Chris de Freitas: Focus should be on plans rather than predictions
Tornadoes cannot be reliably predicted, with warning times in tens of minutes at best. Photo / George Richards
According to Hawaiian climatologist Jen-hu Chang, the Australia and New Zealand region is second to the United States in the frequency of tornadoes. Some estimates for New Zealand exceed 25 twisters a year, but they are smaller and less intense than in the US.
Generally speaking, New Zealand tornadoes are a mass of unstable air rotating at up to about 250km/h that rises swiftly around a centre of a vortex. Typically they have a damage path 10m-20m wide and 1km-5km long, and usually have a life span of less than 15 minutes.
In New Zealand, tornadoes are more frequent in the afternoon and they can occur throughout the year.
Their formation is associated with severe thunderstorms spawned by frontal systems where air masses with contrasting temperatures meet.
Although many things have to come together in just the right way to form an intense thunderstorm, vertical shear is the most important.
This indicates the change in the wind direction from near the ground to high into the atmosphere. This shear helps organise storms and provides the source of rotation for the most severe thunderstorms.
How tornadoes form in tandem with storms is unclear, but they are thought to occur when air near the ground is slowed by friction, but the air aloft is not, so air rotates in circles on a horizontal axis like a rolling drum. When the rotating air meets the updrafts along a front, the rotation shifts to a vertical axis. The rolling drum of air extends vertically and contracts horizontally while wind speed accelerates like an ice skater pulling in her arms to spin faster.
Clues of tornado formation are dark, often greenish sky, hail and a roar similar to a freight train.
The most reliable sign is the growth of rotating wall cloud extending from the base of a rain-free storm cloud. Often tornadoes may form during the early stages of rapidly developing thunderstorms and may be nearly transparent until dust and debris are picked up.
Tornado safety tips for people at home include moving to an inside hallway, away from exterior walls and windows, on the ground floor, or basement if there is one. Avoid places with wide-span roof areas. Get under a piece of sturdy, heavy furniture and use arms to protect head and neck.
It is best not to try to outrun a tornado, and bear in mind they often change direction quickly. Tornadoes cannot be reliably predicted. At best warning times are a matter of tens of minutes rather than hours.
The focus should not be on prediction alone, but on adaptation with disaster planning and crisis management for risk reduction, readiness, response and recovery. Authorities have the responsibility to minimise social vulnerability and have a duty to promote community resilience through enlightened planning. Real crises give a chance to assess community preparedness.
Chris de Freitas is an associate professor in the University of Auckland's School of Environment.
Tornadoes have been reported on all continents except Antarctica. They are most common on continents in the mid-latitudes (between 20° and 60° N and S), where they are frequently associated with thunderstorms that develop in regions where cold polar air meets warm tropical air.
Calculating which country has the most tornadoes per year depends on how this measurement is defined. The United Kingdom has the most tornadoes per land size, most of them weak. On average, about 33 tornadoes are reported annually there. In absolute numbers, the United States has the most tornadoes by far (more than 1,000 per year have been reported every year since 1990). It also has the most violent tornadoes (about 10 to 20 per year).
Tornadoes of this intensity are very infrequent outside of the United States. Canada reports the second largest number of tornadoes (about 80 to 100 annually). Russia may have many tornadoes, but reports are not available to quantify their occurrence. About 20 tornadoes are reported in Australia each year, though the actual number is likely much higher. Many storms occur in uninhabited areas.
Most Southern Hemisphere tornadoes occur in Australia. Many reports come from New South Wales, where there were 173 reported tornadoes from 1901 to 1966. In addition, South Africa and Argentina both reported 191 tornadoes from 1930 to 1979. Because tornado formation is closely tied to the speed and directional shear of the wind with height, tornadoes in the Southern Hemisphere almost exclusively rotate clockwise, opposite to the rotation of their Northern Hemisphere counterparts.
NZ doesn't even get a mention.
Golly! that sounds very scary! should i build a shelter at my house just in case?
but...the daily mail says the UK has the highest tornado rate according to Dr Terence Meaden. who should i believe?
"Dr T Theodore Fujita, an American meteorologist, first recognised the UK as the top site for tornadoes in 1973, and Torro had confirmed his findings ever since, Dr Meaden said."
but what about
http://wiki.answers.com/Q/Which_country_has_the_most_tornadoes
it says the US, canada, UK and netherlands have the greatest amount. no mention of here or oz.
guess i won't be building that shelter just yet.
are you sure these fellas don't just rentaquote when you ring them up?