File:Fog in the Douro Valley, Portugal (MODIS 2024-02-03).jpg

English: On February 2, 2024, valley fog filled the Douro River Valley in northeastern Portugal, creating a widespread blanket of white between the region’s highlands. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired a true-color image on that same day.

The Douro River (known as Duero in Spanish) is one of the longest rivers on the Iberian Peninsula. It rises in north-central Spain, flows 475 miles (765 km) west across Spain before twisting towards the southwest to form the Spain-Portugal border, then turns westward again to cross Portugal and spill into the Atlantic Ocean. Several major rivers join the Douro along its course: the Tormes, Esla, and Pisuerga are the major tributaries.

The banks of the Douro, which are obscured by fog, are widely terraced, creating a spectacular sight when boating down the river. The rich agricultural region is famous for growing grapes, which are used to create fine Port.

Fog, especially in winter, is common along the Douro and its tributary valleys. Fog is essentially low-lying cloud composed of tiny water droplets suspended in the air. As nights lengthen in winter, the atmosphere has more time to cool down and approach the dew point—the temperature at which the air becomes saturated and water vapor condenses into fog. Since cold, moist air is denser than warm air, it sinks and drains into valleys, meaning fog develops there first. The rivers amplify fog formation by providing a ready supply of moisture.