Many parts of eastern China were put on orange alert on December 4, 2016, when heavy smog veiled large swaths of the country. The haze stranded passengers at airports in northern China and slowed down city life in Beijing, which reached orange alert level on December 1.
An orange alert signals heavy pollution—a PM2.5 (particulate matter) density of more than 150 micrograms per cubic meter of air—for three consecutive days. Such high concentration of fine particles in the air can cause lung and heart problems for vulnerable individuals, including asthmatics, children, and the elderly.
The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite acquired this natural-color image of northeastern China on December 6. Photos taken from the ground also showed low visibility—less than 200 meters (roughly 650 feet), according to news reports. On December 5, People’s Daily reported smog blanketing more than 60 Chinese cities.
Low winter temperatures exacerbate smog since they cause temperature inversions. Warm air settles atop a layer of cooler, denser, smog-ridden air, trapping it like a lid. High concentrations of smog frequently appear in cities like Beijing during winter.