In the height of daytime, the sky suddenly blackened, and day became night in Sao Paulo.

Sure, smog is bad in the Western Hemisphere’s largest city, where traffic jams can stretch for dozens of miles. But not this bad. What was going on? Was the end near?

“Apocalypse!” one person cried on Twitter.

“The final judgment is coming!” another added.

“Mordor,” one more person intoned.

Experts tried to puzzle out the cause of the midday darkness on Monday, but their conclusions at times appeared to be conflicting, deepening the mystery. The National Institute of Meteorology said the city, which sits at an elevation of 2,500 feet, was “inside a cloud.” Others explained that it was a cold front. MetSul, a Brazilian meteorology company, said the culprit was smoke that had come in from forest fires in Bolivia, Paraguay and remote parts of Brazil.

In fact, it appeared to be a combination of all three factors — clouds, smoke and a cold front — that ushered in the smoke from distance reaches, plunging the city into darkness in the middle of the day.

Sao Paulo in the middle of the day Monday. ( Sao Paulo in the middle of the day Monday. Courtesy of Helena Artemis)

“The smoke didn’t come from fires in the state of Sao Paulo, but from very dense and wide fires that have been happening for several days in [the state of] Rondonia and Bolivia,” Josélia Pegorim, a meteorologist with Climatempo, said in an interview with Globo. “The cold front changed direction and its winds transported the smoke to Sao Paulo.”

The news highlighted the number of forest fires in Brazil, which rose by more than 80 percent this year, according to data released this week by the National Institute for Space Research (INPE).

“This central Brazil and south of the Amazon Rainforest region has been undergoing a prolonged drought,” Alberto Setzer, a researcher at INPE, said in an interview with local media outlets. “And there are some places where there has not fallen a drop of rain for three months.”

Most of the Amazon was once considered fireproof, but as climate change and deforestation remake the world, wildfires are increasing in frequency and intensity, recent research has shown.

“Wildfires in the Amazon are not natural events, but are instead caused by a combination of droughts and human activities. Both anthropogenic climate change and regional deforestation are linked to increases in the intensity and frequency of droughts over Amazonia,” British researchers wrote this year in the Conversation.

Researchers believe the smoke traveled hundreds, if not thousands, of miles to reach Sao Paulo on Monday. (Researchers believe the smoke traveled hundreds, if not thousands, of miles to reach Sao Paulo on Monday. Courtesy of Juliana Muncinelli.)