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.