Colour

(Section 2.1.2.2)

As light of all wavelengths is almost equally strongly diffused by clouds, the colour of the clouds depends primarily on that of the incident light. Haze between the observer and the cloud may modify cloud colours; for example, it tends to make distant clouds look yellow, orange or red. Cloud colours are also influenced by special luminous phenomena (photometeors).

When the Sun is sufficiently high above the horizon, clouds or portions of clouds that chiefly diffuse light from the Sun are white or grey. Parts that receive light mainly from the blue sky are bluish grey. When the illumination by the Sun and the sky is extremely weak, the clouds tend to take the colour of the surface below them.

When the Sun approaches the horizon, its colour may change from yellow through orange to red; the sky in the vicinity of the Sun and the clouds show a corresponding coloration. The blue of the sky and the colour of the underlying surface may still influence the colours of the clouds. Cloud colours also vary with the height of the cloud and its relative position with regard to the observer and the Sun.

When the Sun is close to the horizon, high clouds may still look almost white, while low clouds show a strong orange or red coloration. These differences in colour make it possible to obtain an idea of the relative altitudes of the clouds (noting that clouds at the same level appear less red when viewd towards the Sun than when viewed away from it).

When the Sun is just above or on the horizon, it may redden the undersurface of a cloud; when this surface is corrugated, its coloration is distributed in bands that are alternately lighter (yellowish or reddish tint) and darker (other tints), which make the relief more apparent.

When the Sun is just below the horizon, the lowest clouds, in the shadow of the Earth, are grey; clouds at the middle levels are rose coloured and those very high may be whitish.

At night, the luminance of clouds is usually too weak for colour vision; all perceptible clouds appear black to grey, except those illuminated by the Moon, which present a whitish appearance. Special illumination (fires, lights of large cities, polar aurora, etc.) may sometimes give a more or less marked colouring to certain clouds.