photo #6001009

Across the valley mist near the village of Coulouma in the Pyrenees mountains of France, Stuart Gregory (Member 48,103) observed the optical effect known as airlight, which gives very distant objects like mountain ranges a bluish tinge. The molecules of our atmosphere scatter the shorter, blue-looking, wavelengths of visible sunlight more than they do the longer, red-looking, ones. This preferential scattering effect makes the sky look blue when looking away from the Sun, but it also meant that the illuminated air between Stuart and the distant Pyrenees brightens the dark background with diffuse bluish light. The more air there is between an observer and the background, the more molecules there are in between to scatter the light. And so, from the dark forest nearby to the far peak of Mount Canigou, some 60 miles (100 km) distant, the layers of foothills, mountains, and valley mist appeared progressively bluer the further from Stuart they were.