Geoscience Reference
In-Depth Information
The first occurred on 18 March 1990 and dropped hail
up to 9 cm in diameter, breaking windows, roofing tiles
and fibro cladding. The damage bill reached $A500
million with houses completely smashed beyond
repair. This event paled in significance compared
to the damage done during the Great Hailstorm of
14 April 1999.
The storm reached the outskirts of Sydney around
7:20 pm, just after the Meteorological Bureau's storm-
watch roster changed and the evening television
stations issued weather forecasts for plenty of sunshine
lasting the next week. The forecasts were ironclad
because the Bureau had already dismissed volunteers'
phoned reports of the storm's intensity up to 200 km
south of the city hours beforehand. Over a distance
of 1 km, between pristine bushland at the edge of
the urban heat island and the first row of houses in
southern Sydney, the storm unleashed its fury. The lack
of warning led to 30 aircraft at Sydney International
airport being damaged to the tune of $A100 million.
Hailstones reaching 9 cm in diameter punched through
car roofs and houses at velocities of 200 km hr -1 .
Hailstones in Sydney had reached this size only twice
before: in January 1947 and March 1990. So intense
was the hail that flocks of seagulls, cormorants, and
flying foxes were killed in the open where they stood.
By the end of the evening, 2500 km 2 of urban Sydney
had been declared a natural disaster area with many
homes uninhabitable as rain poured in through broken
roofs. In the hail zone, 34 and 62 per cent of homes
had broken windows and roofing tiles, respectively,
while 53 per cent of cars were damaged. The storm sta-
tistics were massive: 500 000 tonnes of hail fell, 20 415
homes and 60 000 cars were damaged, 25 000 calls
were made for assistance, 170 000 tarpaulins were
fitted to roofs, 55 000 m 2 of slate and 11 000 000
terracotta tiles were replaced, and 32 000 home and
3000 commercial insurance claims were processed. It
was the costliest natural disaster in Australian history,
resulting in a total insurance bill ($A1.5 billion rising to
$A3 billion when non-insured damage was included)
that surpassed the damage caused by the Newcastle
earthquake of 1989 and by Cyclone Tracy in Darwin
in 1974. Despite this damage, the storm does not
even rank in the top 40 most costly natural disasters in
the world since 1970. It ranks equal fourth with losses
due to tornadoes in the United States over the same
period. The storm actually stimulated the regional
economy, providing work for home renovators and car
salespeople. In the weeks following the storm, the
supply of roofing tiles, tarpaulins, and household
windows was exhausted, and the reputation of the
State Emergency Services was in tatters. Untrained to
climb to the top of 3-4 storey blocks of flats to fix
tarpaulins, and understaffed by the extent of the
disaster, the emergency services had to defer opera-
tions to the Rural Fire Brigade which at least had
extension ladders and professionals trained to skip
across roofs. A week later, the army relieved everyone
as the enormity of the task unfolded. In the end, the
major occupational hazard for emergency services
personnel - besides falling off buildings - was being
stung by the thousands of wasps nesting on the sunny
sides of damaged buildings.
Intro duction
(Miller, 1971; Eagleman, 1983; Nalivkin, 1983)
A tornado is a rapidly rotating vortex of air protruding
funnel-like towards the ground from a cumulonimbus
cloud. Most of the time, these vortices remain
suspended in the atmosphere, and it is only when
they connect to the ground or ocean surface that they
become destructive. Tornadoes are related to larger
vortex formation in clouds. Thus, they often form in
convective cells such as thunderstorms, or in the right-
forward quadrant of a hurricane at large distances
(> 200 km) from the area of maximum winds. In the
latter case, tornadoes herald the approach of the hurri-
cane. Often, the weakest hurricanes produce the most
tornadoes. Tornadoes are a secondary phenomenon, in
which the primary process is the development of a
vortex cloud. Given the large number of vortices that
form in the atmosphere, tornadoes are generally rare;
however, because vortices can be generated by a
myriad of processes, tornadoes have no one mecha-
nism of formation. On average, 850 tornadoes are
reported annually, of which 600 originate in the United
States. This frequency is increasing in the United
States; the increase, however, is due mainly to
increased monitoring. The width of a single tornado's
destructive path is generally less than 1 km. They
rarely last more than half-an-hour and have a path
length extending from a few hundred meters to tens
of kilometres. In contrast, the destructive path of a
tropical cyclone can reach a width of 200 km, spread
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