A sketch of the Sandon Tornado 13/11/76. The Sandon Tornado was an impressive beast which was officially rated at F3, though I suspect it could possibly been as high as F4 (we will never know). It lasted over thirty minutes and was on the ground at  F3 at least for twenty minutes, cloud tops were better than 18.5 kilometres and it was a true full strength supercell.

Clyve Herbert 2002