So how does it pack into a snowball?

Snow is a solid one moment and a liquid the next, and the team behind Disney's snow called it "infuriatingly difficult to model on a computer". Treat it as a solid and a snowball bounces off a wall like rubber; treat it as a liquid and it splashes. Alexey Stomakhin at Walt Disney Animation Studios and Joseph Teran at UCLA, with Craig Schroeder, Lawrence Chai and Andrew Selle, reached for the material point method, built for solids: "To our knowledge, this is the first time MPM has been used in graphics." The bits never touch. Every step they hand their mass and speed to a grid, the grid works out the forces, and new speeds go back. Their snow is elastic until it is pushed past a limit; then it stays packed and gets harder, or stretches and breaks. Disney built it into Matterhorn, a simulator "originally developed for creating snow effects in Frozen". Their snowballs were even harder on the outside: "This mimics our experiments with snowballs in the real world."

Our own snow, simulated here in 2D, and our own cast; no frames from any film.

Where you live, does snow pack or pour?

Sources: Stomakhin, Schroeder, Chai, Teran and Selle, "A Material Point Method for Snow Simulation", ACM Transactions on Graphics 32(4), SIGGRAPH 2013; Walt Disney Animation Studios, "Matterhorn" technology page.
Sounds: our own, made in code (earshot, CC0).