Cold fronts move faster than warm fronts because their cooler temperature increases the density of the air and concentration of molecules. Warm fronts are sometimes nearly motionless when cold fronts take them over. This cold, heavy air mass lifts the warm front out of the way.
Cold fronts often bring extreme weather, including thunderstorms and rain. Because they lift the air instead of pressing it against the air, meteorologists call cold fronts low-pressure fronts. Warm fronts pass over cold fronts, bringing warmer temperatures and more stable weather. They are high-pressure fronts.
Fronts are boundaries that separate warm and cold air. This boundary line usually separates cold, dense air from warm, light air. When fronts pass through an area, the weather tends to change.
Along the Eastern Seaboard of the United States, Nor'easters are common cold fronts. These systems include the collision of a fast moving polar air mass from Canada and the North Atlantic with a tropical warm air mass from the warm ocean. The cold front lifts the warm, moist air, causing it to cool so water condenses. As it passes through the cold air mass underneath, it forms snow. This collision of fronts is responsible for large blizzards that often shut down entire cities.