On the afternoon of August 8, 1988 a tornado ripped through the Auckland suburb of Henderson, leaving a narrow trail of damage three kilometres long. One mans garage disintegrated around him but he escaped serious injury when a large freezer blocked the fall of several rafters. Trees were felled. rooves and fences damaged, and another garage was shifted 20 metres.
A second tornado struck Henderson just after noon on October 11. A chimney crashed through the roof of one house and several greenhouses were demolished, scattering hundreds of panes of glass over a large area.
Luckily, no-one was killed or even badly injured. In fact, the last deaths due to tornadoes in New Zealand were near Franklin in 1948. In the United States, however, tornadoes can grow to awesome dimensions and hundreds of people are killed by them each year.
Tornadoes are violently rotating funnels of air that extend from the base of some cumulonimbus clouds. Usually they last only a matter of minutes but a few have lasted several hours. Speeds of 300kph have been measured in the spinning vortex of air that surrounds the small low pressure area at the centre of a tornado.
The low pressure area contributes to the damage caused by a tornado as the sudden drop in pressure outside a building can cause it to ‘explode’. This low pressure area is usually only metres wide, though in extreme cases it has reached 200 metres across.
This is why the damage path of a tornado is so narrow.
The extreme low pressure inside tornadoes also enables them to lift objects as heavy as cars and throw them hundreds of metres.
Often the same cloud will give birth to many tornadoes and occasionally there will be two or three at once.
Tornadoes are caused by the strong upward motions of air inside cumulonimbus clouds. These giant towers of lumpy, cauliflower-shaped cloud can grow to 10,000 metres in height around New Zealand and 20,000 metres in the tropics. The upward motions inside them are among the strongest in the atmosphere and can exceed 30m/s. Once a parachutist drifted into one and was carried up until he froze to death.
Cumulonimbus clouds are the result of large bubbles of air rising from the earth’s surface due to warm temperatures near the ground and cold air temperatures aloft. Warm atmosphere due to different buoyancies.
The air can be either heated from below by warm land or ocean or it can be cooled aloft by wind changes in the upper atmosphere associated with jet streams.
Jet streams are narrow ribbons of strong winds that occur about eight kilometres above the earth’s surface. Wind speeds in the core of a jet are between 100 and 250kph. Certain areas adjacent to jet streams are favoured sites for upward motion while others are associated with downward motion. In this way jet streams have a major influence on the formation of anticyclones and depressions, as well as cumulonimbus clouds.
Why do the strong updraughts of air cause tornadoes?
As air rises inside the cumulus cloud more air is drawn into the base. If this air is rotating about a vertical axis then the speed of rotation will increase as the air is drawn into the base of the cloud. The same thing happens when a spinning skater brings their arms into their chest and spins faster because the energy has been concentrated in a smaller area.
Sometimes the initial rotation of the air is caused by the difference in wind speed between the earth’s surface and stronger winds just above. This causes the air to roll over about a horizontal axis something like a length of rope being rolled along a table-top by hand. If some of the air rises then the axis of rotation is tilted into the vertical, the equivalent of lifting part of the rolling rope.
The fact that these modest amounts of rotation can be turned into the incredibly destructive power of a tornado is due entirely to the strong up-draughts that exist in a few cumulonimbus clouds. Probably fewer than one cumulonimbus cloud in a thousand is capable of producing a tornado over New Zealand.
Reports of tornadoes are more frequent in the west of both islands of New Zealand, but they do sometimes occur in the east. Reports are also more frequent around populated areas such as Auckland and New Plymouth. This is because most tornadoes are not observed and so go unreported. Since tornadoes are so small and short lived, most New Zealanders will never see one.
Tornadoes are responsible for some bizarre events. When they pass over ponds or estuaries they may pick up quantities of fish or frogs along with gallons of water. These animals are later thrown out of the cloud, often sorted according to size by the different winds inside the cloud.
The sharp pressure drop as a tornado passes is also thought to be responsible for the defeathering’ of chickens. The air pressure inside the hollow shaft of the feather cannot reduce as quickly as the outside air pressure and so the feathers pop out.