The Standard Two/Form One class at Rangiwahia School, asks :-

The room 10 class at Ashburton Borough School, asked:-

What causes tornadoes?

Erick Brenstrum, a meteorologist with MetService, responded.

Tornadoes are violently rotating funnels of air that extend from the base of some cumulonimbus clouds. Usually they last only a matter of minutes, but a few last several hours. Speeds of 300 kph have been measured in the rotating air.

Tornadoes are caused by extremely strong vertical air currents that occur in a tiny fraction of thunderstorms. These upward motions can be faster than100 kph. When the air moves up very fast, surrounding air is sucked in very quickly to replace it. If this air is rotating, it will spin faster as it is sucked in towards its axis of rotation.

This phenomena can be seen when a spinning ice-skater brings their extended arms into their chest and so spins faster, or when water goes down a plug hole.

Air drawn into the base of a thunderstorm will often be rotating about a horizontal axis because of friction at the Earth's surface. As this air rises into the cloud the axis of rotation changes to become vertical. This can be demonstrated with a thick piece of rope. If you roll the rope along a table-top, the rope rotates about a horizontal axis. If, as you roll the rope, you lift one piece of it while most of the rest stays on the table-top, you can see the change to the axis of rotation.