What is "black ice" and when is it most likely to form?

1. Thick visible ice that forms on shaded curves; it occurs only below 0°F
2. A thin, nearly invisible layer of ice on the road surface; it forms when temperatures are around 32°F, especially on bridges, overpasses, and shaded areas
3. Ice that forms only at night when temperatures drop below 20°F
4. A type of ice found only in northern states during winter months