Paxboy - in simple terms, fr fog is composed of small suspended 'supercoolled' water droplets which freeze instantly on meeting any (hard) surface. This makes a significant icing accretion rate. Freezing rain is larger, falling drops of same which do the same and REALLY accrete! Aircraft can very rapidly acquire a heavy coating of ice in fr rain with sometimes dire consequences. The rate of accretion can often defeat any deicing system on the aircraft and obscure windows. Best avoided if possible.