The apocalyptic red skies that turned Western Australia's skies blood red have captured global attention. The phenomenon was caused by crimson dust whipped up by the strong outer winds of Severe Tropical Cyclone Narelle.
Such spectacular events are rare in northwest Australia and require specific conditions: a very dry landscape with red soils, lack of preceding rain, very strong winds ahead of rain bands, and a particular wind direction. The region's iron-rich soils, which support a multi-billion dollar mining industry, give the dust its deep red colour.
According to the Bureau of Meteorology, the six weeks before Cyclone Narelle saw only 10-50 mm of rainfall, leaving the landscape very dry. This was crucial for the size and magnitude of the dust storm. In the Southern Hemisphere, tropical cyclones rotate clockwise due to the Coriolis Force, meaning strong winds south of the cyclone's eye blew off the dry landscape.
As the cyclone moved, humidity increased rapidly, followed by dense cloud and heavy rain, washing the dust out of the atmosphere. The red soil particles scattered shorter wavelengths of light, making the sky appear deep orange-red or blood coloured. The cyclone's approach in the early morning, when sunlight travels through more atmosphere, intensified the red tones.
Mega dust storms are a regular feature during prolonged droughts in central, southern, and eastern Australia. A notable example was the 'Red Dawn' dust storm in Sydney on 23 September 2009, when residents woke to an eerie red dawn due to a huge dust cloud.
