What causes fire rainbows?

(Image credit: Guillaume Piolle)

Question from Isla Jenkins

Fire rainbows are so called because the wispy clouds look like bright flames licking the sky.  Officially known as circumhorizontal arcs, they only occur in very specific conditions, so you will be very lucky if you do manage to spot one. Firstly, you must be within 55 degrees north or south of the equator in the summer months. Then, there must also be cirrus clouds in the sky, which are thin and wispy and exist at high altitudes where the temperature is very low.

Due to their location, cirrus clouds are formed of plate-shaped ice crystals. When the sun rises to higher than 58 degrees, the ice crystals act like prisms and refract the light, bending the rays into their individual wavelengths.

Each visible wavelength of light has a colour, from red, orange and yellow to green and finally blue. When all of these wavelengths reach our eyes from the same angle, we see the light as white, but when refraction causes them to scatter and reach us from different angles, we see their individual colours. This then makes a colourful rainbow appear in the cirrus clouds, creating a fire rainbow.

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