Note 1: When used with FG, the qualifier '
PR' is used for fog banks, i.e. an extensive area of fog impinges upon an airfield, reducing visibility over part of same to less than 1 km; 'BC' [ patches ] would be used when a discrete, small-scale area of fog drifts/forms over the airfield, again reducing visibility below 1 km but not in all directions. In practice, it is not easy to tell the two apart!
Voila!