STS031-73-59
The NASA photos STS31-73-54 and STS31-73-59 show a second dust front over the Beijing region (Beijing lies under the northern margin), situated to the north of the main dust.
The source of the dust is the vast loess (wind-laid dust) sheet of Inner Mongolia that stretches west from Beijing 1400 km to the Sinkiang border. The climatic gradient is characterized by rapidly decreasing rainfall west from Beijing, from 500 mm/yr to 250 mm/yr only 300 km upwind. Below 250 mm of yearly rainfall, vegetation density is low enough to allow wind deflation of surface dust. Air masses over the Takla Makan Desert of Sinkiang are usually dust laden to some degree. Occasionally, the dust loading becomes heavy and moves as far as Korea (as shown here), and then offshore over the Pacific.
