What are (and how do) rainbow clouds form?


Our reader Annalisa Criscenzo sent us this photo taken last June 11, 2023 in San Leone in the province of Agrigento. And they wonder what kind of atmospheric phenomenon it is.

The expert says… We talked to our friend Serena Giacomin, a climatologist and meteorologist, who explained to us: “It is an iridescent cloud, a high cloud (that is, cold) composed of very small ice crystals, capable of breaking sunlight into different colors thanks to the phenomena of refraction and reflection. The optical process is similar to that which gives us the rainbow.”

so what happens Sunlight passes through many small crystals that behave like many optical prisms: all the wavelengths of light, which together appear as white light, are spread out into a “fan” of different colors, from violet to red. How many colors exactly? The answer is less simple than it seems.

How many colors? I don’t know! In the past, Aristotle, who was among the first scientists to study the rainbow, spoke of only three colors in Book III of his Meteorology: red, green, and blue. Dante Alighieri instead claimed that there were seven (in Purgatory he actually wrote “…seven lists remained distinguished, all in colors that make the bow the Sun and Delia the girdle”). Early Islamic scholars instead saw a tricolor rainbow: red, green, and yellow.

In the Renaissance it was found that there were 4 colors (red, blue, green and yellow), while in the 17th century five were assumed: red, yellow, green, blue and violet.

All right. Isaac Newton later convinced European scientists that there were 7 colors (associating colors with notes). Actually No there is an exact number of colors in the rainbow. Each shade smoothly transitions into the next. Some scientists think that it is our brain that divides the spectrum into distinct bands, because we humans tend to “order” everything, including the colors of the rainbow.


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