E. Linacre and B. Geerts
Data on the occurrence of hail over 77 years in New South Wales have been collected from insurance claims after cereal crops have been damaged (1). It is apparent that some areas are particularly prone to hail damage, chiefly as a result of local topography. There was a period of relatively high incidence of hailstorms between the mid 1940’s and early 1970’s, with an especially disastrous 1947/8 season when claims were double those at other times. This period, and other summers with high hail damage, were times when other major climate fluctuations were observed in New South Wales, e.g. pressure, temperature and rainfall. There were fewer insurance claims during times of drought, but the individual claims were larger, either because the crop has a higher value during drought, or because during droughts the fewer storms tend to be more severe.
Sydney has been hit hard by hail several times during the last few decades. An extraordinary hailstorm in Sydney on 14th April 1999 created damage estimated to cost 1.5 billion dollars to repair (1). The damage notably affected roofs, windows and vehicles. Panels of cars were dented slightly where hailstones were 1.0 - 2.9 cm in diameter, and windows of houses and vehicle windscreens were broken where hailstones were larger. The largest hailstones collected were about 9 cm in diameter. A hailstone of 8.4 cm weighed 132 grams, and would have fallen at over 140 km/h on reaching the ground!
The occurrence of hail in Sydney over the period 1791-1994 shows a slight maximum in November (i.e. early spring), and hail events are fewest in April - July. Elsewhere in New South Wales, the peak hail occurrence in late spring is more marked (pers. comm, Natural Hazards Research Centre). In Argentina, the USA, and elsewhere, it has similarly been observed that hail, especially large hail, is most likely is spring.
There is some correlation with the Southern Oscillation Index (2), at least in the Sydney area: there tend to be more hailstorms when there is an increase of SOI until November, and then a decrease. In other words, the tendency of the SOI between June and October can be used as a predictor of the likelihood of hail during the following few months. There also appears to be a weak relationship between El Niño's and the occurrence of hailstorms elsewhere in Australia. Hail damage is slightly higher when the SOI is high - the La Niña phase, although there also have been El Niño years during which severe hail damage occurred.