shore or between shoals. Vertical fluctuation may be observed during changes of sea-level. Fast ice may be formed in situ from sea water or by freezing of drift ice to the shore, and it may extend a few meters or several hundred kilometres from the coast. |  | Ridged or hummocked ice (f = number of ridges / nautical mile). Ice piled haphazardly one piece over another in form of ridges or walls. |  | Rafted ice (C = concentration). Type of de-formed ice formed by one piece of ice overriding another. |  | Ice edge or ice boundary. The demarcation at any given time between the open water and sea ice of any kind, whether fast or drifting (cf. ice boundary). It may be termed compacted or diffuse (cf. ice boundary). |  | Fracture: Any break or rupture through very close ice, compact ice, consolidated ice, fast ice, or a single floe resulting from deformation processes. Fractures may contain brash ice and/or be covered with nilas and/or young ice. Length may vary a few meters to many kilometers. |  | Lead: Any fracture or passage-way through sea ice which is navigable by surface vessels. |  | Water temperature isotherm, °C |  | Thickness measured in cm |
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