This image from space shows widespread smog in eastern China on Nov. 30, 2015. (NASA's Earth Observatory)

For millions of Chinese residents, a cold front brought exactly what they'd been waiting for: a chance to see the sky again.

Thick, choking, dangerous smog blanketed the city for days, forcing people to stay inside. Pollution levels were off-the-charts awful in China's second-largest city, and officials raised the alert level to a code orange– the second-highest level– for several days.

What caused this sudden clearing of the air? The winds picked up when a cold front came through and blew the smog out of the city, according to senior meteorologist of weather.com

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As the skies cleared Wednesday, residents rejoiced and shared photos on social media to show the rest of the world how bad Beijing's pollution had been.

Smog gathers during periods of stagnant weather due to a combination of pollution from power plants, automobiles and other industrial activity. Terrain can also keep the pollution locked in place, Wiltgen said. Beijing sits in a valley.

The most recent round of smog settled in the city last Friday and worsened over the weekend, just as world leaders met in Paris to discuss solutions for climate change.

Weather.com meteorologist didn't see much blue sky during a five-day visit to Beijing in late July.

"I saw a tiny sliver of blue sky among the clouds after a thunderstorm cluster moved through the city for only about 1-2 hours," said Erdman. "Otherwise, it was a hazy cloak the rest of the time I was there."

"I could feel it in my lungs after just a few hours outside, and could sense it even in our hotel room. Those days, air quality levels for particulates (PM 2.5) were only about 150, still very unhealthy by U.S. standards, but a small fraction of what has been witnessed in recent days in Beijing, about 600-700."

Erdman says there weren't many Beijingers wearing masks outdoors in late July. "However, I can't imagine not needing to wear them outside during this recent smog event."