The only actual measurements currently available are for USA tornadoes. Indirect methods of estimating the wind speeds have generally been used. There are several different methods of classifying tornadoes. The intensity is based on the apparent damage to structures, the extent of the path and other descriptors from which wind speeds are then inferred. The tornado which occurred at Harrismith on 15 November 1998 was classified as F2 due to the severity of the damage.
Tornadoes can occur basically anywhere where a thunderstorm is possible.
The seasonal distribution of South African tornadoes is given in figure 3. Figure 3 Forecasting Tornadoes Meteorologists rely on weather radar to provide information on developing storms. Storm structure can provide clues about the existence of a tornado, clearly shown in the radar images for the Harrismith tornado. The Bow echo shape in the Harrismith area is typical of severe storms.