The bush fires now threatening Sydney in Australia bring a sense of dΘjα vu. Events were very similar eight years ago and the world watched in fascination as the fires of January 1994 swept down through New South Wales to encroach upon the suburbs of that same city. On that occasion the conflagration was contained.
Australia is probably the most fire-prone country in the world. At any given time, bush fires are raging somewhere on the continent, its dry, sparse vegetation and its seasonally hot and arid climate combining to create ideal conditions for the fires which have been endemic since long before mankind first arrived some 40,000 years ago.
In those primeval days, the major cause of forest fires was lightning; even today, it ignites some 30 per cent of Australian bush fires, but human activity, intentional or otherwise, has increased their incidence dramatically.
The Australian climate fosters a well-established annual pattern.
In the tropical dry season of the antipodean winter and spring, the fires are concentrated in the north of Queensland and in the countryside just south of Darwin; as spring turns to summer, the tropical rains extinguish the northern fires and, at this time of year, from December until March, the southern and western parts of the continent become the most vulnerable.
Fires in the south-eastern states, in the neighbourhoods of Sydney and Melbourne, are typically intense and short-lived, but in these densely populated areas the toll in terms of lives and property can be very high indeed.
As one might expect, a brisk wind greatly affects the rate of spread of bush fires. For wood to ignite, it must first be heated to a temperature of about 300
But very large fires sometimes produce sufficient heat to generate their own weather. When high temperatures at ground level make the air above exuberantly buoyant, powerful updrafts develop in the atmosphere - great invisible fountains of hot air surging into the sky for thousands and thousands of feet above the flames. This in turn causes air from the surrounding countryside to be sucked in on all sides to the centre of the fire, fanning the flames to even greater heights.
Sometimes, in a process similar to that which occurs naturally on a hot and thundery day, tall cumulus clouds may be seen to form above the conflagration.