Freezing overnight temperatures are forecast for many parts of New Zealand. Photo: RNZ / Jean Edwards
A high-pressure system over New Zealand is likely to mean clear skies but could also lead to freezing temperatures that last all day in parts of Central Otago.
NIWA says a record high pressure system is currently over New Zealand and it is set to strengthen on Thursday.
"Air pressure is the force exerted by the weight of air above a given point on the Earth's surface," NIWA says.
"It decreases with altitude and varies with weather conditions."
The high pressure system that is currently over the country is set to peak in intensity tonight and on Thursday, potentially breaking the national mean sea level record of 1046 hectopascals (hPa), NIWA says.
NIWA said in a high-pressure system air descended and spread outward "typically leading to clear skies, calm conditions, and stable weather".
Despite the lack of rain it could cause frosty and / or foggy conditions, NIWA said.
"In parts of Central Otago like Alexandra and Clyde, a phenomenon called an inversion may cause the fog and freezing temperatures to last all day!"
MetService forecasts below zero temperatures for many parts of Canterbury and Otago on Thursday morning.
MetService meteorologist John Law said there would be a cold, frosty morning in the South but also in parts of the North Island such as Waikato and Taupo which was forecast to have an overnight low of -2C.
"So a very chilly old start first thing on Thursday but with plenty of clear sky around."
A cold night was forecast for parts of the South Island where the skies were clear with a low of -4C forecast for Mount Cook, he said.
