The red sky effect explained

(File: Tom Coates)

The orange haze from a dust storm that enveloped Sydney and other parts of NSW is caused by the same scattering process that normally makes the sky blue.

The orange haze from a dust storm that has enveloped Sydney and other parts of NSW is caused by the same scattering process that normally makes the sky blue.

University of Sydney physicist Tony Monger says dust storms were common in Sydney decades ago but have been relegated more to rural parts of the state in recent times.

"If you live out in the country you see this all the time," Dr Monger told AAP.

"But in the city it's more unusual and has been more unusual in the last 20 or 30 years."

Dust storms that hit parts of NSW yesterday arrived in Sydney today just before dawn, turning the sky to a deep orange.

Dr Monger, who studies atmospheric optics, says the same process that keeps the sky blue has contributed to the change in its hue.

"When astronauts look down at the earth and they see a blue earth and when we look up and see a blue sky it's because there's blue light being scattered around all over the place by the air molecules," he said.

Sunlight is a mixture of colours from blue to red, but it is the blue that is predominately scattered across the sky, Dr Monger said.

"Now when the atmosphere is suddenly swamped with a whole bunch of particles, which are other than just air, like lots of dust and aerosols, then they scatter light differently," Dr Monger said.

The dust particles are rich in iron, which gives them a reddish colour and is reflected across the sky when bombarded with light.

"Therefore they preferentially scatter the light and that's the colour that we predominantly see in the sky while they're in it."

Where does the dust come from?

Central Australia is a very dry place and dust storms are a common occurrance.

Associate Professor Michael Box from the School of Physics at the UNSW says that while dust storms originating in this region are common, it's far less common that the dust is carried 1,500 km to Sydney.

But very strong westerly winds pushed dust from the Lake Eyre Basin area to the east coast.

Professor Box says Australian dust is much redder than other dust, for example from the Sahara Desert, because of its higher iron oxide content.

When this type of dust blows over the ocean it has been attributed to helping fertilize plankton, due to the high iron oxide content. These "plankton blooms" may in turn help to absorb carbon dioixde in the atmosphere, experts say.

Source AAP

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