Smoke from Canada wildfires has colored the skies. Why the orange hue?

Millions of horrified Americans have watched clear blue skies morph into thick orange fogs in recent days as wildfires in Canada and a stalled low-pressure weather system have sent thick plumes of smoke into the United States.

Tens of millions of Americans were under air quality alerts Thursday, and, in parts of the Northeast, officials urged residents to stay inside and limit or avoid outdoor activities for the third-straight day.

While familiar to many Americans living in the western part of the country, the eerie orange hues have frightened and confounded some in regions of the nation less accustomed to the effects of wildfires.

The cause? In short: Smoke particles from the fires allow sunlight's longer wavelength colors, like red and orange, to pass through while blocking the shorter wavelengths, like yellow, blue and green.

"That's similar to during sunrise and sunset times, when the sun is near the horizon and sunlight has to travel through more of Earth's atmosphere to get to you," NASA explained in the past.

"The additional atmosphere filters out the shorter wavelengths and allows the longer wavelengths to get through, providing reds and oranges during those times," the agency said.

Live updates: Air quality alerts, wildfire smoke from Canada may linger in US for days

What's the solar spectrum?

The sun emits light at a range of different colors, something called the "solar spectrum," explained Jochen Stutz, an associate professor in the department of Atmospheric and Oceanic Sciences at the University of California, Los Angeles.

Scientists describe light as an electromagnetic wave, and different colors are associated with a particular wavelength – the distance between the tops of a wave, Stutz said. For example, while the color blue has a wavelength around 400 nanometers (or one billionth of a meter), red is around 600 nanometers, he said.

"The sum of light at the different wavelengths coming from the sun is perceived as the color white by the human eye," Stutz said. "Any process that alters the intensity of specific wavelengths in the solar spectrum will be perceived as a color by our eye."

Why is the sky typically blue?

What we see as the "sky" is sunlight that bounces off air molecules, Stutz said. The "sky" on the moon, for example, is black because the moon does not have an atmosphere, he noted.

Blue colors in the solar spectrum – light that travels as short, smaller waves – are bounced off air molecules more efficiently than green or red wavelengths, which are longer. "Our eye perceives this as a blue color," Stutz said.

Wildfires are impacting air quality. Here's how to protect yourself.

Why are many Americans now seeing orange skies?

Smoke plumes consist of tiny particles that interact with light, Stutz said. When sunlight passes through the smoke plumes, the particles either bounce off or absorb the sunlight.

Blue colors, and to a lesser extent green, are more strongly scattered or absorbed, Stutz said. That leaves us with the yellow and red part of solar light, making the sky look orange.

This article originally appeared on USA TODAY: Why does the sky look orange when there's smoke in the air?