The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured this natural-color image of eastern China on January 25, 2017. Milky, gray smog shrouds many of the valleys and lowlands. The brightest, whiter areas (left, top, and bottom edges) are likely clouds or fog.
Outbreaks of smog and haze like this tend to occur during the winter because of temperature inversions. Air naturally cools as it rises in altitude; but during an inversion, warm air masses settle over a layer of cool air near the surface. The warm air acts like a lid and traps gases and pollutants near the surface, especially in basins and valleys.
NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response. Caption by Mike Carlowicz.