Around the twist: Facts about tornadoes
21 December 2015
A tornado and a twister are different names for the same type of weather event—a violently rotating column of air in contact with land or water.
Tornadoes range in diameter from metres to hundreds of metres—some are even more than a kilometre—and can last from a few seconds up to half an hour or longer. They have an intense updraught near their centre, which is why they can lift heavy objects such as cars and trees as well as cause enormous damage.
Do we get tornadoes in Australia?
We certainly do get tornadoes in Australia. They are more common than you might think, with dozens of sightings per year. Many more may occur in remote areas and hence are unreported. Many of the stronger tornadoes in Australia are associated with a type of thunderstorm known as a supercell.
Strong tornado near the highway between Nimmitabel and Cooma New South Wales, 23 December 2008. Photograph courtesy of Heather Leckie.
Why do tornadoes occur?
Strong tornadoes occur with severe thunderstorms. The key conditions for the formation of a tornado are:
- an intense, sustained updraught; and
- strong wind shear, i.e. turning and strengthening of the environmental winds with height.
These conditions allow the thunderstorm's air-flow pattern to interact with the environmental winds and produce rotation within the updraught.
What is it like near a tornado?
Tornadoes mostly occur near the back side of supercells. Heavy rain and hail may arrive before the tornado, but once the storm's main updraught is overhead, an eerie calm may accompany the tornado’s approach. The tornado itself produces a violent wind that begins and ends quickly. It will last from several seconds to at most a few minutes and be accompanied by a variety of sounds caused by the damage to buildings, trees, etc.
If the sound is irregular it is likely the result of damage occurring nearby. A separate source of a roaring sound that originates in the sky or toward the main storm cloud may be caused by large hailstones hitting the ground or colliding in mid-air. After the passage of the tornado, some light rain or hail can occur, along with cooler winds, before clearing takes place.
When is the tornado season?
There is no defined tornado season in Australia. Tornadoes can occur at any time of the year, although they frequently occur in late spring to early summer. Winter time tornadoes are relatively regular visitors to southern coastlines and they even form in the outer bands of thunderstorms associated with tropical cyclones.