Asian brown cloud, a large atmospheric brown cloud that occurs annually from about November through May over eastern China and southern Asia. The Asian brown cloud is caused by large amounts of aerosols (such as soot and dust) produced in the combustion of fossil fuels and biomass across the region. It has been linked to decreases in summer monsoon rainfall in India since 1930, the southward shift of the summer monsoon in eastern China, declines in agricultural production, and increases in respiratory and cardiovascular problems in the people inhabiting the region.
The first observations of this phenomenon were made in the late 1990s as part of the Indian Ocean Experiment (INDOEX), in which coordinated air pollution measurements were taken from satellites, aircraft, ships, surface stations, and balloons. The INDOEX observations surprised researchers by revealing a large aerosol formation over most of South Asia and the northern Indian Ocean. Because of the effects of the Asian brown cloud, India and China are dimmer at the surface today by at least 6 percent compared with their state in preindustrial times.