The Met Service isn’t willing to promise sun for the Upper Clutha before Sunday (June 28) at the earliest.

Wanaka and its surrounds have been shivering under an inversion layer of cloud for the past week.

The inversion is an annual weather event which results from rapid cooling at the surface under clear sky conditions, forming an inversion at low levels due to the temperature difference of the air just above the surface of the land and the air higher up.

“It’s ironic - to get the fog and low cloud forming you get the clear skies to start with,” Met Service meteorologist Stephen Glassey told the Wanaka App.

This image, from Japanese Meteorological Agency’s Himawari 8 satellite 36,000km above the earth, shows extensive low cloud around the lower South Island. IMAGE: NZ Met Service

Stephen said the lack of heating at this time of the year, when the sun’s at its weakest, combined with a lack of wind to mix dry air from above, means the moisture stays in basins and valleys.

“Usually under a ridge of high pressure you’ve got light winds and generally clear skies, and that allows rapid cooling to take place at the surface but if there’s moisture trapped at low levels that condenses into fog, which can stay trapped in these valleys for days.

“It’s quite extensive - it’s not just Wanaka, but also lots of areas through Central Otago, inland Southland, and around Fiordland,” he said.

Stephen said the cloud layer won’t change until a decent front comes through, and yes, there is one forecast for the weekend.

“Rain is forecast to move across the South Island on Saturday and Sunday, so I think the low cloud and fog will hang around for the next couple of days. There should be some rain on Sunday moving over that should help clear it out,” Stephen said.

“I wouldn't like to say you’ll definitely see the sun on Sunday, but the low cloud will clear at least. There may still be higher cloud. I think you will start to see some sun on Monday - hopefully.”

The forecast for Monday is fine, apart from evening cloud.