it should be noted that scientists don’t truly understand how tornadoes are formed, but we do know some aspects of what causes development. Tornadoes are the opposite of microbursts, in that they are caused in the updraft regions of thunderstorms, and indicate inflowing, violently up drafting air. Wall clouds and funnel clouds are actually not technically lowering, per se, but are building down from the cloud base, due to condensing rising air.

For tornadoes to form you need strong updrafts in the rain-free portion of the thunderstorm (usually to the southwest of the cell), to combine with wind shear in the atmosphere levels above the cloud base. Wind shear references both changes in wind speed, and/or wind direction. In the example of speed shear, you could have a 10mph wind out of the west at 3000 feet, and a 30mph wind out of the west at 6000 feet. This causes the air above the cloud base to start “rolling”, and becomes a rotating horizontal column of air parallel to the ground. When this rotating column of air is close enough to the ground and encounters powerful updrafts, the column gets twisted, and part of it is directed down below the cloud base, which can form a funnel cloud, and if it continues to the ground, a tornado. Wall clouds are indicative of the area of strong updraft, and condense below the cloud base, through which funnel clouds and/or tornadoes can form.

Tornadoes usually form along dry lines, which are frontal boundaries where moist humid air meets and makes contact with dry arid air. This usually happens when air from the southwest deserts or Rockies meet with air from the Gulf of Mexico, but can also happen with dry Canadian air meets up with moist air from the Gulf, or from the Atlantic Ocean.
There are 28 damage indicators, and 8 scale degrees for each indicator in the Enhance Fujita Scale that was instituted in 2007 that help to define whether or not the tornado fits into one of six categories based on its severity that range from EF-0 to EF-5

Here is a link to the Enhanced Fujita Scale