There are several factors that combine to create extreme fire weather. Wind speed is the primary factor and the temperature and dryness of the air are the other factors. These affect the moisture content of fuels such as twigs, bark, and leaves. The lower the moisture content, the easier it is for the fuel to ignite and burn. Under the extremely dry conditions the led up to the Victorian fires, the fuel was very dry and could burn very quickly.

Once a fire begins, fire weather and topography affect the behaviour of bushfires in terms of the rate of spread, the height of the flames, the intensity of the heat released and so on.

This question answered by CSIRO scientists,
drawing on internationally respected bushfire,
climate and urban development research.