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 wider 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.
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.
Fig 1: Small tornado
Strong tornadoes occur with severe thunderstorms, mostly supercells. The key conditions for the formation of a stronger tornado are:
These conditions allow a supercell to produce rotating air flow in the lower parts of the storm and for outflows to remain reasonably warm (a necessary ingredient for tornadoes to form).
Tornadoes mostly occur on the trailing 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 (known as "hail roar"). After the passage of the tornado, some light rain or hail can occur, along with cooler winds, before clearing takes place.
There is no defined tornado season in Australia. Tornadoes can occur at any time of the year, although they more commonly 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.