CAN FOG FREEZE?

The phenomenon of freezing fog is familiar to anybody who lives in a cold mountainous area. It occurs when liquid fog freezes to hard surfaces and coats them in frost called rime. It is typically seen on mountaintops during very cold weather when there is low cloud cover. People in the western United States call this “pogonip,” a name that is derived from a Shoshone word meaning “cloud.” It’s especially common on the Columbia

Plateau that spans Washington, Oregon, and Idaho when the temperature falls to between 30 and 10°F. At times freezing drizzle can accompany the freezing fog, coating surfaces with a thin film of ice. But ice fog is another matter: When the temperature drops below –20°F, all liquid droplets in the air can solidify into tiny ice crystals. Because of the extremely low temperatures required for ice fog to form, it’s mostly seen in polar regions. But it is also found where human habitation adds large amounts of water vapor to the air because of hydrocarbon heating, steam vents, and vehicle exhaust emissions.