How are tornadoes formed?
Tornadoes develop out of thunderstorms, forming when changes in wind speed and direction cause air to spin horizontally within a storm cell.
When air begins to rise up through the thunderclouds, this spinning effect extends vertically towards the earth's surface, creating a funnel of rapidly spinning air – a tornado.
At least 44 people were killed by tornadoes in the US over the weekend.
Large thunderstorms called supercells cause the most violent tornadoes.
This video posted on YouTube shows a classic funnel.
Tornadoes' funnel clouds, often called twisters, are transparent - but when water droplets from a storm condense, or when the funnel takes up dust and debris, they become visible, with winds reaching up to 400 kilometres an hour.
Tornadoes move at speeds of around 15 to 30 kilometres per hour, although they've been recorded moving at up to 110 kilometres per hour.
Most tornadoes do not move far, usually travelling under ten kilometres in their lifetimes.
How are they different to cyclones, hurricanes and typhoons?
Tornadoes are smaller than cyclones (also called hurricanes and typhoons).
Cyclones are giant, spiralling tropical storms with gale force winds near their centre, which in turn may spawn tornadoes.
These gale force winds may extend hundreds of kilometres from the cyclone centre, and can reach speeds over 250 kilometres and hour. Cyclones can cause more than 9 trillion litres of rain a day.
They begin above warm tropical ocean waters with surface temperatures around 26.5 degrees Celsius, gaining energy from them.
Cyclones spin around a low-pressure centre known as the 'eye' and can persist over many days, but dissipate over land or colder oceans.
