Thunder is rare when it snows, but during this week’s so-called polar explosion in a phenomenon known as thunder snow.
It was unusual to get thunderstorms and snow at the same time, but about a dozen lightning strikes were recorded on Wednesday in parts of Southland, Otago and Canterbury where it had snowed, MetService service forecaster Heath Gullery said.
thunder snow is unusual for the South Island, and “extremely rare” for the North Island, where snow rarely falls to sea level.
So what is it?
The U.S. National Oceanic and Atmospheric Administration (NOAA) describes thunderstorms as a rare weather phenomenon, where thunder and lightning were accompanied by snow instead of rain.
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“The ingredients that create thunderstorm snow are similar to the dynamics of a typical thunderstorm — moisture, instability and a lifting mechanism.”
JOHN BISSET/Things
Snow in South Canterbury on Wednesday.
Driven by cold air, warm seas
Niwa Weather Meteorologist Ben Noll said: weather processes of the past few days was driven by the difference between the temperature of the ocean and the temperature of the atmosphere somewhere far from the surface.
“It’s all kind of interrelated. All of these processes are driven by the fact that we had an extremely cold air mass coming out of Antarctica.
“For New Zealand, we’ve had warmer than average seas for a while now.
“The difference between the air mass above and the seas below is what causes things like the ocean effect, as well as thunderstorm snow,” Noll said.
The ocean effect involved cold air moving over relatively warmer waters, resulting in cellular snow showers.
Thunderstorm events were similar to thunderstorms, except that the cloud tops were usually not as high.
Thunderstorm likely during heavy snowfall
The prospect of thunderstorm snow was usually accompanied by a high accumulation of snow. “We saw that in parts of Southland, in Dunedin in the hills, and to a lesser extent in the eastern hills of Christchurch.”
Snow, hail and graupel — a kind of grain snow — had fallen during the thunderstorms.
Thundersnow was no stranger to New Zealand. It was supposed to happen in the Southern Alps in winter, but no one would be around to hear it, Noll said.
“I think the time of year makes this event more unusual.”
Delivered
Images provided by Jason Herrick, a farmer from northern Southland, of snowfall on his property on October 6, 2022.
NOAA said air rarely became unstable when it snowed because air temperatures were consistently cold during winter snow storms, both at high and lower points in the atmosphere.
Air was considered unstable if it continued to rise on its own after being pushed by a cold front or warm front.
“In some winter storms, shallow layers of warm air are lifted and continue to rise on their own — more snowfall and enough electrical charge separation for lightning to occur,” NOAA said.
“Thunder snow can be found where there is relatively much instability and a lot of moisture above the surface, such as over a warm front.”
Lightning seems brighter
The UK Met Office said: lightning seemed brighter during a thunderstorm, as the light reflects off the snowflakes.
But the sound of the thunder was muffled by the snow in the thunderstorm. While the thunder of a typical thunderstorm would be heard many miles away, during a thunderstorm, the thunder would only be heard within about 3 miles.
Niwa’s Noll said the past few days’ event ticked just one of two boxes needed for heavy snowfall.
“This time we had a southerly airflow. There was some moisture, but air particles coming from the ice sheet are colder and drier.
“For example, if we had the same air mass from the ice sheet, but interacting with moisture from the subtropics, that would be a recipe for a lot of snow,” Noll said.
Lightning in Wellington tonight, but no thunderstorm snow
MetService meteorologist Amy Rossiter said no more thunderstorms had been recorded overnight or Thursday morning.
There was some lightning over Wellington around 11pm to midnight, and there were reports of some snow flurries at the same time, but that was not considered thunderstorm snow.
There had also been a significant amount of thunderstorms far offshore, to the east.