Hot air can lower the moisture content of forests and grasslands to around 5 per cent and in extreme cases to 2-3 per cent, greatly increasing the speed of the fire. Very low relative humidity of, say, less than 20 per cent, causes fuels to dry out and become more flammable. Air movement provides the oxygen the fire needs to keep burning. Higher winds mean more oxygen and more intense flames. Doubling the wind speed will quadruple the rate of spread of the fire.
Winds also carry burning embers downwind, which can start new fires.
This is known as spotting. Dry grass, parched native shrubs and dead leaves and twigs are fire's basic fuel. During droughts and in very hot, windy weather, even heavy fuels like large logs and the green leaves and smaller branches of large trees can become dry and flammable. Bushfires in Australia spread as a thin front of flame, with flames usually about as thick as they are high. 100-l50 metres high and thick.