Water molecules tend to absorb all colors except blue, giving snow its ethereal hues at depth. Thousands of avalanches occur in Northwest mountains each winter, and they usually kill several people in Washington. About 90 percent of avalanches occur on slopes of between 30 and 45 degrees. On slopes steeper than that, snow sloughs off before it can accumulate, but on shallower slopes the tug of gravity is not so great. The bottom line is that winter recreationists should greet a big dump of snow with as much caution as celebration.
Snow travels downhill another way, too - from sudden melting in rain brought by warm winds.
Serious flooding in the Pacific Northwest is almost always a combination of rain and dissolving snow, occurring most frequently at the beginning and end of winter. Scientists have been arguing for almost a century about whether the problem has been worsened by logging and other land-clearing. The problem is that they do both. Snow will often linger in the shade of trees, melting slowly in early summer. On the other hand, the snow depth under trees is up to six times less than in open areas because branches intercept snow, and it evaporates back into the air.