English: A high-pressure system over Italy brought crystal-clear skies to Northern Italy in early October 2023, but also contributed to the dingy gray haze that covered the Po Valley. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a true-color image of hazy skies on October 9. The Po River, Italy’s longest river, runs approximately 650 kilometers (400 miles) from the southwestern Alps to the Adriatic Sea, passing indirectly through Milan, and through Turin. Both cities are industrial centers and heavily urbanized, creating pollutants that often cloud the skies overhead. However, hazy skies result from more than just the industrial emissions from the area around major cities. Smog is often trapped at the base of the Alps by high atmospheric pressure, especially in autumn.