Four major types of air masses that influence the weather in North America are: Maritime Tropical (mT), Maritime Polar (mP), Continental Tropical (cT) and Continental Polar (cP). The climate of North America is influenced during winter by the Continental Polar air mass. The most common example of continental polar air entering the U.S. comes in winter, when the jet stream dips southward, carrying cold, dry cP air, sometimes as far south as Florida. When it moves across the Great Lakes region, cP air can trigger lake effect snow.