Mainlanders get a tornado twist

A weather expert is baffled by the recent spate of tornadoes on the West Coast and will investigate them further.

Two houses in Runanga lost part of their roof when a small tornado blew through the town on Monday.

The Yvonne Rust Trust house, on Seven Mile Rd, was the worst affected, with the twister pulling off half its roof and dropping it on a nearby section.

Runanga chief fire officer Tom Griffin said about 10 crew and a local builder worked for about two hours to secure the house but there was some water damage.

"The house next door also had half a dozen sheets and half the garage roof taken off."

On March 10 a tornado left a 4km trail of damage.

National Institute of Water and Atmospheric Research spokesman Dr Warren Gray said the recent tornadoes were interesting because they did not fit the two types of causes.

Traditionally tornadoes were caused either by the land surface heating, causing thunderstorms, or when cold air settled on a warm ocean.

"But they seem to be different than that ... they had no thunder with it."

One possible explanation was the significant turbulence caused by the surrounding hills and Dr Gray believed the Southern Alps also had a part to play.

"The Alps are playing a role in changing the environment associated with a weather front to make convection in tornadoes more likely."

- NZPA