Weather conditions today and through tonight will be ideal for the formation of thick and troublesome "radiation fog", experts say.
As for the rest of the day Met Eireann warn that the fog will linger in many parts of the country, and the day will remain cloudy and misty with little sun breaking through. Afternoon temperatures will range from 5° to 9°, while it will be coldest in the north.
Friday night will be windy and very wet over the western half of the country, with heavy possibly thundery rain in many areas and some flooding. Top afternoon temperatures of 3° to 7° are given for the day.
An icy arctic blast is set to batter Britain with snow and gale force winds as temperatures could plummet to -10C in some parts of the UK. Lowest temperatures are expected to be 4° to 8°.