Alpenglow is an atmospheric optical phenomenon which makes mountains appear to glow in a light pink or red hue. It occurs just after the Sun sets or just before it rises.
Facts about Alpenglow
- It occurs when red light waves in sunlight reflect off of mountains back to an observer
- There are two stages of Alpenglow, it can be caused by direct or indirect sunlight
- It is an optical phenomenon that occurs just before sunrise or just after sunset
- It is very similar to the Belt of Venus
- The German word for Alpenglow is Alpenglühen. Glühen in English simply means “to glow” or “to smolder”.
How does Alpenglow occur?
It is important to note that there are some discrepancies in books and internet sources on how Alpenglow occurs and what it is. Some sources state that it is a result of direct light from the Sun that is hidden behind the horizon. Other sources that it is caused by indirect light that has been reflected off of atmosphere and other particles.
Because Alpenglow isn’t a well-defined term, each observer or reader may have different ideas of what causes it!
Direct Light. When Alpenglow is caused by direct light, the Sun has set behind the horizon from an observer’s point of view who is standing on the ground, far below the mountain tops. In this case, direct sunlight is still reaching the mountain tops, and if the observer were standing at the top of the mountains, they would see that the Sun was still above the horizon.
The direct sunlight that is reaching the mountains has traveled through significant amounts of atmosphere, due to the Sun’s position low on the horizon and the curvature of the Earth. When sunlight travels through more atmosphere, the small blue light waves in the sunlight get scattered, and only the longer red light waves get through.
When these red light waves reflect off the mountains, it gives them a bright pink glow. If an instance of Alpenglow is caused by direct light, shadows will be present and well defined.
Indirect Light. When Alpenglow is caused by indirect light, much of the mechanics are similar to when it is caused by direct light. The Sun has similarly set behind the horizon for the observer on the ground, but it has also set for the observer on the mountain top.
Sunlight from the Sun still has to travel through significant amounts of atmosphere, due to the Sun’s position near the horizon and the curvature of the Earth. Therefore, the light waves reaching the mountains are still red. But light from the Sun is no longer directly hitting the mountains. The only red light waves that are hitting the mountains have been reflected off of elements in the atmosphere.
If Alpenglow is being caused by indirect light, its effect is much less pronounced than when it is caused by direct light. The hue of the mountains are a dull pink to almost purple. There are also few if any shadows. According to Wikipedia, this is the only true Alpenglow, but that is very much open for interpretation.
Is True Alpenglow Direct or Indirect Light?
It is the opinion of this writer that Alpenglow can be caused by both direct and indirect light. If one glances through photos of this phenomenon, it becomes quickly apparent that there are few instances where at least some direct light isn’t present. Similarly, there are enough instances where indirect light is causing Alpenglow to say it isn’t solely the result of direct light.
When to view Alpenglow
Alpenglow is best seen near summer months, when the air is relatively still and clouds are minimal. It is best seen just after sunset, but can also be observed before sunrise. Mountains are of course a requirement for this particular phenomenon, but its sibling, the Belt of Venus, can be seen around the world if mountains are not close by.