Freezing fog occurs when fog exists at temperatures below 0 °C with droplets remaining liquid (supercooled). On contacting cold objects (trees, cables, roads), they freeze instantly, forming rime.
It is extremely dangerous for traffic (invisible ice patches) and for power and telecom lines (ice weight). In Spain, it occurs on the plateaus and interior valleys during cold spells, especially in Castilla y León, Aragon, and Navarra.