The purple "scattering" as it is known is a phenomenon that occurs when heavy rain displaces larger atmospheric particles which absorb light. What remains is a fine spray of particles that act as a prism to rays passing through, particularly at dawn or dusk.
In this case the sky reflected the shorter wavelength colours giving the clouds an unnatural purple hue.
It's not the first time the meteorological event has been observed.
Ahead of last year's Hurricane Michael in Florida, observers in the US described bright pinks reds and purples in the sky.
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"As sunlight shines down to Earth, most of the colours of the spectrum are able to reach the surface uninterrupted," explained meteorologist Lauren Rautenkranz at the time.
"The light scatters around the moisture in the air, causing the magical purple colour."
More superstitious observers took it as an omen for the approaching storm.
"A beautiful scene, indeed," wrote one. "But beneath it lies a big catastrophe. Pray for Japan, everyone."
By Monday the storm which dumped almost half of the country's average annual rainfall had passed leaving at least 40 dead.
Over 110,000 rescuers joined the effort to reach those affected by the storms.
