For tornadoes to form, two factors are needed. The first of these is an unstable atmosphere with rapidly rising air, this air then cools and condenses, forming thunderstorm clouds.
The second important factor is wind shear. This is the difference between wind speed on the ground and higher up in the atmosphere. A thunderstorm cell can form if there is a speed difference of 30-40 km/h between winds found at the surface and those found about 3,000m in the air.
The wind shear causes air to spin in a tube along a horizontal axis. The updraft of the thunderstorm then sucks this rolling air upwards so its axis is vertical.