English: On January 2, 2017, the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of fog and haze over eastern China.

Fog (low cloud) appears bright white while haze (smog) appears gray. Vegetation appears green and water should appear blue, although much of the water visible in this image, especially the Bohai Sea in the northeast, appears tan and blue due to suspended sediment which has washed into the Sea from land. The fog and haze obscures most of the land and ocean, making it very difficult to discern where land ends and water begins.

Outbreaks of haze in this region are common and are especially tenacious and thick in the wintertime when temperature inversions can keep pollutants – and even fog – close to the ground. Air normally cools with altitude, but during an inversion warm air settles above a layer of cool air near the surface. The warm air acts like a lid and traps pollutants beneath it, especially in basins and valleys.

Many of the particles in the haze are sulfate aerosols produced by burning coal. Coal supplies a majority of China’s energy, and the northern half of the country uses coal widely to heat buildings in the winter. In addition to emitting carbon dioxide, coal fires release sulfur dioxide, a gas that combines with water vapor to make small droplets and crystals of sulfuric acid and other sulfates, which can be detrimental to health.