large, persistent, atmospheric low-pressure centre that forms between Iceland and southern Greenland. It often causes strong winter winds over the North Atlantic Ocean. In winter, the ocean is considerably warmer than the continents, and this difference is responsible for the location of the Icelandic low, which dominates the wind circulation over the North Atlantic. In summer, the low weakens and often divides into two separate cells. Its January mean sea-level pressure is below 996 millibars (29.4 inches of mercury).