The Met Office has issued a yellow weather warning for fog across Herefordshire, Monmouthshire and Shropshire.
Areas of fog, some of it dense and freezing, will develop during Monday evening and overnight, persisting into Tuesday morning in places.
Visibility may dip below 100 metres in some locations.
Fog will tend to lift into low cloud in many areas on Tuesday, though a few spots may stay foggy into the afternoon.
Forecasters said the warning will be in force from 7pm on Monday until midday on Tuesday.