Why did the sky turn to orange and red during wildfires?

Dozens of wildfires, raging in California, Oregon and Washington, has blanketed the west coast in dense smoke this week

As nearly 100 wildfires burned across the America West, the skies on Wednesday turned ominous shades, from a brownish orange to blood-red.

It led to thousands of social media posts comparing the scenes to a vision of Mars or the post-apocalyptic sci-fi movie, Blade Runner. 

The eerie scenes were caused by the thick blanket of smoke and ash from the blazes which hung over much of the region.

Smoke particles allow sunlight's longer wavelength colours - such as red and orange - to filter through but block shorter wavelength colours of yellow, blue and green, according to NASA.

The dense smoke, which has led to health warnings, was expected to remain over western states until at least the weekend.

By Thursday, the orange and red hues had started to dissipate, with the sky turning grey due to stronger winds coming off the Pacific Ocean.

“The wind has been blowing the smoke up more offshore so, the sun is not being blocked as much as it was yesterday,” Steve Anderson, a meteorologist with the National Weather Service’s Bay Area office, told Associated Press.

But as the wind scattered the smoke and a layer of air blocking it from reaching the ground, smoke particles began mixing with ground-level air worsening air quality in much of California and parts of Oregon where wildfires are raging.

Satellite photos on Thursday showed a towering band of smoke hovering along the Pacific coast.

The Bay Area Air Quality Management District issued its 25th consecutive “Spare The Air” alert, which makes it illegal for residents and businesses to use fireplaces or any other wood-burning devices during a time of unhealthy particulate pollution.

It is the longest stretch of alerts since the program began in 1991. The alerts began 18 August after thousands of lightning strikes ignited three massive wildfires to the north, south and east of San Francisco.

A San Francisco street pictured during the day on Wednesday (Philip Pacheco/Getty Images)

Air quality warnings also were issued throughout the Pacific Northwest, and people in communities from southern Oregon to north of Seattle reported hazy skies and choking smoke.

This summer has brought a punishing  fire season across many states with the climate crisis driving more intense and frequent blazes. 

At least seven people have died including a one-year-old boy.

The California Department of Forestry and Fire Protection reported on Thursday that wildfires have scorched nearly 4,844 square miles (12,545 sq km) so far this year. Six of the top 20 largest fires in state history have occurred in 2020.

More than 900,000 acres have burned across Oregon in the last several days – nearly double the amount of land that usually burns in a typical year.

Almost half a million acres have burned in Washington state, according to officials there, with some communities completely decimated.

Wires contributed to this report