OPOD - Rays Shadows Colours

Rays - Shadows - Colours

Mario Freitas (site) of UTFPR Curitiba, Brazil caught this unusual aspect of cloud rays and shadows during a flight from Bogota to Cartagenia. Sunset was approaching.

The cloud tops responsible for the shadows are clearly discernible as is the shadows' true nature – tubes of unlit air stretching for tens and sometimes hundreds of miles. Compare the appearance of mountain shadows. ©Mario Freitas, shown with permission

Close to sunset the shadow tubes cast by clouds slope gently downwards. They are parallel but appear by perspective to converge to a point. Viewed sunward they converge on the sun itself as 'crepuscular rays'. Viewed in the opposite direction (as above) they converge at the antisolar point as 'anticrepuscular' or 'antisolar' rays.

From the ground in Mario's upper image the rays and shadows might look something like this - shadows radiating outwards from the near setting sun and appearing to project upwards into the sky.

Below: A study in cloud and atmospheric colours. Cloud tops still in direct sunlight have yellow and rosy hues reflecting the reddened light of the low sun. Aerosol and dust in the sunlit atmosphere backscatters the same light to form the 'rays' between the dark shadow tubes. At right, lower clouds deep in shadow look blue grey because they are lit purely by blue skylight. In contrast, shadowed clouds at right is a purplish pink - they are lit by a combination of blue skylight and yellow/red rays reflected from the sunlit cloud surfaces around them. The distant sky near the horizon is almost white because the aerosol and dust haze is approaching an optical thickness when incident sunlight is multiply scattered.