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The science of sunsets and the secrets of alpenglow

What makes sunsets in the mountains and desert so unique?

FARMINGTON – No matter how many times we have seen the sun set in a blaze of reds and oranges, we are often left amazed. We take pictures, call friends and family and post the images to social media.

But what is it about the setting sun – what science – explains the variation in colors and intensities we see evening to evening?

“At sunset, the atmosphere acts like a color filter, scattering blues and greens out of our view and leaving behind reds, oranges and yellows,” said Joanna Casey, visiting instructor of physics and engineering at Fort Lewis College.

A recent sunset seen from the west edge of Florida Mesa.

In Southwest Colorado, reflective surfaces along and above the horizon, like the mountain faces and the clouds that surround them, act like projection screens, Casey said. That environment makes for an “extra spectacular sunset venue.”

In the desert, sunsets often have deeper reds and oranges. That’s because the lower horizon means there’s more atmosphere to pull out blues and violets, said Michael Ottinger, dean of the School of Science, Math and Engineering at San Juan College.

A sunset on Feb. 6 on the outskirts of Farmington.

Other factors can affect the colors and intensity of a sunset, whether in the mountains or desert.

For example, moisture or pollution particles in the air can block a lot of the sunlight, dulling and diluting the colors, Casey said. Therefore, when the air is very dry, as it often is in the desert, there will be brighter sunsets.

Winter in particular can have spectacular sunsets because freezing weather pulls even more moisture out of the atmosphere, Ottinger said.

Clouds also affect sunset colors. In particular, when light hits low-lying clouds, sunsets may produce brighter oranges and yellows, Ottinger said.

A sunset on Feb. 6 on the outskirts of Farmington.
What is alpenglow?

Search Instagram for “alpenglow,” and you’ll likely see spectacular images of blaze-red mountaintops.

But Casey said there’s some disagreement about what the phenomenon is. For some, the reddish glow occurs when direct sunlight illuminates mountains, while others say alpenglow occurs only after sunset, when light reflected off atmospheric elements turns mountaintops red.

Either way, we see the same effect for the same reason.

“The light on the mountains is rosy and warm because of the low angle of the sun in the sky and the long path sunlight must take through the atmosphere to reach them,” Casey said.

While Durango is known for rosy, glowing mountains, in-town skygazers often miss out on the excitement.

“When you’re down in a low valley, the changes and hues have already happened, and you don’t really get to see the full sunset and sunrise,” Ottinger said.

A 2012 sunset view at Mount Rainier National Park in Washington.


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