A Jetstar spokesman said an early flight from Sydney was delayed by 30 minutes and a Melbourne flight was delayed by an hour.

The flights departed on schedule but had to circle Launceston until the fog lifted enough to land.

Virgin said a morning Melbourne-Launceston return flight was delayed by 30 minutes because of fog.

A weather bureau spokesman said a combination of a cloudless night, little wind and cold ground temperatures combined on Saturday night to blanket pockets of the state in fog.

He said a cloudless night let the heat from the day escape with air cooling enough through ground contact to allow moisture to condense.

Fog is caused by tiny water droplets suspended in the air.

It is understood that fog is a much bigger problem for Launceston airport than in Hobart.

The capital city has on average about five days a year when fog affects flights, with 40-45 days affected in the North.

Police did not report any increase in road incidents due to fog, but warned motorists to drive to the conditions and not to put their lights on high beam in fog.

Today's forecast for Launceston is for rain and a temperature range of eight to 14 degrees, but no fog. Rain is expected for the rest of the week.

On Ben Lomond, the forecast is for snow and wind each day this week, and a minimum of minus seven on Wednesday.