Cold fronts occur when a mass of cold air moves in and collides with an existing mass of hot air. Because cold air is heavier than warm air, drive cold air mass underneath the mass of warm as a wedge, lifting it higher in the atmosphere. The mass of heat rising in the end high enough to begin condensing, forming clouds and rain. The cold air is forcing the warm air to lift upwards, often leads to the development of thunderstorms or heavy snow in winter. For this reason, the majority of severe weather occurs near cold fronts.