Some days everything is just right for tornadoes. All of the elements can be in place for thunderstorms to form, but one very important ingredient must be added for tornadoes to form. That ingredient is wind shear.
What is wind shear? Wind shear is simply a change in wind direction with height. The wind may be from the southeast on the ground. A little higher up it is from the south. Go a little higher and it is from the southwest. Higher still, the wind is coming from the west. Wind speeds can also increase as we move up into the sky.
All of this turning creates a horizontal spinning effect in the atmosphere. Thunderstorms have a rising updraft, but now you have a rising updraft that is rotating and spinning high up into the thunderstorm. As the storm stretches higher and higher, this column of air spins faster and faster.
Imagine a figure skater spinning on the ice. She pulls her arms in and triples her speed. Strong and violent tornadoes can form within this small area of rotating air.