In the air, part of the sunlight is scattered. The small particles (molecules, tiny water droplets and dust particles) scatter photons the more, the shorter their wavelength is. Therefore, in the scattered light, the short wavelengths predominate, the sky appears blue, while direct sunlight is somewhat yellowish, or even reddish when the sun is very low. Goethe believed this to be the basic phenomenon to generate colours ("Urphänomen").
Due to the scattering of light the wood in the background is seen like behind a bluish veil ("aerial perspektive").
The fraction of the light which is deviated by scattering increases with increasing path length, so that at sunset the shorter wavelengths are depleted in direct sunlight and the sun appears orange or red, depending on the amount of haze or dust in the air.