Clashes of warm and cold air sparked thunder and more than 200 lightning strikes around eastern parts of central New Zealand yesterday.
MetService severe weather forecaster Eric Brenstrum said such events happened in spring when the sun was warming the ground but conditions could still present chilly air.
He said the storms were sparked by cold air moving over warm ground, causing warm air near the ground to rapidly rise and expand.
Processes then occurred which led to turbulent conditions.
"You get an enormous electric potential across a short piece of air. The air then ionises and becomes a conductor and you get lightning."
Mr Brenstrum said there had also been hail in the Wellington area, with reports of hailstones 1cm across in the suburb of Johnsonville.
There were similar conditions yesterday in Canterbury, where snow recently fell to low levels. After heading north the stormy conditions continued through parts of Wairarapa.
Mr Brenstrum said severe frosts left vineyard owners and orchardists in Otago, Canterbury, Marlborough and Hawkes Bay exposed to potential damage to tender young fruit.
Hundreds of helicopters were expected to be on standby to push down warmer inversion layers and prevent frost from settling on vulnerable crops.
- NZPA
