February 7, 2011 - Low clouds and snow in western Europe
Snow and clouds covered Western Europe on January 31, 2011 when the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite passed over the region and captured two images. The first image, seen here, is a false color image, used to differentiate snow from clouds.
In this false color image, snow appears orange-red, land appears green and clouds are white. The ocean water appears dark red. Using this particular false coloration scheme enables snow-covered areas, such as the peaks of the Alps seen in the lower right corner of the image, to stand out in brilliant contrast to the bright white of the clouds which stretch across the same mountain range.
Rolling the mouse over the false-color image brings the true-color image into view. In this image, vegetation appears green, soil appears brown and water, such as the North Sea (upper left corner) appears deep blue. Both snow and clouds appear a very similar bright white.
The MODIS instrument has 36 “bands”. Each band is a filter which only allows light of a specific, narrow wavelength interval to pass through and reach the detectors behind the filter. Therefore, each band allows collection of data in very specific wavelength. When displaying the data, each band can be assigned a color so the data collected by a band appears as a specific color in the final image.
By choosing the bands to display, and by carefully assigning colors to the bands, the scientist can choose the most effect imagery to visualize his or her particular area of interest. In true-color images, bands 1, 4 and 3 are assigned the primary colors, red, green and blue in that order. These bands allow only light in the visible spectrum to reach the detectors, so the display appears the same as we see our Earth through our own eyes.
This false-color image assigns red, green and blue to bands 3, 6 and 7, respectively. Band 3 allows detection of a portion of the visible wavelength, but bands 6 and 7 only allow detection in the middle-infrared range, which is not visible to the human eye. So this scheme not only gives a different coloration, but can also allow visualization in variations of reflectance within non-visible wavelengths which would otherwise be unobservable. This combination of bands is especially useful for distinguishing clouds from snow.