The Met Office has issued a yellow weather warning for fog covering large parts of Wales, with the alert in place from around 6pm on Sunday until 11am on Monday.
The warning affects Blaenau Gwent, Caerphilly, Cardiff, Conwy, Denbighshire, Flintshire, Gwynedd, Merthyr Tydfil, Monmouthshire, Newport, Powys, Rhondda Cynon Taf, Torfaen, Vale of Glamorgan and Wrexham.
Visibility down to 50 metres
According to the Met Office forecast: "Fog patches will form readily again during Sunday evening and overnight becoming extensive by Monday morning. Some dense patches of fog are expected with visibility down to around 50 metres or less in some places. Fog thinning slowly later on Monday morning."
The national weather and climate service advises that journey times will be slower, with possible delays to bus and train services, as well as a chance of delays or cancellations to flights.
Travel advice issued
The Met Office adds: "Give yourself the best chance of avoiding delays by checking road conditions if driving, leaving extra journey time, or amending plans if necessary. Make sure you know how to switch on your fog lights, and check they are working before setting off on your journey. Bus and train services, as well as flights and ferry travel may also be affected; check for updates from your travel company and follow their advice."
The service also recommends staying up to date with the weather forecast in your area, as weather warnings can change.
