Air Masses, Fronts and Synoptic Systems
Air masses
An air mass takes its properties from its source region: polar or tropical, maritime or continental. Cold air moving over a warm sea becomes unstable and showery; warm air moving over a cold surface becomes stable with fog and stratus.
Fronts
A cold front is steep, moves fast, and brings a narrow band of heavy showers, squall lines and a sharp wind veer with a temperature fall. A warm front has a shallow slope, and its approach is marked by thickening cirrus, then cirrostratus, altostratus and nimbostratus, with steady rain and a wide belt of low cloud. An occluded front forms where cold air catches the warm sector; it often brings complex, widespread cloud and rain. A stationary front lingers and can produce prolonged rain.
Synoptic systems
Depressions form on fronts along the polar front and are driven by upper-air divergence. Pressure systems are shown on surface charts with isobars at 4 hPa intervals. Ridges and cols are relatively stable; troughs are unstable. In the tropics, easterly waves and the intertropical convergence zone dominate.
Traps
In the northern hemisphere, wind veers (backs) ahead of and behind a cold front; a common exam trap is to confuse veering (clockwise) with backing (anticlockwise). Behind a cold front, expect good visibility but gusty winds and cumulus. Do not assume a warm front gives thunderstorms; it usually gives steady rain.
Worked example
As a cold front passes, the wind veers from southwest to northwest, temperature drops 5°C and pressure rises. That pressure rise is the classic post-frontal signature.