Opposing weather patterns to blame

Once-in-a-generation event has brought snow and all its relatives, scientists say. Photo / Supplied

Snow, sleet, graupel, hail and ice - New Zealand has seen the entire range of frozen conditions over the past few days of Arctic weather.

The extreme conditions have been caused by opposing weather patterns, said Niwa principal scientist James Renwick.

"We've got quite a big storm that's a low pressure centre east of the country. That's come down from the North Tasman Sea and has brought quite a bit of moisture with it.

"And that's met up with a cold air stream coming from the far south, so that's cooled down that moist air and caused all the precipitation - be it rain or snow or whatever."

He said the air stream is flowing all the way from Scott Base to New Zealand around the edge of an anti-cyclone.

The air stream is wedged between a low to the east of the country and a high pressure system to the west.

He said Kiwis had seen the entire range of "frozen precipitation" across the country.

"Graupel is like soft hail that hasn't frozen properly. And sleet is sort of like snow that hasn't frozen properly."

Dr Renwick said the main difference was between snow and hail, which were both types of ice that formed in different conditions.

Hail typically forms in a thunder storm with violent updrafts, he said. Water droplets were quickly pushed up through the cloud where they froze and collided with other drops and particles.

Snow was formed in a gentle kind of way in very flat layer clouds, with a slow updraft of air over a wide area.

"The air cools down slowly in a measured kind of a way. You get these crystals forming, typically hexagonals, and the little tendrils grow off those crystals in a typical hexagonal shape."

The ground air temperature had to be four degrees or lower to stop snow melting as it fell, he said.

People within one city or town can see variances in snow patterns because the slopes and wind determined where and how snow fell.

New Zealand has experienced similar weather conditions this year, on June 21, and in 1976 and 1939, Dr Renwick said.

The South Island had been hit with big snowstorms more often - in 1945, 1992 and 1996.

"It's maybe not quite once in a lifetime but it's once in a generation at least."

Chris Noble, MetService forecaster, said reports of snow had been varied.

"It can still be snowing but not settling. And that's very common with the onset of snow. We say it is snowing but not settling, and that's the way we qualify it."

He said Wellington had seen a mixture of passing snow flurries and settled snow yesterday.

MetService duty forecaster Rachel Kirkman said the white weather conditions had continued because the low and high pressure blocks had remained fairly still.

Dr Renwick said there would be a gradual eastern migration of the weather patterns and the weather should settle by the end of the week.