See the paper Computer Modelling of Fallen Snow published in SIGGRAPH 2000:
In this paper, we present a new model of snow accumulation and stability for computer graphics. Our contribution is divided into two major components, each essential for modelling the appearance of a thick layer of snowfall on the ground. Our accumulation model determines how much snow a particular surface receives, allowing for such phenomena as flake flutter, flake dusting and wind-blown snow. We compute snow accumulation by shooting particles upwards towards the sky, giving each source surface independent control over its own sampling density, accuracy and computation time. Importance ordering minimises sampling effort while maximising visual information, generating smoothly improving global results that can be interrupted at any point. Once snow lands on the ground, our stability model moves material away from physically unstable areas in a series of small, simultaneous avalanches. We use a simple local stability test that handles very steep surfaces, obstacles, edges, and wind transit. Our stability algorithm also handles other materials, such as flour, sand, and flowing water.
Its project page contains explanations and example images. A PDF is here.
An older paper is Simulating Dust Accumulation, published in IEEE Computer Graphics & Applications in 1995:
This paper describes a dust modeling technique. An empirical method is used to visually simulate the effect of dust accumulation on object surfaces. The dust amount is first predicted based on the properties of the surfaces: the surface inclination and stickiness. This predicted amount is then adjusted according to some external factors: surface exposure to wind and scraping off by other objects. The calculated dust amount is finally perturbed by a noise function on rendering to give a fuzzy visual effect.