Frozen sea in winter. The sea ice covers with snow and becomes about 1 to 3 meters thick. In spring a combined raise of temperature, change in ocean currents and strong winds break it up and carry it away as floating chunks ('pack'). Sea ice is dangerous to walk on as it contains cracks ('rivers'), can be thin or very soft and can be carried away in its entirety by strong katabatic winds in a matter of hours.
1. Specifically, ice formed by the freezing of seawater; as opposed, principally, to land ice. In brief, it forms first as lolly ice ( frazil crystals), thickens into sludge, and coagulates into sheet ice, pancake ice, or into floes of various shapes and sizes. Thereafter, sea ice may develop into pack ice and/or become a form of pressure ice. 2. Generally, any ice floating in the sea.