UK tornado climatology and the development of simple prediction tools

The principle features of tornado climatology in the UK are presented based on the 5-year period from January 1995. Just over one third of reported tornadoes occurred in the south-east region of England, and most tornado activity took place during the spring and summer while the least activity occurred during autumn. This was different to the seasonal distribution for the period from 1960 to 1989 when autumn had the greatest number of tornadoes. The reported tornado distribution was shown to be significantly affected by topography and the density of potential observers. Of the ground-based meteorological variables tested, air temperature was most closely related to tornado occurrence with a peak at 13 deg;C. An equation incorporating air temperature, dew-point temperature, wind speed and pressure was shown to predict a tornado day with an accuracy of 86. 2%. The probability that a tornado would occur on a predicted day was 81. 2%. The model was used to predict actual tornado occurrences across England, Wales and Scotland during the 5-year study period, and it was estimated that just over five-times as many tornadoes occurred than were reported. The model results suggest that the bias induced by population density was not greater than the combined influence of topography and spatial setting. This is important in the UK, because most tornadoes are reported in lowland areas which are heavily populated and it has been difficult until now to determine the extent to which tornado reports are biased by the density of potential observers.