I remember valley fog fondly. When I was a youngster I’d ride the bus and looking down into the valley it would actually look like the valleys were filled in with a solid white mass and we would have to descend into it to get to the school. It’s one of those things you have to experience to appreciate I suppose.
This image was taken by the the Suomi NPP satellite (credit: NASA/Joshua Stevens/Adam Voiland). One of the most striking thing about the image is the “light pollution”.
It’s autumn in the Northern Hemisphere, which means many people living in mountainous areas are awakening to fog-filled valleys.
As nights lengthen with the season, 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 air is denser than warm air, it sinks and drain into valleys, meaning fog develops there first. Many valleys also have rivers and streams, which amplifies the process by providing a ready supply of water vapor.
The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured a glimpse of this process at work in the mountains of West Virginia on October 24, 2018. The sensor acquired the nighttime image at about 2 a.m., when fog had filled many valleys of the Cumberland Mountains.