The Asian brown cloud
is a layer of air pollution
that covers parts of the northern Indian Ocean
, and parts of South Asia
, Southwest Asia
, and China
. Viewed from satellite photos
, the Cloud appears as a giant brown stain hanging in the air over much of Asia and the Indian Ocean.
In some humidity conditions, it forms haze
. It is created by a range of airborne particles and pollutants (e.g. woodfires, cars, and factories), characteristic of biomass
burning and industrial emissions due to incomplete burning. The cloud is associated with winter monsoon (December to April) during which there is no rain to wash pollution from the air.
This pollution layer was observed during the Indian Ocean Experiment (INODEX) intensive field observation in 1999. Subsequently the United Nations Environment Programme (UNEP) has been supporting a project called ABC (still ongoing in 2007).
Although aerosol particles are generally associated with a global cooling effect, recent studies have shown that they can actually have a global warming effect in certain regions such as the Himalayas. The warming effect of the Asian Brown Cloud may be an important factor in the retreat of the Himalayan glaciers. The United Nations Environment Program is worried that Asian glacial melting could lead to water shortages and floods for the hundreds of millions of people who live downstream.
A recent CSIRO study found that the Asian Brown Cloud is also affecting rainfall in Australia.