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The series of tornadoes that have hit the west coast of the North Island in recent days has raised questions about why there has been a sudden concentration.
According to MetService, the reason is fairly simple - a combination of Antarctic cold air hitting warmer seas and creating thunderstorms.
New Zealand experiences, on average, about 20 moderate to strong tornado events each year and most have been reported in the North Island, particularly around the Bay of Plenty and Auckland.
Taranaki has also had it's fair share with 12 tornadoes between 1961 and 1975.
But this week it has had as many as eight already, according to reports.
MetService Forecaster Oliver Druce said large thunderstorms were currently coming off the Tasman sea and the storms were "a breeding ground for tornadoes".
But he said today's tornado risk would be the last for a while: "It will pretty well clear up everywhere by Sunday."
Colleague Bob McDavitt said: "The whole atmosphere is kind of sorting itself out after a break of cold air off Antarctica.
"It's accentuated a couple of fronts in the Tasman Sea and when they move onto the North Island, Taranaki takes the brunt of them.
"It's very cold air over the South Island, and the temperature difference is much stronger than normal. The temperature difference is what's causing the problem. It's slowly unravelling, but its going to take a while."
He said every thunderstorm had a "chimney, or bubble of rising air, in the middle".
When that rising air gets into the cold air above it turns to rain, so that causes some falling air.
Mr McDavitt said: "In a normal thunderstorm that process ticks over reasonably well. But in a thunderstorm breeding a tornado, the cold air falls and encloses around the bubble of rising air and contracts it. So it squeezes the chimney of rising air into a tornado.'
"It's unusual for us to get them all together, but it can happen."
How they happen
Tornadoes are caused when air is drawn into the base of a large thunderstorm cloud and accelerated upwards.
Any rotation in the air is concentrated - much the same as when a skater or dancer spins faster when they pull their arms in towards their torso.
The wind that is sucked into the storm begins to swirl and form a funnel, or tornado. The air inside the funnel begins to spin fast, creating a low air pressure which pulls even more air in.
Tornadoes are usually associated with clouds that have a strong up-draught, and clouds that have a strong up-draught are normally associated with thunderstorms.
Winds that change direction with height help to induce the spin of the air as it gets sucked into the thunderstorm.
With wind speeds as high as 300 kilometres per hour, a tornado can be very destructive when it reaches the ground.
Not uncommon in NZ
Tornadoes, or "twisters', are not uncommon in New Zealand, but most of the time they don't strike populated areas.
A series of small tornadoes tore off roofs and downed trees in parts of Greymouth in May this year.
Roofs were blown off, iron strewn along streets, letterboxes blown from their bases and fences knocked over.
Residents there recall a far worse tornado sweeping in from the sea and hitting the town in March 2005.
New Zealand's worst tornado struck Frankton in the Waikato on August 25, 1948.
The twister killed 3 and injured 80 others. The tornado winds lifted buildings, snapped off chimneys, took the roofs off houses, and uprooted trees.
The air was reportedly filled with flying corrugated iron, branches of trees, timber and other debris.
The twister damaged or destroyed 163 buildings and 50 businesses.
In the US, tornadoes such as the one in this report strike the flat landscapes of the continent's midwest with frightening ferocity, often causing a high death toll and levelling whole towns.
- NZHERALD STAFF, NZPA
