May 31, 2019 3:12 pm

Why does the sky look yellow during a wildfire?

WATCH ABOVE: The wildfires in northern Alberta impacted Edmontonians on Thursday as smoke created air quality concerns. Kendra Slugoski reports.

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It’s become a common sight in recent summers in Alberta: while wildfires rage across the province, cities and towns hundreds of kilometres away wake up to a yellowy-orange apocalyptic sky.

But what is it that causes the sky to look that colour? In short, the smoke is blocking shorter wavelengths of light, and the yellow and orange wavelengths are the ones that make it through.

READ MORE: Friday’s air quality forecast for Alberta amid wildfires

To explain it further, air molecules scatter light. The sun shines white light down on Earth, but the blue end of the colour spectrum is closest to the size of the air molecules. So when the air molecules scatter white light, we see a blue sky.

During a sunset, the sun’s light has to pass through more of the Earth’s atmosphere, leaving the longer red wavelengths as the ones visible — which is why sunsets are red and pink. The scientific name for it is “Rayleigh scattering.”

During an event where there are wildfires and a lot of smoke, the particulate matter — organic carbon and other things that are in the Earth’s atmosphere — blocks some of the sun’s light.

File photo of smoke from the B.C. wildfires over the city of Edmonton on Wednesday, August 15, 2018.

THE CANADIAN PRESS/Jason Franson

READ MORE: Wildfire smoke can harm your lungs, your heart, and maybe even your mental health: research

When that happens, the blue light gets scattered away and only the longer wavelength colours make it through. Orange and red are at the longer end of the spectrum, which is why the smoke and the light takes on that eerie yellow tinge.

“Because they have a longer wavelength, they can get by there and then the material then scatters that,” Frank Florian of the Telus World of Science Edmonton told the Ryan Jespersen show on Friday.

“It’s the amount of scattering that dictates how dark it gets too. If you get a lot of scattering and not that much light getting through, it appears darker outside — like yesterday.”

LISTEN BELOW: Frank Florian from the TWOSE explains why the sky appears yellow during wildfire season

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It seems when the wind brings that wildfire smoke across the province, everyone starts hoping for rain to not only help the firefighters, but to clear the smoke.

But would that really help? Yes, Florian said, because the smoke particles would latch on to the water molecules and bring that particulate matter to the ground.

In simple terms: the rain cleans the air.

“It would really clean up the atmosphere, the air that we breathe and things like that immensely,” he said.

And once the sky clears up, astronomers and amateur star gazers will see lingering effects for a few days.

READ MORE: 10,000 people forced out, 16 homes destroyed by Alberta wildfires

“When you take a look at sunsets — even after the sky clears — there’s a lot of particulate matter in the Earth’s atmosphere. Sunsets will still be very, very red in colour. And moon rises, when you look at the full moon rising, it will also take on not that grey appearance — it’ll be reddish in colour as well.

“It’s little shifts in colour of the things that we look at in the sky, the sun, the moon, and even the stars that we look at.”

Smoke across Alberta caused a number of communities to reach as high as 10+ on the Air Quality Health Index scale. Residents were urged to stay inside and strenuous activity until air conditions improved.

READ MORE: ‘Lovely morning in the apocalypse’: Edmonton wakes up to orange, smoky sky

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