The Met Office has issued a yellow warning of fog for south west Hertfordshire tomorrow, meaning the public should be aware of the potential for disruption to travel and the potential for difficult driving conditions.
The chief forecaster at the Met Office said: "Light winds and clear skies overnight and during Thursday morning will allow fog to form in places. The fog will then slowly clear through the middle of the morning."
Fog patches are expected to becoming dense in places with visibility below 200 metres at times.
The warning is in place from 2am until 10am.