Question: how does a tornado start

Daniel Parsons answered on 12 Nov 2014:

Tornadoes are caused by the collision of two distinct air masses, a lower one containing warm humid air and a higher one containing cooler dry air. As they crash into one another unstable humid warm air moves upward and is stopped by a the layer of cool dry air. This layer acts like a cap on the warm air below and causes pressure to build as more humid air rises. Eventually this cap gives way under the pressure of rising warm air and lots of warm air shoots upwards.

Air currents move in from the sides to replace the rising warm air and as it does so it begins to spin forming a vortex. As more air piles in it forms a bigger vortex. Its a bit like pulling the plug in your bath…but upside down and with air rather than water!

Tornadoes can happen anywhere, but are most frequent in the plains of the USA, where warm air from the Gulf of Mexico meets cold air fro Canada. I once saw one when I was working in the USA, I was on Interstate-55 (a bit motorway) near Mississippi.