The story uses the image of thick, opaque white snow to represent the erasure of the past and the purification of the world. Details, dirt, and deeds done are all expunged from view. The moral differences that might have existed between the outcasts and the other townsfolk are literally "covered up" by the deep snowfalls:
But all human stain, all trace of earthly travail, was hidden beneath the spotless mantle mercifully flung from above.
The snow that falls on the outcasts in ever-increasing quantities covers good and bad people alike, erasing all differences under a freezing white blanket of purity. Snow like this, a "spotless mantle" which seems to come "mercifully" from "above," is often used in the Christian tradition to denote forgiveness and a soiled slate being wiped clean. The snow, like forgiveness in Christian belief, comes from heaven down to earth and changes what it touches.
In the final stages of the story, fallen snow also becomes incorporated into imagery about the characters themselves, showing that they're more than mere criminals. When all hope is lost in the whiteout, the narrator gently describes Piney and the Duchess lying down together against the cold:
And so reclining, the younger and purer pillowing the head of her soiled sister upon her virgin breast, they fell asleep.
The Duchess and Piney fall asleep together in the blizzard, “pillowed” together never to awaken, and because of the snow making everyone look the same, it is impossible to tell which of them is the prostitute and which is the innocent girl. As all of the “outcasts” are killed by the effects of the snow and their bodies frozen and covered with it, the snow itself becomes associated with erasure and purity. The snow has metaphorically purified Oakhurst and his companions of their sins, and does not differentiate between virtuous and immoral people, rendering everyone equal.
