japan typhoon hagibis
© Twitter photo: @ara_to1
Purple sky in Japan
At least 10 people were killed and around 140 injured as Typhoon Hagibis made landfall in Japan at 7 pm Saturday (local time), with wind speeds of 144 kmph and heavy rains.

Hagibis, which means "speed" in the Filipino language of Tagalog, is reportedly the worst storm Japan has seen in 60 years. The same day, Japan was also struck by an earthquake, measuring 5.7 on the Richter scale, off its south-eastern shore.

As Hagibis wreaked havoc, social media was flooded with shocking images of the storm's impact — trucks falling like dominoes, roofs flying off buildings, and flooded streets.

Hagibis raised the water level by a metre along several parts of the Japanese coast. Six million people have been affected by the typhoon, which is expected to head out to sea again by end of the day Sunday.

Purple sky

However, until just a day before, another flurry of images from Japan was causing immense curiosity on social media: A dark pink, almost purple sky.

While some people sought to derive spiritual meaning from the colour, there's a simpler explanation for the phenomenon, which is seen as the harbinger of storms.

The colour of the sky can be attributed to refraction, but it isn't as simple as that either.

When sunlight hits the atmosphere, gas molecules and dust tend to scatter all the different wavelengths of light. However, red, orange, yellow, green from the visible spectrum (VIBGYOR) can reach the surface largely uninterrupted because they have longer wavelengths and are scattered less. But shorter wavelengths like blue and violet get scattered everywhere, and thus don't really reach our eyes.

An intense storm, especially a typhoon coming in from the ocean, tends to modify the composition of the atmosphere. Storms wash away the larger particles that typically absorb more light and scatter multiple wavelengths uniformly. This makes the colours of the sky appear more vivid.

During a typhoon — also called a cyclone or hurricane in other parts of the world — warm air rapidly rises over the sea and causes evaporation, which leaves higher amounts of salt molecules in the atmosphere. These, in turn, cause a more widespread scattering of shorter wavelengths, enabling us to see them.

But there's more.

Often, what we see aren't really purple skies, but pink skies superimposed on the blue behind.

At sunrise or sunset during a storm, there can be greater scattering of longer wavelengths because of increased humidity, causing pinkness. When superimposed against a dark blue sky, the colours can mix and we see the sky as purple.