Freezing precipitation (FZ)

FZ is the descriptor for freezing precipitation: liquid water, already below 0 °C, that freezes on whatever it touches. FZRA is freezing rain, FZDZ freezing drizzle, and FZFG freezing fog.

FZRA and FZDZ produce clear ice, which is dense, hard to shed and follows the contour of the surface it forms on. It builds fast on wings, control surfaces and propellers, and it contaminates runways. Freezing rain typically forms where rain falls through a shallow sub-zero layer near the surface, so it can occur while the air aloft is warmer than the air on the ground.

FZFG is a different mechanism and easy to underrate. It is fog made of supercooled droplets which deposit rime on exposed surfaces — so it reduces visibility and ices the airframe at the same time. Because it forms in calm, clear, cold conditions, it often appears in otherwise benign-looking reports. Any FZ group is a stop-and-think item regardless of how mild the rest of the observation reads.