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Going down: The sun sets Thursday evening looking west toward Audubon. BRIAN BASHAM/DL NEWSPAPERS

Big fires mean brillant skies

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Wildfires in the western part of the United States can make for spectacular sunsets and sunrises in this area.

“It’s been periodic and not continual,” said WDAY meteorologist John Wheeler, “but a few days over the past few months — it’s not every night — it’s been brilliant.”

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The outstanding sunsets are caused by high altitude smoke from the west, he said.

“People need to understand this isn’t smoke you can smell,” he said. “It’s 10,000-20,000 feet up.”

And, he said, the presence of smoke from fires in Utah, Oregon or California “is kind of a snaky thing — the fires are not that huge. Imagine a winding road through the mountains to get to us … It’s not just the wind direction here, you need high-altitude winds to bring the smoke to us.”

So when should you expect a brilliant sunset?

Wheeler said to look for “clear blue sky days, but there’s not blue sky but white sky from the haze — usually that’s an indicator we’re going to have one of those nights.”

The smoke particles don’t seem to have much effect on the weather. It mostly just produces “an interesting sky,” he said.

With much of the western United States very dry, wildfires have been an ongoing problem.

The USDA Forest Service lists 14 large fires, with the two biggest— both in California — mostly contained.

The largest by far is the Rim fire, near Yosemite, which is now 80 percent contained and has burned over 237,000 acres.

According to the Weather Channel, the glowing red and orange skies that sometimes appear at sunset are due to what is called Rayleigh scattering (named for British Scientist Lord Rayleigh).

Smoke particles act as a filter for sunlight by scattering shorter wavelengths such as blue and leaving longer wavelengths of the light spectrum behind.

This allows more orange and red colors to pass through. The effect is enhanced at sunset, since the sun is at a lower angle in the sky.

As a result, light has to pass through more of the atmosphere, including, in this case, smoke particles.

Many other particles in the atmosphere such as dust, smog and clouds also contribute to the scattering of sunlight, leading to the colorful sunsets that you may see on some evenings.

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