At least as far back as King David’s psalms and Isaiah’s prophecies, the snow has been characterized as a symbol of purity. “Lawn [linen] as white as the driven snow” was how Shakespeare once described it.
Beyond the symbolism, the purity of snow has consequences for society. Snow is a key source of water for drinking and irrigation. It is also Earth’s best method for reflecting sunlight back into space.
Tom Painter is very interested in the purity of snow. Based at NASA’s Jet Propulsion Laboratory and the University of California, Los Angeles, he studies how much water is stored in snowpack and how that snow looks to satellites. He also studies the effects of light-absorbing impurities—dark-colored particles like dust and soot that coat snow.
Dirty snow usually melts faster than fresh snow because it absorbs more energy from the Sun, and that’s not just a problem in sooty, gritty cities. Except for some mountains and high plateaus, snow cover naturally retreats from Earth’s surface in the spring and early summer. Dust on top of that snow significantly accelerates the process.