A polynia is an area of open water or water covered by a thin layer of ice within pack ice. It is formed during the summer and is generally found in the same location from one year to the next.
Polynias can be seen in the Arctic and in Antarctica and are caused by local conditions, by warming due to vertical marine currents (upwelling) that bring warmer water to the surface and also by the break-up of pack ice due to the effects of winds or currents.
Their size varies from several square kilometres to 50,000 square kilometres, as is the case for the large polynia in the Weddell Sea.
In the Arctic, these areas of open water are the home for many species, plankton (plant or animal), bears and marine mammals.