Adam Burton/Alamy
Why do rainbows refract light into several different colours, but we don’t see the same effect with cloud inversions?
Mike Follows
Sutton Coldfield, West Midlands, UK
The formation of a rainbow requires the observer to be between the sun and falling rain, with the rain in front and the sun behind. Sunlight passes from behind the observer, refracts as it enters each raindrop, reflects off the back surface, and then refracts again on its way back to the viewer. The raindrops need to be relatively uniform in size and well separated. In contrast, the water droplets in a cloud are typically less uniform and more densely packed. Even if light undergoes the refraction-reflection-refraction process required for rainbow formation, it is scattered multiple times before it can reach the observer. This is why clouds appear opaque.
In inversion conditions, the temperature increases with altitude, rather than decreasing as it usually does. This allows people at a high vantage point to look down on fog or low stratus clouds. The suspended water droplets in the fog or clouds are typically 10 to 100 times smaller in diameter than raindrops, making them light enough to remain airborne and not fall as precipitation. At this droplet size, diffraction becomes more dominant than refraction. Each droplet creates its own diffraction pattern, consisting of alternating bands of constructive and destructive interference for each wavelength of light. As the droplets get smaller, these bands broaden. When the droplets are tiny enough, the interference bands from different colours spread out so much that they overlap, mixing the colours and producing a white arc.
A Brocken spectre is the shadow of an observer cast onto cloud or mist. It is often surrounded by a glory, which is centred on the shadow and results from the backscatter of sunlight, resembling a halo or colourful ring. A glory forms when the cloud or fog consists of uniformly sized water droplets, typically between 5 and 20 micrometres across. The physics is quite complex but intriguingly, can be explained using either classical or quantum models. The smaller the droplets, the larger the glory appears, though its colours tend to be less intense.
To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
Â鶹´«Ã½ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.
