This question is one that has been a bit uncertain among people who study weather, but here is an explanation that many understand is the closest possible cause of tornadoes.

Tornadoes are simply borne out of supercell storms (Supercell tornadoes are more powerful than those that do not come from supercells). A supercell storm is a thunderstorm characterized by powerful updrafts. Examples of non-supercell tornadoes are ‘gustnadoes’ and ‘landspouts’. 

Here is how they form:

Take a look at this illustration and find the notes below.

Step 1: Like all winds and storms, tornadoes begin when the sun heats the surface of the land. As the warm, less heavy air begins to rise, it meets the colder, heavier air above it. Note that wind shears make it even easier to set them off. Wind shear is when two winds at different levels and speeds above the ground blow together in a location.

Step 2: The faster-moving air begins to spin and roll over the slower wind. As it rolls on, it gathers pace and grows in size.

Step 3: At this stage, it is an invisible, horizontal wind spinning and rolling like a cylinder. As the winds continue to build up, stronger and more powerful warm air forces the spinning winds vertically upward, causing an updraft.

Step 4: With more warm air rising, the spinning air encounters more updraft. The winds spin faster, vertically upwards, and gains more momentum.

Step 5: At this stage, the spinning winds create a vortex and the wind has enough energy to fuel itself.

Step 6: The tornado is fully formed now and moving in the direction of the thunderstorm winds. When the pointed part of the tornado touched the ground from the cloud, it is often referred to as ‘touch down’. As it moves it rips off things along its patch.