How tornadoes form is one of the giant mysteries of science, and it has led to great efforts to understand both the supercell thunderstorm and tornadic vortex behavior.
While not every tornado form from mesocyclones, the strongest tornadoes are typically formed from Supercell storms with mesocyclones. A supercell’s typical environment is typically characterized by strong directional shear, which results in strong horizontal vorticity in the lower parts of the atmosphere. As we discuss on the supercell page, these horizontal rolls are tilted upwards by thunderstorm updrafts, causing them to rotate and become supercells.
It is likely that tornadic circulations originate in the mid-levels of the atmosphere, where the storm’s updraft and mesocyclone are both at their strongest point. From that point, the circulation builds both upwards and downwards within the storm.
The tornadic circulation builds downwards because of what is believed to be rapidly lowering near-surface barometric pressures. Coincident with the rotation building downwards, the rear-flank downdraft intensifies and moves downwards towards the surface.
As the RFD reaches the ground and rotates around the mesocyclone, it helps focuses the storm’s area of rotation under the wall cloud. This, in turn, creates a low pressure center on the surface — as the area which air is being sucked in and lifted into the updraft shrinks. As this occurs, the rotation continues to intensify from cloud base to ground level producing the tornado.
Typically, the RFD reaching the ground and tornado formation usually occurs within a few minutes of each other. Warm and moist air will continue to flow into the tornadic circulation and up into the updraft until the RFD completely wraps around the tornadic circulation. It is at that point the circulation occludes and the tornado dissipates. (NOAA)