File:Fog in Rogue Valley (MODIS).jpg

English: A weak weather system hung over Oregon on November 15, bringing cloudy, cool weather with a few light showers bringing drips and drops across much of the state. With plenty of moisture in the atmosphere, dense fog began to form in the Willamette and Rogue Valleys. Thanks to a strong inversion and little wind in low-lying areas the widespread fog lingered through November 17.

Fog is common in the region in the autumn and winter, as moist Pacific air moves inland, cools, and sinks into the vast valley between the Cascade Mountains and the Oregon Coast Range. As that air gives up its heat to the upper atmosphere, the layer near the surface becomes saturated and essentially forms clouds at ground level.

This is also the season when temperature inversions become common. Inversions occur when warm air high in the atmosphere moves in over the cooler, denser air in the lowlands. Normally, temperatures are warmer at the land surface, cooling higher up in the sky. This allows for most of the rising air and pollutants to continue to disperse out into the atmosphere. But an inversion acts like an atmospheric “lid” that can trap moisture, fog, and air pollution in a valley for days.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite acquired a false-color image centered on the Rogue Valley in southern Oregon. In this type of image, vegetation appears bright green, water looks blue, and cloud, including low cloud (fog) is usually white, but may be tinted with light blue. Fog typically has a finely granular appearance and forms “fingers” at the edges as it creeps into small mountain valleys.