An air mass that forms in the north Atlantic ocean in winter over cold water and then comes ashore is a Continental polar (cP) or continental arctic (cA).
These air masses are chill, dehydrated, dry, and stable. These develop over northern Canada and Alaska as a consequence of radiational cooling. They drive southward, east of the Rockies into the Plains, then eastward.
Air masses are also determined and established on whether they develop over land or over water. Maritime air masses form over the water and are moist. Continental air masses form over land and are dry. Thus, an air mass that extends over northern Canada is named a continental polar air mass and is cold and dry.
