A stunning round solar halo caused a social media frenzy – people were out in the streets, taking photos, sharing them, while scientists were also excited to see such a rare phenomenon. But what are solar halos, and why did this one appear in Mexico?

Generally speaking, a halo is an optical phenomenon produced by light interacting with ice crystals suspended in the atmosphere. Basically, millions of ice crystals in the atmosphere refract the light, and when light hits them at just the right angle, a halo is formed. The most common type of halo, as was the case here, is called the 22° halo.

It’s called that because it forms a circle with a radius of approximately 22° around the Sun, or occasionally the Moon. As light passes through the 60° apex angle of the hexagonal ice prisms it is deflected twice resulting in deviation angles ranging from 22° to 50°. The angle of minimum deviation is about 22°. As no light is refracted at angles smaller than 22° the sky is darker inside the halo.

In folklore, such halos are generally a bad omen – either a sign of a storm to come, or just of… something bad (superstitions, yeah).

“I have received some 30 phone calls from people who are scared and think that it’s a bad omen, others think it means it will rain more than expected,” the coordinator of the National Weather Service, Juan Manuel Caballero, told said in an interview.

Of course, the manifestation is quite benign; it is possible that it’s followed by a storm, but the halo can appear without an ensuing storm. Some people took it more lightly, and even made memes with it: