Many people have noticed the reddish-pink glow of mountaintops on winter evenings. This is known as alpenglow - a natural phenomenon that has to do with how we perceive visible light. Terry Deshler, a professor emeritus in atmospheric science at the University of Wyoming, explained that to understand why we experience alpenglow, we must first know why we see any light.
“You have to kind of step back a little bit and think about how light from the sun interacts with the earth's atmosphere,” said Deshler. “To begin with, you have to kind of say, ‘Why is the sky blue?’ And the reason is because the solar radiation that enters the atmosphere has a wide range of wavelengths.”
Visible light is made up of different wavelengths and ranges from violets to greens to reds.
“The shortest of the visible wavelengths are the blues,” said Deshler. “And so they're scattered most effectively. So, we look at the sky, we see blue.”
This is what occurs when the sun is overhead. However, this changes when the sun rises and sets due to the angle of the light, which makes the path of the sunlight much longer. As the wavelengths get stretched, the color we perceive changes.
The effect of alpenglow is often increased in the mountains. Deshler explained that in the mountains, light from the sun is still visible even after the sun goes below the horizon. This is because the atmosphere changes the direction of the light.
“The light gets bent,” said Deshler. “So, you can see the sunlight coming from the sun, even though the sun disc is below the horizon.”
Deshler explained that the surface of the mountains, especially in the wintertime due to the snow, can increase the reflection of alpenglow.
This phenomenon can also be enhanced by a dirty or polluted atmosphere, including by smoke. In Wyoming, Deshler explained that increased alpenglow can indicate a fire nearby.
“When you can wake up in the morning in the summertime around here and you see kind of a reddish glow to the light around you, you know there's a fire somewhere in the vicinity causing the [pollution] particles to get larger,” said Deshler.
As the particles get larger and scatter more light, the amount of blue light decreases and only the red light remains, giving the sky its red glow.
Weather can also affect alpenglow. Deshler explained that when you have more clouds between you and the sun, the effect of alpenglow can be increased.
In ideal conditions, alpenglow occurs during twilight and can be seen between five to ten minutes before fading.
