Fronts Explained

An air mass is a large body of air with fairly uniform temperature and humidity, taken from wherever it spent time sitting still — polar continental, tropical maritime, and so on. A front is the boundary where two of those masses meet. Because their densities differ, the lighter (warmer, moister) air always ends up above the denser (colder, drier) air. What varies is the geometry — how steeply the boundary tilts and how fast it moves.

A warm front is a warm air mass advancing on a retreating cold one. Because warm air is less dense, it overruns the cold wedge — riding up its shallow upper surface at a slope of roughly 1:100 to 1:200. That gentle ramp produces a long, layered cloud stack that a pilot sees hours before the front arrives.

A cold front is a cold air mass advancing on a warm one. The cold air undercuts abruptly at a slope of roughly 1:40 to 1:80, lifting the warm sector violently. Where the warm air is moist and unstable, cumulonimbus builds along the frontal band — sometimes as an organized line of thunderstorms called a squall line, 50–100 nm ahead of the surface front itself.

When a fast cold front catches a warm front, the warm sector is lifted clear of the surface: an occlusion. Occluded fronts carry the broad stratiform deck of a warm front and the embedded convection of a cold front, which is the worst combination for a VFR pilot. A stationary front is a frontal boundary that has stalled; it can hang across a route for days, holding steady precipitation and low ceilings in place until the pattern moves.