Freezing rain is one of the most dangerous winter weather phenomena. It occurs when raindrops — which are liquid — fall through a shallow layer of sub-zero air near the surface and freeze instantly on contact with any object: roads, power lines, trees, and aircraft. The result is a smooth, transparent coating of glaze ice that can be virtually invisible — the dreaded "black ice" on roads.
The atmospheric setup required is very specific. A warm layer aloft (above 0 °C) melts falling snowflakes completely into rain, but a thin cold layer near the ground (below 0 °C, usually less than 500 m deep) supercools the drops without re-freezing them. If the cold layer is deeper, the drops freeze into ice pellets (sleet) instead. This delicate balance means that freezing rain zones are often very narrow — just 50–100 km wide — along a warm front.
The impacts of ice storms can be catastrophic. Ice accumulations of 10–25 mm can topple trees, snap power lines, and collapse structures under the sheer weight. Canada's Great Ice Storm of 1998 left millions without power for weeks. In Spain, freezing rain is uncommon but occurs occasionally during cold-air intrusions in northern mesetas and mountain passes. Aviation treats freezing rain as an extreme hazard because ice accretion on wings destroys lift — aircraft are prohibited from flying through it. Weather radar and radiosondes are essential for detecting the warm-over-cold layers that produce this phenomenon.