What causes tornadoes? That’s a good question and is still actively researched with lots of questions still remaining. There are many detailed theories but one of the remaining unanswered questions is whether tornadoes form from the top down or bottom up.
Honestly it may be in some cases both are true. An example of the top down theory would be this: a supercell thunderstorm (a rotating storm that acquires it’s rotation from an updraft interacting with environmental wind shear) has some large rain drops falling around the updraft. As these raindrops fall into a warm and moist environment, a minimal amount of evaporational cooling occurs and downward momentum and vorticity is concentrated and re-ingested back into the updraft. Over time conservation of angular momentum allows for a concentrated area of vorticity to form under the updraft and a tornado is formed. The next theory is the bottom-up approach: here an already existing area of vertical vorticity or sheet of vorticity already exists. This could be from a pre-existing outflow boundary or along the rear flank downdraft boundary. The vorticity accumulates and gets stretched under the updraft and a tornado forms. It’s likely that either one of or both of these theories contribute to tornado formation depending on environmental conditions.
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