Air Masses, Fronts and Clouds

An air mass adopts the temperature and moisture of the region where it forms, and a front is the boundary where two air masses meet. A cold front, where cold air shoves warm air aside, is usually fast and narrow: expect a compact line of showers or thunderstorms and rapid clearing with cooler, drier air behind it. A warm front, where warm air rides up over cool air, is broad and gentle: expect widespread low clouds, steady precipitation, and lowering ceilings well ahead of it. A stationary front barely moves, and an occluded front forms when a cold front catches up to a warm front. The clouds tell the story too. Cumuliform (heaped) clouds mean rising, unstable air with possible turbulence and showers; stratiform (layered) clouds mean stable air and smoother flying. The lowest broken or overcast layer defines the ceiling, which is often what decides whether a VFR flight is legal and safe.