In 1986, the Tornado and Storm Research Organisation (TORRO) developed an international Hailstorm Intensity Scale and have used this to characterise around 2,500 hailstorms known to have occurred in Great Britain since the first documented such event of 1141 AD. The most intense British hailstorm, in Hertfordshire on 15th May 1697, reached intensity H8 on the TORRO international scale which extends from intensities H0 to H10.
The 50 most intense British hailstorms (TORRO intensity H5-6 or more) since 1650 have all occurred between the months of May and September with a well-defined peak during July. Geographically, these very intense events have most frequently affected the east Midlands, East Anglia, Greater London, and counties adjacent to Greater London. These exceptional storms have typically followed a track from the South or South-West to the North or North- East with a swath length of 25 km or more and a swath width sometimes in excess of 10 km. The longest British hailstorm, on 22nd September 1935, reached 335 km in length; sweeping from the Bristol Channel and Monmouthshire north-eastwards across central England, the Fens and Norfolk and out to the North Sea!
Historically, perhaps the most destructive British hailstorm, taking account of intensity and aerial extent, occurred on 9th August 1843 between the Cotswolds and the Norfolk broads. In North Oxfordshire slate roofs were "pounded to pieces", in Cambridge an enormous quantity of glass was smashed at colleges and in public buildings, and in Norfolk the devastation of agricultural crops was so complete that a local council levied a special voluntary rate.
In the past decade the most widespread occurrence of severe hail in Britain occurred on 7thJune 1996. Following an early heatwave with temperatures of 33 C in London, spectacular thunderstorms deposited huge hailstones, typically 25 to 50mm across, along a series of swaths which included several meduim to large towns such as Sherborne (Dorset), Salisbury (Wiltshire), Cambridge and Luton. There was extensive damage to glasshouses, vehicle bodywork, etc. Indeed in most years there are at least one or two incidents of locally severe damage by hail over 25mm diameter. In 2001 hailstones up to 30 to 40mm across caused considerable damage to car bodywork and horticultural glass in parts of Kent and East Sussex on 27th June, and in Lincolnshire on 25th August.