Solar halo
Halo appeared around the sun in Mexico City AFP

A weird weather phenomenon has caused a halo to appear around the sun in Mexico city, sparking fears of UFOs and the apocalypse.

Luckily, the end is not nigh, nor are aliens invading the planet. Instead, it was an optical phenomenon – sun halos occur when light passes through ice crystals in the atmosphere, bending the light and creating a coloured or white ring.

The ice crystals responsible are suspended in clouds high in the upper troposphere. However, in colder weather they can float closer to the ground. When this happens, the crystals behave like prisms, refracting the light.

sun halo
Social media users suggested the halo was everything from aliens to bad omens AFP

Juan Manuel Caballero, coordinator for the National Weather Service, reassured concerned citizens of the science behind the phenomenon, but some remained unconvinced.

"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," he told AFP. "I've had to repeat several times: 'No, it doesn't hurt if you don't look at it directly'.

"It's very common. The thing is that people never look up."

Despite this, social media lit up with ideas of what the halo could be: "The aliens have finally arrived!" one Facebook user said.