Thunderstorm distribution and frequency in Australia

Thunderstorms are spectacular but hazardous weather phenomena which, together with tropical cyclones and bushfires, top the list of concerns in the Bureau of Meteorology’s Severe Weather Warning Services Program. Insurance-related claims arising from thunderstorms (wind, hail and flash flood damage) from 1967 to 1999 amount to about $A5 billion (Insurance Council of Australia 2000). The Sydney hailstorm of 14 April 1999 inflicted over $A1.7 billion of insurance losses, and was but one of a series of severe hailstorms to affect Sydney and other parts of the eastern seaboard over the past 100 years. There have been 650 registered fatalities in Australia in the period 1803-1992 caused by lightning strikes (Coates 1996). Protection of people and property from thunderstorms is therefore of great concern. The Bureau of Meteorology was invited by Standards Australia (Committee EL/24 – Protection Against Lightning) to participate in revision of the standard for Lightning Protection (AS 17682002/NZS/AS 1768-2002 Lightning Protection, 2002). The Committee’s attention was particularly focussed on overhauling the risk assessment procedure which, of course, depends heavily on lightning incidence. Recently, high quality lightning data has become available from a variety of sources (described below), but as yet there is an insufficient long period of data on which to base reliable averages. For this reason, lightning activity continues for the present to be represented by thunderstorm frequency, which is routinely recorded at meteorological observation sites. Thunderstorm occurrence at a particular location is usually expressed as the number of days in a calendar year when thunder was heard, averaged over several years. The Bureau of Meteorology last produced an Average Annual Thunder-Day Map for Australia in 1967, based on the period 1954 1963, and it is the purpose of this paper to present and discuss an updated map.