With the fog we had 2 or 3 days ago I thought it would be worthwhile explaining why does it happen exactly.

Fog is similar to a run of the mill cloud but on the ground. If the air temperature falls below the dew point temperature that then leads to condensation of water vapour into tiny water droplets, fog then occurs at this saturation point. The dew point temperature is a measure of moisture in the air, with these clear nights the temperature normally just starts dropping at this time of year and when it encounters a moist atmosphere the moisture in the air can’t go anywhere so pops into fog if that makes sense. Like popcorn almost, the temperature and right conditions are met then boom you have fog.

Moisture in the air the last few nights has been cleared out of the system by a light south to southeasterly airstream moving around an incoming anticyclone, hence we’ve had cool night’s but no fog in most places. Still, inland areas that are quite sheltered may still be getting some fog as the temperature can cool a fair bit and reach a saturation point still.

- By weather analyst Aaron Wilkinson, WeatherWatch.co.nz
http://www.weatherwatch.co.nz/content/what-causes-fog

It is precisely that that allows basin fogs at Alexandra and similar locations. Radiation loss exceeds input even if the days are clear and sunny if there's a run of hard frosts - until the air in the basin is so cold that it reaches its dewpoint even though the original moisture content was low. I watched the whole scenario from a hillside south of Alex. township once when I was a schoolkid, but didn't understand why it was happening - the condensation steadily spread up to near the inversion level, and afterwards a slight stirring of warmth at the lower layers removed some of the lowest fog. The rest stayed overhead for 3 days. The Met Service calendar page for July comments on this.