When air masses collide

In a cold front, the leading edge of an advancing cold air mass meets less dense warm air and forces it up sharply like a blade, causing instability. Typically associated with low-pressure weather systems, cold fronts develop rapidly, often producing large cumulus and cumulonimbus clouds and triggering heavy rain and thunderstorms (Anatomy of a thunderstorm). Rainfall and winds are heaviest along the front. In a warm front, the leading edge of a mass of warm air meets a stationary, cold air mass and gradually rises above it along a slope that can stretch for hundreds of kilometres. As the warm air rises, it cools, forming cirrus clouds. If higher clouds form, condensation will follow, causing widespread precipitation accompanied by strong winds.