Crepuscular Rays over the Roof Tops of Manchester
December 17, 2020
Photographer: Rob Lawley
Summary Author: Rob Lawley
As shown above, rays of light from the setting Sun rise up above roof tops in Manchester, England. Gaps in the clouds in the western sky confine the rays to bands in a fan-like form. In this image dust, smoke and other dry particles in the atmosphere more effectively scatter blue and green light (shorter wavelengths) allowing more yellow and red light (longer wavelength) to pass through. The dusty atmosphere acts to make the rays easier to detect. Light from the setting Sun has to pass through up to 40 times as much air as light from an overhead Sun. This effect is known as Rayleigh scattering, named after the nineteenth-century physicist Lord Rayleigh.
Whilst the bands appear to radiate from a point behind the roof, this is nothing more than an optical illusion. The Sun is about 93.6 million miles from Manchester, so the bands are near parallel. The fan-like form results from the observer’s perspective, in much the same way as railway lines appear to merge to a single point in the distance.
At the time the image was made the Sun was still above the horizon, so technically the bands are not crepuscular rays (from Latin crespuculum which means twilight or after sunset). Photo taken on September 4, 2020, at 7.29 p.m.