File:British Columbia (MODIS 2021-12-22).jpg

English: Fog filled the valleys of British Columbia, Canada on the last day of autumn, 2021. The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua satellite acquired a true-color image of the scene on December 20.

Near the center and the southeast portion of the image, a bank of dense fog obscures the land from view. In several other locations, light gray fog flows through narrow valleys, flanked by bright-white snow that tops the high elevations, creating a vein-like structure across the region. In other locations, valleys appear green, thanks to the vegetation on the valley floors.

Fog is simply a cloud lying on the ground. Like all clouds, it forms when the air reaches its dew point, which is the temperature to which an air mass must be cooled for the water vapor in it to condense into liquid droplets.

Valley fog is common in the Pacific Northwest. On clear winter nights, the ground and overlying air cool off rapidly, especially at high elevations. Cold air is denser than warm air, and it sinks down into the valleys. The moist air in the valleys gets chilled to its dew point, and fog forms. If undisturbed by winds, such fog may persist for days.