I’ve written previously about having seen the Northern Lights on a visit to Canada many years back, but until today, I’d never heard of a “circumhorizontal arc”. Then I received this email from a family member, double checked it’s authenticity, and damned if this is not true. It’s pretty amazing and from what I’ve just learned in the last few minutes, thanks to Wikipedia, it’s considered quite rare.
A circumhorizontal arc, also known as a fire rainbow, is an optical phenomenon similar in appearance to a rainbow, caused by the refraction of light through the ice crystals in cirrus clouds. It occurs only when the sun is high in the sky, at least 58° above the horizon, and can only occur in cirrus clouds. The phenomenon is quite rare because the ice crystals must be aligned horizontally (instead of the more typical vertical alignment) to reflect. It is often thought of as the rarest natural occurrence in the world.
And while researching these, I also stumbled across a similar phenomenon, “Nacreous Clouds“, on theAtmospheric Optics website, where you can see other images like this one.