Winter storms or snowstorms happen when warm, wet air meets with cold air. The warm, wet air mass and the cold air mass can each be 1000 km or more in diameter. Snowstorms affecting Northeastern United States often get their moisture from air moving north from the Gulf of Mexico and cold air from air masses coming down from the Arctic. In the Northwest United States warm, wet air from the Pacific Ocean cools when it is pushed upward by the mountains. Many different things can affect the direction of movement, content and temperature of air masses. All of these differences affect the type and severity of the snowstorm.