Most significant tornadoes come from supercells: thunderstorms with a persistent rotating updraft called a mesocyclone. The rotation starts horizontally, generated by wind shear (wind changing speed and direction with height), and gets tilted vertical by the storm's updraft. When that rotation tightens and reaches the ground, it's a tornado.
The ingredients are the ordinary thunderstorm recipe plus strong shear: moisture, instability, lift, and winds that turn and accelerate with height.
The EF scale rates damage, not wind
Ratings are assigned after the fact by surveying damage, because measuring wind inside a tornado is rarely possible. A powerful tornado over open field may be rated low simply because it hit nothing to damage.
Sheltering, in order of preference
- Underground: basement or storm cellar, under something sturdy.
- Small interior room, lowest floor, no windows: bathroom, closet, or hallway. Put as many walls between you and outside as possible.
- Cover your head. Most injuries come from flying debris, not the wind itself. Helmets, bike, sports, any kind, are genuinely recommended by emergency managers.
- Never shelter under a highway overpass. It accelerates wind and debris and offers no protection.
- Mobile homes are not shelter at any rating. Leave for a permanent structure before the warning, not during it.
If the tornado is distant and traffic is clear, drive at right angles to its path. If it's close, abandoning the car for a sturdy building is better; a ditch below road grade with your head covered is a last resort. Vehicles are among the most dangerous places to be.
Warning signs the storm gives you
A dark greenish sky, a wall cloud lowering from the rain-free base, large hail, a persistent roar, and an eerie calm as inflow winds die can all precede a tornado. But visual confirmation is a bonus, not the plan, rain-wrapped tornadoes are invisible until they arrive, and at night you may hear one with no warning at all. The alert is the plan.