English: Snow falling across the Pacific Northwest in November 2015 brought excellent skiing conditions at many of the major resorts across the region. Although the mountain tops were capped with white snow, another type of white filled the valleys late in the month: fog.

Low clouds (fog) often fill the valleys of the Pacific Northwest in winter, especially where rivers lie in the valley and increase humidity. Sometimes warmer air can move over the chillier valleys, creating an inversion that can keep fog trapped below for days at a time.

NASA�s Terra satellite flew over western Canada and north-western United States on November 29, allowing the Moderate Resolution Imaging Spectroradiometer (MODIS) on board to capture this false-color image of fog filling the valleys of British Columbia, Washington, Oregon, Idaho and Montana.

A typical true-color image, using MODIS bands 1, 4 and 3, would show both snow and cloud as white, making it hard to tell one from the other. By using a different combination of data (the 7, 2, 1 bands) which include non-visible wavelengths, it becomes extremely easy to tell the two apart. In this false-color image, snow and ice are bright red, while cloud and fog are white or pale peach. Ocean water appears deep blue-black and vegetation is green. Using this combination, the location of fog is strikingly evident.