I remember reading a hard sci-fi novel. somewhere along the way the protagonist is part of a group that finds itself trapped in a building by what they originally mistake as a heavy snowfall. The "snow" is revealed to be made up of microscopic alien creature skeletons that fell (I think) from space. I think the snow was something of a harbinger of a menacing alien threat, and was a prelude to a showdown with a much nastier baddie. I wish I remembered more; I recall the book having a dark, desperate overtone, my like my search for the identity of the book itself.
Sorry for vagueness, any guess appreciated.
P.S. Fisk's Trillions wasn't it, but thanks for the suggestions.