The green pixels are land regions without snow, blue pixels represent water, and dark gray shows where no data were collected. In November the average snowline runs north of the U.S.-Canadian border except in the Rocky Mountains, where it dips to the south and extends all the way into northern Arizona and northern New Mexico. In the west, snow covered large parts of Colorado, Utah and even parts of Nevada, providing banner conditions for ski resorts, which had most of their trails open by Thanksgiving. This winter's increased snowfall was a nice change for the ski areas that suffered from last year's lesser snowpack. Because snow has such a high reflectivity compared to other surfaces on Earth, snow covered areas appear much brighter in satellite imagery than most other surface types.
Clouds appear darker than the white snow.
More than 40 percent of the Earth's land surface in the Northern Hemisphere can be covered with snow during the winter months. The highly reflective nature of snow combined with its large surface cover make it an important factor in the Earth's radiation balance, which includes incoming solar energy and energy reflected back into space. Because the Earth is in a steady-state balance of incoming and outgoing energy, its temperature undergoes small change, but the mean temperature stays nearly the same. According to the National Snow and Ice Data Center, snow may reflect up to 80 and 90 percent of incoming solar energy, whereas a surface without snow would only reflect 10-20 percent. Retained solar energy means increased warmth.