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Why does snow help make precipitation? Two reasons.
First, the saturation point of water vapor with respect to liquid is higher than the saturation point of water vapor with respect to solid ice. In other words, air that is saturated with respect to liquid water is supersaturated with respect to solid ice.
Second, there are a lot fewer condensation nuclei that support ice than condensation nuclei that support liquid water. So ice will start to form on just a few particles, rather than a large number of particles like water does.
Remember our rapidly-cooling cloud? You might expect that once it cools below 0°C, the liquid cloud droplets will all freeze. Actually, they don't. It's hard for water to freeze without something solid to freeze onto. Instead the water becomes supercooled, colder than the freezing point but still a liquid.
As the temperature within the cloud continues to drop past -10°C or so, some of the existing dust particles in the air start becoming able to serve as condensation nuclei for ice, so you get a few ice particles forming. And once that happens, the rush is on. Now that ice is present, and the air is supersaturated with respect to ice, water vapor starts condensing onto the ice crystals. As the air loses water vapor, it is no longer saturated with respect to the liquid water, so some of the liquid water evaporates. And so on, and so on. Eventually, you get a few large ice crystals big enough to fall as precipitation. And away they go, leaving the rest of the liquid water to feed the next set of ice crystals that comes along.
The meteorologist Tor Bergeron discovered this mechanism for precipitation formation while walking in the mountains in the midst of a cold fog. He came upon a train tunnel, which was somehow clear of fog. He realized that the smoke from the engine allowed ice to form on the smoke. The ice grew and drifted to the ground, while the fog droplets evaporated.
Trick question: is ice heavier or lighter than liquid water?
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