Tornadoes
A rotating column of air in contact with both ground and cloud.
A tornado is a violently rotating column of air touching both the ground and the base of a cumulonimbus cloud. Most form from supercells — thunderstorms with a persistent rotating updraught called a mesocyclone.
Formation
Wind shear, wind changing speed or direction with height, sets a horizontal tube of air spinning. A strong updraught tilts that tube vertical, creating the mesocyclone. If the rotation tightens and reaches the ground, it becomes a tornado. Only a minority of supercells produce one, which is why warnings cover far more storms than deliver.
The Enhanced Fujita scale
| Rating | Wind estimate | Typical damage |
|---|---|---|
| EF0 | 105–137 km/h | Branches down, gutters damaged |
| EF1 | 138–178 | Roof surfaces stripped, windows broken |
| EF2 | 179–218 | Roofs torn off, large trees snapped |
| EF3 | 219–266 | Storeys destroyed on well-built houses |
| EF4 | 267–322 | Well-built houses levelled |
| EF5 | over 322 | Structures swept from foundations |
The scale is assigned from damage after the fact, not measured wind. A violent tornado crossing open farmland may be rated low simply because it hit nothing.
Watch versus warning
A watch means conditions favour tornadoes — be alert. A warning means one has been sighted or indicated on radar — take shelter now. Mixing these up costs lives, and the words are not interchangeable.