Where it sits
The strict definition requires the sun to be gone. Most photos labelled alpenglow aren't.
Tell it apart
| Aspect | Alpenglow | Belt of Venus |
|---|---|---|
| Where | On a mountain | In the sky |
| Direction | Opposite the sun | Opposite the sun |
| Light source | Backscattered | Backscattered |
| Needs mountains | Yes | No |
Both are backscattered twilight red, and they're two halves of one scene. The Belt of Venus is the pink anti-twilight arch in the sky opposite the sun, sitting above the blue band of Earth's own shadow. Alpenglow is that same light landing on a mountain face.
What it doesn't mean
In the strict optical sense no — though dictionaries also record the looser everyday sense, which is the one most photography writing uses. True alpenglow occurs when the sun is ALREADY BELOW the horizon, so no direct sunlight reaches the peak at all. The illumination is backscattered from particles low in the atmosphere. Direct low-angle sun on a summit is a different and more ordinary thing.
The colour is atmospheric, not from the surface. Long red wavelengths survive the long low-angle path and are reflected from ice crystals, water droplets and particulates; the snow is simply a bright neutral screen.
What it really means
Most photographs captioned alpenglow aren't. They show low-angle sunlight on a summit — which is beautiful and entirely ordinary, since the sun is still up and doing what suns do.
True alpenglow happens after the sun has set. No direct sunlight reaches the peak at all. What lights it is backscatter: red wavelengths that have survived a long, low path through the atmosphere, bouncing off ice crystals, water droplets and airborne particles and arriving at the mountain sideways, from a source that is no longer above the horizon.
That's why it looks impossible in person. Everything below is in shadow, the valley has gone blue, the sun is demonstrably gone — and the mountain is still burning pink, lit by nothing you can point at.
The colour isn't in the snow. Snow is doing the least interesting job available: it's a bright neutral screen, showing you what the atmosphere is sending. The same light hitting dark rock would barely register.
And the word is more literal than it looks. German Alpenglühen — the Alps glowing. The first element is a proper noun, so the term originally described one mountain range and got generalised afterwards. Every alpenglow outside Europe is, etymologically, borrowing the Alps' name for something the Alps were simply first to be described doing.
Where it comes from
German — the Alps
German — glowing
The first element is a proper noun. The word literally means 'the Alps glowing', though in use — in German as in English — it applies to any mountain.
- 1871
English partial calque of German Alpenglühen — glühen is translated as glow, while Alpen is borrowed unchanged.
- 20th c.
Atmospheric optics accounts for it as indirect illumination — light reddened by its long low-angle path through the air, then backscattered onto the peak by airborne ice crystals, water droplets and other particulates low in the atmosphere.
In real life
- Twenty minutes after sunset, with the valley already in blue shadow, the snowfields high on the mountain are still glowing pink.
- 'Look up — alpenglow,' the guide said, as the summit turned rose-red above a lake that had already gone dark.
- The looser everyday sense: a holiday photo captioned 'alpenglow' that actually shows the last direct rays of sunset turning a peak orange.
Questions
Q01When does alpenglow happen?
Just before sunrise or just after sunset, when the sun is below the horizon and cannot illuminate the peak directly. If the sun is still up and shining on the summit, that is direct low-angle light. Dictionaries count that as alpenglow too; the strict optical sense does not.
Q02Is alpenglow the same as the Belt of Venus?
Related but distinct. Both come from backscattered red light; the Belt of Venus is the pink arch in the sky opposite the sun, above the blue band of Earth's own shadow. Alpenglow is that light striking mountains.
Q03Do you need snow?
It's most apparent on snow-covered mountains, since snow makes an ideal bright neutral screen. The atmospheric effect exists regardless — snow just shows it best.
Sources
- Celestial phenomena: STEVE, Belt of Venus, Earth's shadow — CU Boulder.
- The phenomena that brighten dawn and dusk — CU Boulder.
- alpenglow (etymology) — Wiktionary.