This west-looking oblique view shows the Po River valley under a thick layer of yellowish industrial air pollution. The polluted air is effectively trapped by the high walls of the Alps (center through mid-right margin of the view) and Appenine Hills (left), which rise, amphitheater-like, around the valley. As the plume exits the valley it flows eastward into the Adriatic Sea (lower left), which from the space shuttle often appears almost as polluted as the Po Valley itself. The Mediterranean Sea appears at top left and the mountains in Yugoslavia at lower left.
One of the four most vibrant industrial regions of Europe lies in the Po Valley, centered on such cities as Turin and Milan. This industrial region generates great quantities of smog haze. Surrounding mountains protect the valley from winds that might otherwise disperse such plumes. Such pollution plumes remain trapped over the Po River Valley during certain times of the year, when atmospheric conditions prevent dispersal. Other areas where air pollution is concentrated in a similar manner by mountains include the Great Basin of California and the Red Basin of Sichuan in western China. On a smaller scale, similar pollution concentration occurs around Los Angeles and Mexico City.
NASA-MIR 6, image NASA-6-701-99.