A low-level temperature inversion is to blame for parts of Dunedin recording the lowest temperature of the year so far, with the mercury plunging to -6.7C at the airport on Thursday morning.
MetService meteorologist Lewis Ferris said the phenomenon was caused by a combination of clear skies, calm conditions, and Dunedin’s topography.
“Cold air gets trapped near the ground, while warmer air sits above it.”
Swampy Summit, at around 700m above sea level, recorded 8C, significantly warmer than Dunedin Airport despite its much higher elevation.
“It’s a little bit backwards to how people would typically think about temperature, we typically think temperature declines with increasing height,” he said.
Temperature inversions like this were common across the lower South Island during winter in places like Central Otago and the Mackenzie Basin, but Ferris said Thursday’s example was “unusual” due to the steep temperature difference of 14.7C.
The event was notable because there was stillness and lack of cloud, which allowed the ground to radiate heat into the atmosphere unimpeded.
Dunedin’s coastal location might also have played a role in mitigating the temperature at Swampy Summit, Ferris said.
“There is kind of a warm air force nearby, I know you wouldn’t consider the waters around Dunedin to be warm, but compared to the air temperature.”
Similar temperature patterns were also occasionally observed in Christchurch, with the airport recording low temperatures and the surrounding Banks Peninsula hills being warmer.
