Skies over Indonesia turned a shade of blood red this weekend, leaving some locals frightened by their surroundings.
The phenomenon, which occurred mainly in the nation's Jambi province, was caused by the ongoing forest fires that have endangered large parts of the country. It's an effect known as Rayleigh scattering, where light is dispersed by particles in the air, which filters out certain wavelengths.
Rayleigh scattering is the reason the sky usually appears blue, but in this case, particles released by the fires turned everything a deep, dark red. The forest fires also brought reduced air quality that made it difficult for many people to breathe properly.
Kiki Khairi, a Jakarta, Indonesia, resident who posted videos of the incident to Twitter, said she received the footage from Jambi, where her mother lives. She told the U.K.'s PA Media that her mother was "terrified" of the sight, as well as the potential danger of the haze.
"She said it's hard to breathe," Khairi told the PA. "The burning forest happens every year, but this is the first time the sky turned red and the air quality is too dangerous for people."
“It was terrifying to witness such a scary phenomenon," she added.
Khairi was just one of many social media users to share videos of the sky. A Twitter user named Zuni Shofi Yatun Nisa, who also shared footage, responded to comments by others questioning whether or what they were seeing was even real.
"This is not Mars. This is Jambi," she said in one of her posts. "We humans need clean air, not smoke."
Ini sore bukan malam. Ini bumi bukan planet mars. Ini jambi bukan di luar angkasa. Ini kami yang bernafas dengan paru-paru, bukannya dengan insang. Kami ini manusia butuh udara yang bersih, bukan penuh asap.— Zuni Shofi Yatun Nisa (@zunishofiyn) September 21, 2019
Lokasi : Kumpeh, Muaro Jambi #KabutAsap#KebakaranHutanMakinMenggilapic.twitter.com/ZwGMVhItwi
Koh Tieh Yong, a professor at the Singapore University of Social Sciences, told the BBC that Rayleigh scattering would cause the redness to seem more extreme when people are looking directly into the sun. He noted that since many of the videos were taken close to noon — when the sun is in the center of the sky — the red shade would have been amplified.
"If the sun is overhead and you look up, [you will be looking] in the line of the sun, so it would appear that more of the sky is red," he told the BBC.
Forest fires are a yearly occurrence in Indonesia, with the majority of blazes taking place during the dry season from April to October, according to the World Resources Institute. This year's fires have caused damage across the country, including canceling hundreds of flights and displacing endangered animals such as orangutans.