Weather systems affecting Newfoundland usually originate from the west, over mainland Canada, or from the southwest, from the east coast of the United States. Cyclonic storms consist of an area of low atmospheric pressure characterized by inward spiraling winds that rotate counter clockwise in the northern hemisphere. Such storms passing to the south of the island bring strong northeasterly winds sweeping in off the open North Atlantic Ocean. These storms are sometimes referred to as Nor'easters and are responsible for the worst of Newfoundland's weather. High winds sweeping over a large surface of ocean can build up very large waves. The frequency and severity of storms is greatest between November and March, although they may occur at any time of the year.