Anatomy of a firestorm

In the evolving history of Victoria there will be some days never forgotten because of the magnitude of lost lives and devastation from uncontrollable fire.

Black Thursday was February 6, 1851, when one-quarter of the emerging colony of Victoria burned. February 14, 1926, does not have a special name but 60 lives were lost in widespread fires. Seventy-one lives were lost on Black Friday, January 13, 1939, and vast areas of forest were destroyed in fires that continued for another eight days. On Ash Wednesday, February 16, 1983, fires erupted from the Adelaide Hills to east of Melbourne and 47 Victorians died.

Black Saturday, February 7, 2009, is now added to this dreadful roll call.

These were not the only days of fire devastation, just the worst. The climate of Victoria with its winter rains and summer heat sets the scene for annual danger. Grasses and forest vegetation grow prolifically in the warm moist conditions of spring and dry out over summer into a tinderbox, just waiting for an ignition. And the ignition generally comes, either naturally as in lightning strikes, through accidents or from arson. But not all fires reach the intensity and spread with the ferocity of our worst.

Two characteristics mark the historic fire days. The first is a long period of significantly below-average rainfall; the second is special meteorological conditions on the day, with a combination of high temperatures, low humidity and strong winds.

Bureau of Meteorology published records show that since 1900 the average annual rainfall for Victoria has varied from 356 millimetres (in 1967) to 919 millimetres (in 1973). The same records also show that the rainfall for the year before each major fire day ranked among the lowest: 1925 ranked 16th lowest at 511 millimetres; 1938 ranked fifth at 412 millimetres; 1982 ranked second at 362 millimetres; and 2008 ranked 15th at 504 millimetres.

Below-average rainfall during the preceding winter and spring does not allow the soil moisture to become fully replenished. As a consequence, the grasses cure early and the forests become extraordinarily dry during the following summer.

Rainfall is highly variable from year to year and more frequent low or high annual rainfall years have been characteristic of given decades. The decade from the mid-1930s to the mid-1940s was particularly dry and this pattern has been repeated over the recent decade.

In contrast, some very high rainfalls in the 1950s and the 1970s made those particularly wet decades. Broadly speaking, the first half of the 19th century was drier than the second and rainfall has now returned to the early 20th century pattern.

There is no simple explanation for the varying rainfall, although the changing patterns of surface temperature over the warm Indian and Pacific oceans are known to be important but complex influences. To a large extent it is the changing tropical patterns of ocean surface temperature that regulate the transfer of heat and moisture to the atmosphere, and also the location and intensity of tropical convection.

The weather systems responsible for Victoria's rainfall rely on a supply of moisture from the tropics. When the focus of tropical convection shifts from the western Pacific to the central or eastern Pacific, such as during an El Nino event, the weather systems affecting Victoria can be starved of their essential moisture. Conversely, La Nina events, with their focus of tropical convection more to the north and west of Australia, are a more favourable source of moisture.

In addition to an appropriate source of tropical moisture there is need for a favourable air current to bring the tropical moisture across Victoria to feed into passing rain systems. Ideally, the moist air ascends and converges into the clouds of the rain system and widespread rainfall results. Ideal conditions are the exception rather than the rule.