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Particles originate from launch sites belonging to the underlying 3D model, potentially reaching the sky plane unimpeded and contributing to the growth of the local snow surface. We use a simple bucketing and filtering scheme to allocate the successful flakes to the total mass of the sky's available snow, while ensuring that small local areas of sky do not over contribute. 
This is important, since the number of particles hitting any particular area of the sky may vary dramatically depending on the complexity of the underlying surfaces.  We must ensure that a large concentration of flakes (say, directly above a tree), draws the same total snow as would the sky above a sparse flat surface. Furthermore, importance ordering implies that not all launch sites shoot the same number of particles.  Our approach allows to easily manipulate the amount of snow the sky produces, allowing us to "draw" on complex, curved objects using snow.
Non-constant allocation of snow mass to sky bucketing can be used to ``write'' the SIGGRAPH 2000 logo with snow.
Click to enlarge.
Two more examples of writing with snow.
Click to enlarge. Click to enlarge.