SYDNEY • Vast clouds of smoke from Australia's historic bush fires are expected to circle the earth and return to the country, the National Aeronautics and Space Administration (Nasa) says.
The US agency said satellites have been monitoring the movement of the smoke high in the atmosphere as it swirled east towards South America and beyond.
By last Wednesday, "the smoke had travelled halfway around earth, crossing South America, turning the skies hazy and causing colourful sunrises and sunsets", Nasa said on its website.
"The smoke is expected to make at least one full circuit around the globe, returning once again to the skies over Australia."
The agency, in a posting last Friday, said that "Nasa satellites have (over the past week) observed an extraordinary amount of smoke injected into the atmosphere from the Australian fires and its subsequent eastward dispersal".
More than 11 million ha of Australian forests and farmlands have been scorched since the fires began last September, several months before the traditional start to Australia's summer fire season.
The fires have been particularly bad in the states of New South Wales and Victoria along the country's forested east coast.
As the thick smoke crossed the Tasman Sea, it brought severe air pollution to parts of New Zealand and turned snow in the mountains brown. Days later, it was reported over South America.
Fire-generated storms, called pyrocumulonimbus, injected large amounts of smoke into the stratosphere more than 16km above the earth, Nasa said.
Ash, smoke and burning material are lifted into the atmosphere via super-heated updrafts.
"Once in the stratosphere, the smoke can travel thousands of miles from its source, affecting atmospheric conditions globally," Nasa said.
Agency staff had counted 20 pyrocumulonimbus firestorms during one week this month.
These storms, usually dry thunderstorms, also generate lightning that can spark new fires.
"By our measures, this is the most extreme pyrocumulonimbus storm outbreak in Australia," Nasa quoted Dr Mike Fromm and his colleagues from the Naval Research Laboratory in Washington as saying.
Dr Fromm tracks pyrocumulonimbus storms using satellite imagery from Nasa.
Two instruments aboard the National Oceanic and Atmospheric Administration and Nasa's Suomi National Polar-orbiting Partnership satellite provided data on the smoke cloud.
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