Snow fog and low clouds in Central Europe
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Snow, fog, and low clouds in Central Europe

November 21, 2007
A series of images demonstrate how satellite imagery can been used to distinguish -and map - between snow and clouds. The first image that displays is a “true color composite” image of snow, fog, and low clouds in Central Europe. With this type of image, colors are assigned according to the intensity with which the land or ocean surface reflects sunlight, much in the same way as an ordinary digital camera. For example, objects or phenomena that reflect the red portion of the spectrum are assigned the color red; things that reflect red very strongly are given a higher intensity than things that reflect red less strongly. The same is true for blue and green, the other primary colors. However, unlike the human eye, satellites also collect information in other parts of the spectrum, such as the near infrared. By assigning these other bands that humans are not able to see to colors that we can see, a false color composite it formed; this is the type of image that displays when you place your mouse over the first image. Often, these images are useful for highlighting certain features. In this image, longer wavelength sunlight (short-wave infrared) is assigned to the color red; objects that strongly reflect the short-wave infrared (such as bare soil) will appear deep red. The near infrared is assigned to the color green; vegetation is highly reflective of near infrared light and thus appears very bright green. The red portion of the spectrum is assigned the color blue; together, this combination of bands is called a “7, 2, 1” false color composite. The satellites bands are numbered somewhat arbitrarily; the short-wave infrared band is number 7, near infrared is called band 2 and red is band 1. In this image, clouds appear to be a very light blue, while ice and snow appear to be a much darker and more vibrant blue.

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