We first thought of a return of the Care Bears and we liked to believe in it. The reality, however, turns out to be much more rational. Monday June 3 around 2 p.m., a motorist and his passenger noticed a strange phenomenon while they were driving near Combourg on the dual carriageway between Rennes and Saint-Malo. In the blue sky of Brittany (we place it here discreetly), a cloud (there are sometimes) turned out to be completely different from its congeners by displaying different colors of the rainbow. How can we explain that a cloud can display blue, yellow and even pink hues as can be seen in the photos sent by the motorist to 20 minutes ?
To find out, we interviewed Jérôme Dreano, boss of Météo Concept which notably manages the Météo Bretagne site. “It’s a phenomenon of iridescence. We are talking about an iridescent cloud, here an iridescent cirrostratus,” explains the specialist. According to him, it is “quite rare to see it” because very specific conditions must be met to obtain such a phenomenon. But then how does it work? According to our specialist, “it is the diffraction of sunlight when encountering the ice crystals of the cloud which allows these colors to appear”.
To put it more simply, we can say that the sun’s rays tinted the drops of this very fine cloud as they passed through, thanks to an improbable combination of circumstances: the droplets must be relatively spaced and distributed uniformly so that the light is reflected there.
Rainbow, halo, mirages…
This is what we call a “photometeor”, that is to say an optical phenomenon that we can sometimes find by observing a soap bubble. The best known is the rainbow but we can also cite the lunar halo or mirages.
Although the phenomenon of iridescent clouds is quite rare, it occurs more frequently in Asia, where much more spectacular cases have already been observed. At the end of May, an Internet user had already published photos of the same phenomenon occurred in the sky over Collioure (Pyrénées-Orientales).