Why Alpenglow Happens (and How to Predict It)
Alpenglow — the rosy glow that lights mountain peaks at dusk and dawn — is one of the most photographed and least understood optical phenomena in alpine photography. It looks like the mountain is generating its own light. It's actually indirect skylight scattered by particles in the lower atmosphere.
The mechanism: at low sun angles (below the horizon, or just above it), direct sunlight is strongly filtered by the atmosphere, removing most blue wavelengths and leaving red and orange. This already-red light then bounces off clouds and haze above the observer's horizon and illuminates the mountain faces from above. The mountain itself is in shadow from direct sunlight — it's lit from the reddened sky.
The best alpenglow conditions: clean air with some moisture in the lower atmosphere (but not overcast), clear sky at higher altitude, and mountains that face roughly east or west so they're perpendicular to the sunrise/sunset direction.
For prediction: I use the combination of meteorological visibility data (European Meteorological Service SYNOP reports) and clearsky index from Open-Meteo. When visibility at valley stations is 30-50 km (enough moisture for scattering, not enough for thick cloud) and clearsky index is above 0.7 at summit level, alpenglow is highly likely. This combination occurs roughly 40-60 days per year in the Alps — maybe 20-30 of those are accessible for photography given weather windows and logistics.
Last season I successfully planned 8 alpenglow shoots using this method. 6 delivered strong colour. The 2 misses were both cases where cloud built faster than forecast overnight.