91 Spotting tornadoes at a distance is harder than it looks. Even without haze, showers, or low cloud bases, any distant object in the sky quickly becomes tiny beyond about 10 km. You will have to depend on knowing exactly where to look for it in the storm structure. This F2 tornado followed soon after photo 86. There are two main tornado types - weak and strong.
Weak ones are brief events causing only limited damage.
They are formed primarily by the tightening-up of a rotating updraft as the storm intensifies to a maximum,and will be found right under the updraft core,sometimes without a significant lowering. They are more common on the Prairies or with mid-summer storms. There is a much stronger variety that occurs with supercells. The updraft becomes a mesocyclone, or rotating column,within the cloud which you see at the bottom as a rotating wall cloud. The mesocyclone derives its rotation from a complex process in which two forces combine.