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Tornadoes, or "twisters", are not that uncommon in New Zealand, but most of the time they haven't struck populated areas and the damage they cause is minimal.
For example, a series of small tornados tore off roofs and downed trees in parts of Greymouth in May this year.
A torrential downpour and lightning shortly before 1pm on Friday May 11 caused damage in isolated pockets across the West Coast town.
Roofs were blown off, iron strewn along streets, letterboxes blown from their bases and fences collapsed.
Residents there recall a far worse tornado sweeping in from the sea and hitting the town in March 2005.
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.
New Zealand experiences, on average, about 20 moderate to strong tornado events each year.
Many others happen over water, and are known as water spouts.
Most tornadoes in New Zealand have been reported in the North Island, particularly around the Bay of Plenty and Auckland.
Another concentration of tornadic activity is found at the site of today's tornado, on the western coast of Taranaki near New Plymouth.
No less than 12 tornadoes struck this part of the North Island between 1961 and 1975.
New Zealand's worst tornado struck the community of 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 rooves off houses, and uprooted trees.
The air was reportedly filled with flying corrugated iron, branches of trees, timber and other debris. In total, the twister damaged or destroyed 163 buildings and 50 businesses.
HOW A TORNADO IS CAUSED
As 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.
Tornados 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.
In the case of New Plymouth, the thunderstorms had stopped producing lightening off-shore (severe thunderstorms often produce lightening) and so had died off somewhat before moving on-shore according to John Crouch, severe weather forecaster for the MetService.
With wind speeds as high as 300 kilometres per hour, a tornado can be very destructive when it reaches the ground.
- NZHERALD STAFF
