File:Low clouds in central China (MODIS 2016-12-11).jpg

English: Low clouds stretched over central China’s Sichuan Basin and settled into valleys across central China in early December 2016. In addition to low clouds (fog), a thick gray haze, caused by pollution, lays across the northeastern section of the image, obscuring the land from view. Thinner haze stretches over much of the image mixing with the fog in the Sichuan Basin. The mountain peaks in the west of the image are topped with snow, and the crispness of the view indicates that the air above these mountains is clear.

Both clouds and haze are likely trapped close to the Earth by a temperature inversion. Normally, air is warmest near the surface of the Ear. The warm air rises, cools, then sinks back down. This vertical circulation is important in weather patterns and movement of pollution and moisture away from the ground. In a temperature inversion, however, a mass of warm air moves over cooler air, creating a situation where the atmosphere warms with altitude. Because cool air does not have the energy to rise through the warm air, vertical circulation stops and lower layer of air is trapped at the surface.

The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA’s Terra satellite captured this true-color image on December 9, 2016.