The prime difference between a dust devil and a killer tornado is humidity. Most of the time plant respiration and photosynthesis provides the bulk of that humidity. A recent light shower on dry ground can provide the same ground level high-temperature, high-humidity air, but the circumstances have to be exactly right.
The vertical weather tornado is a local phenomenon often associated with thunderstorms. They are especially prevalent over agricultural areas where plant-photosynthesis, plant- respiration and/or evaporating surface moisture create a high humidity layer close to the ground. This ground level high-temperature, high-humidity layer is covered by a stagnant relatively low-humidity, medium-temperature stagnant air layer extending to just below the cloud layer.
- Plant respiration and photosynthesis maximizes humidity close to the ground.
- Plants dampen local air movement minimizing horizontal and vertical air movement.
- Plants encourage thorough thermal mixing.
- Maturing plants have near blackbody adsorption of solar radiation. The result is humid, still, hot air captured close to the ground.
I’m rather specific about saying ground level humidity. Recent drone footage of tornadoes shows conclusively that high-energy, high-humidity air within a few feet of the ground fuels tornadoes.
Bottom driven tornadoes tend to be smaller because their energy source is smaller.
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