large atmospheric low-pressure (cyclonic) centre that frequently exists over the Aleutian Islands region in winter and that shifts northward and almost disappears in summer. Although the Aleutian low is associated with smaller eastward-moving low- and high-pressure centres, the region's average pressure is low. It is the source region of the maritime polar air masses that influence the climate of the western U.S. The low-pressure centres that persist in the higher latitudes of the Northern Hemisphere consist of the Aleutian low and the Icelandic low.