Figure 3. Air pressure at sea level in July and January for the northeast Pacific region. In winter, when the continental land mass is cold and the ocean relatively warm, a large-scale counterclockwise low pressure gyre (the Aleutian Low [AL]) develops, dominating the weather of the entire northeastern Pacific. In summer when the land mass is hot and the ocean relatively cool, a large-scale clockwise high-pressure cell (North Pacific High [NPH]) moves up from the south to dominate the weather in the northeast Pacific. Arrows indicate prevailing wind direction. Values on the diagram +10,000 divided by 10 gives pressure in millibars (modified after Favorite et. al. 1976).