Tiny, supercooled liquid water droplets in fog can freeze instantly on exposed surfaces when surface temperatures are at or below freezing. Some surfaces that these droplets may freeze on include tree branches, stairs and rails, sidewalks, roads and vehicles. Extreme caution should be taken if travel is necessary. Freezing fog can cause black ice to form on roadways. Black ice is difficult to see and so particularly dangerous. Drive more slowly when you suspect icy conditions. For those flying, a thin layer of ice can form on aircraft, making flight very dangerous unless the aircraft is treated or has effective de-icing equipment.