The Met Office has issued a weather warning for fog in Northern Ireland.
Visibility will fall to below 100m in places, and fog will thicken towards dusk, the national weather service said.
Motorists are being advised to slow down, use their lights, and check mirrors.
The warning will remain in place from 15:00 GMT on Wednesday until 11:00 on Thursday.
Wednesday's fog meant many flights to and from Belfast City Airport were diverted or cancelled.
Slower journey times expected
By Barra Best, BBC News NI weather presenter
Fog lingered across many parts of Northern Ireland on Wednesday leading to reduced visibility on roads and footpaths.
Authorities issued advice to road users about using fog lights and slowing down as a result.
This evening and tonight it will once again become more widespread and quite dense for some.
Visibility will fall below 100m in places.
Although it will thin out for some areas, for others it will last for much of the day on Thursday.
As a result, slower journey times can be expected and there is a chance that flights could be delayed or cancelled.
Police have received reports of drivers not using headlights while driving in the fog. Be visible. Fog lights should only be used where visibility is seriously reduced as they dazzle other drivers and obscure your brake lights. Remember to turn off when visibility improves.— PSNI M&EA District (@PSNIMEADistrict) January 10, 2018
The Department of Agriculture, Environment and Rural Affairs has also issued a pollution warning for Wednesday and Thursday.
It outlined the possibility of "moderate to high levels of air pollution in urban areas across Northern Ireland".
The department expects winds to strengthen in the early hours of Friday morning, which "should disperse pollutants".
While healthy people are unlikely to experience any ill effects, people with lung or heart disease may find their condition worsens, it said.