The Alps
The Alps serve as a natural barrier, trapping smog and haze in the valleys of northern Italy. In this image, taken on December 20, 2003, by the Moderate Resolution Imaging Spectroradiometer ( MODIS ) on NASAs Aqua satellite, a veil of thin grey haze hangs around the base of the towering mountain range, filling in the canyons on the south side of the range. Further south, dense white fog covers the Plain of Lombardy. Smog is a frequent problem in northern Italy, where air is hemmed in by the Alps on the north and the Apennine mountains to the south.