"Special circumstances" are to blame for the fog that caused flight chaos in Wellington, a meteorologist says. The wind blew it in, then it got trapped - and it happens about six times a year.
Fog is defined as when one cannot see more than 1000m and when cloud comes in contact with the land.
The type that disrupted flights in the capital today and yesterday is called "advection fog" (as opposed to the other main one - radiation fog).
That is where winds blow warm air over the cool sea, chilling it as it goes. In other words, sea fog.
This fog came from the north of New Zealand, MetService meteorologist Peter Little said.
The problem was, when it hit Wellington, it got trapped.
High cloud stopped the sun penetrating it, while a southerly off Cook Strait stopped it blowing on by.
It was the "perfect set-up", Mr Little said.
With enough wind, warm air and cold seas, the fog could "keep hanging around for some time".
"In this case, there's often not much to stop it."
There were usually about six "advection fogs" in Wellington each year, he said.
It "lingered" after it came in overnight Tuesday and into Wednesday morning, but the wind was "turning around".
"We may even see the sun this afternoon in Wellington."
As far as weather events went, fog was "good when we can forecast it", but thunderstorms were more exciting for meteorologists, Mr Little said.