Thursday's bitter early-morning cold triggered rare forecasts for "freezing fog" in Western Maryland. "What in the world is that?" one puzzled Ruxton reader asked. Plain old fog occurs when rising humidity reaches 100 percent, and water vapor in the air condenses into fine droplets - a cloud at ground level. With nothing to freeze onto, those droplets will stay liquid down to 14 degrees. But when they touch a surface at that temperature, they form beautiful, feathery ice crystals called "rime." That's freezing fog.