File:Fog and Snow in Canadian Rockies (MODIS 2022-11-11).jpg

English: On November 9, 2022, the Canadian Rockies were washed in shades of white and gray in this true-color image acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Terra satellite.

These shades are typical on snowy days in the Rockies, when a light coat of snow coats vegetation, heavy snow sits in high elevations, and fog fills the valleys. In true-color images, it is sometimes easier to use texture rather than color to sort out white-toned surfaces. For example, fog (low cloud) is liquid water while snow is frozen ice crystals. Fog droplets are much smaller than crystals of ice, and these differences can be seen in the softer texture of the fog. Deep snow tends to have a relatively smooth surface, so it reflects light back strongly and appears bright white. Both the round droplets that make up fog and the variable texture of light snow sitting atop grasses or trees reflect light in varying directions, and less directly back to the sensor, so they appear much less bright than deep snow and may even look gray in true-color images.

Although the texture and brightness in true-color images are useful, when scientists want to more clearly understand where snow stops and fog begins, they turn to false-color images. The image directly below our true-color Image of the Day is the false-color version of the same image. The false-color image uses infrared and visible light (MODIS bands 7,2,1) while the true-color image is created by visible light (MODIS bands 4,3,1). While the true-color image shows shades of white and gray, the false color image is quite bright and it is simple to separate fog, cloud, liquid ground water, and vegetation. Fog looks white, snow appears electric blue, while deep water (rivers, streams, and lakes) looks deep blue. Vegetation looks bright green and open soil would appear tan. Where light snow sits atop vegetation, the colors of the predominant land cover may appear strongest or the area may be tinted blue-green.