Air Masses, Fronts and Frontal Weather
Air masses are classified by origin and track: continental or maritime, tropical, polar or arctic. As an air mass moves it is modified by the underlying surface. Warm air moving over a cold surface becomes stable (stratiform cloud, poor visibility); cold air moving over a warm surface becomes unstable (cumuliform cloud, showers, good visibility between showers).
Fronts
A cold front has a steep slope (about 1:50 to 1:100), fast movement, narrow active weather belt, squall lines, heavy showers, hail and a sharp wind veer with a temperature fall. A warm front has a shallow slope (about 1:100 to 1:150), a wide belt of layered cloud (cirrus, cirrostratus, altostratus, nimbostratus) and steady rain, with a gradual temperature rise. A stationary front barely moves; an occluded front forms when the cold front catches the warm front, giving mixed weather.
Traps
Candidates frequently confuse which front has the steeper slope, and forget that the warm front's approach is signalled by falling pressure and high cirrus. Fronts are strongest aloft; the polar front jet stream is associated with the polar front.
Worked example: a sequence of cirrus, then lowering altostratus with continuous rain and a slow temperature rise means a warm front is approaching; expect the wind to back and pressure to fall before passing.
Trap: after a cold front passes, pressure rises, temperature falls and visibility improves.