Black ice forms on the roadway when the surface temperature falls to 32 degrees Fahrenheit or below.  This phenomenon usually occurs suddenly.  A driver can be traveling down a wet road without any difficulty and suddenly the situation changes.  Ice.  Almost as quickly as the road turned into a skating rink, it can change back to an innocent wet roadway.  Freezing conditions occur at 32 degrees and can thaw quickly when the surface temperature goes up slightly. Black ice is invisible.   It is difficult or impossible to tell black ice.

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