Thunderstorms can produce torrential rain over a small area in a short space of time, causing streams and rivers to rise rapidly in what is known as a flash flood. The most lethal in New Zealand occurred in February 1938 in the Kopuawhara stream, between Gisborne and Napier. In the middle of a Saturday night, the stream rose rapidly by 5 metres after thunderstorms dropped an estimated 130 millimetres of rain in an hour in the coastal hills. Carrying logs and rolling boulders, it became an 80-metre-wide torrent, inundating 47 miners’ huts in a railway construction camp and drowning 21 people. In another stream, north of Gisborne, the flood rose to almost 20 metres, leaving debris tangled in the top of a telephone pole.
As air surges up inside a cumulonimbus cloud, some of the surrounding air is drawn inside the cloud. This air is often much drier than the air inside the cloud. Consequently, cloud droplets evaporate into the air, making it colder and denser. The dense air then begins to fall inside the cloud and may become a large-scale down burst of wind that hits the ground and spreads out in all directions, much like a cup of water poured onto the ground from a height.
In New Zealand, these winds can briefly reach gale force and have destroyed small buildings and fences, flipped over small aircraft parked on runways and snapped off trees as thick as 40 centimetres in diameter. Down bursts cause squalls that are hazardous for small boats. They are also dangerous for aircraft approaching a runway.
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. In certain circumstances, the rotation inside the cloud produces a narrow, tightly spinning funnel of air known as a tornado, which extends below the cloud. With wind speeds as high as 300 kilometres per hour, a tornado can be very destructive when it reaches the ground.
An uplifting story
Just after finishing the morning milking in September 1990, Manukau dairy farmer Laurie Coe drove off on his four-wheel bike to see what was worrying his cows, who refused to leave the milking shed. Unable to see the approaching tornado because of heavy rain, he was suddenly thrown into a fence, then lifted into the sky. He was carried 100 metres before being slammed down into another paddock. He suffered only a stiff neck, sore back and a few minor bruises.
In August 2004 a tornado swept across Taranaki uprooting trees and destroying a house near Waitara. Two of the occupants were killed. The worst tornado in New Zealand history struck Frankton and Hamilton in 1948. Almost 150 houses were wrecked, three people were killed and dozens injured. However, New Zealand tornadoes are neither as common nor as destructive as those that occur over the plains of the United States, where in 1974 over 300 people were killed and over 6,000 were injured.