What's behind our dramatic sunsets

Spectacular reds and oranges flooded the evening skies Sunday and Monday, prompting people to pull over and step onto balconies to snap striking photos posted across social media.

So what's the reason for such eye-catching sunsets? Clear skies coupled with high, thin and wispy clouds called "cirrus clouds," said Jeff Frame, teaching assistant professor of atmospheric sciences at the University of Illinois at Urbana-Champaign.

"That orange and pink and red light associated with the sunset illuminating the bottom of these clouds can yield very nice, very beautiful, very stunning sunsets," Frame said. "That happened Sunday and Monday."

That's especially true in autumn because the air is cooler and brisker, Illinois State Climatologist Jim Angel said. "In summertime, everything is hazy with high humidity levels mixed in with some air pollution, so you don't really get a clear evening sky," he said.

Sunsets are red because air molecules scatter blue light about four times as much as they scatter red light, Frame explained.

In the middle of the day, when the sun is high above your head, the sky looks blue. But at sunrise and sunset, when the sun is low in the sky, light takes a much longer path to reach us. By the time it does that, all the blue has scattered and we're left with that arresting mixture of red, orange and yellow, Frame said.

But there's something more simple at play this time of year, Angel pointed out.

Sunsets take place earlier, when people are getting off work and more likely to be outside to see them, as opposed to during dinner time or later in summer.

"You're more apt to notice them because you're in traffic and you have nothing else to look at," Angel said, "so you look at the pretty sunset."

If you want to know ahead of time whether you can witness a great sunset, check out sunsetwx.com, which has daily maps projecting sunset quality across the United States.