Ask the meteorologist: How do clouds form and create snow?

Question: How do clouds form and create different types of precipitation? — Qwest Cockman

Answer: Clouds form when sufficiently moist air is cooled to the dew point temperature of the air or below, so that either liquid water droplets form on cloud condensation nuclei, or in the case of cirrus clouds made of ice crystals, water vapor deposits directly onto ice nuclei.

Most clouds are formed by cooling that occurs due to expansion of air that is lifted to a higher altitude having lower pressure. This can occur due to surface heating causing convective updrafts, or due to other lifting mechanisms like upslope flow, convergence along seabreeze boundaries, and by way of the passage of organized systems like low pressure centers, upper-level troughs and air flowing in the vicinity of frontal boundaries.

Once clouds form, precipitation can develop through additional growth of droplets or crystals in moist air, by collision and coalescence of smaller existing droplets into larger ones, and most efficiently, by something called the Bergeron-Findeisen process in which ice crystals that form in the presence of super-cooled droplets rapidly grow due to low vapor pressure above the ice surface compared to higher vapor pressure above the droplets. This results in a transfer of water from many droplets to a few crystals, allowing them to grow quickly to the point where upward motion of the air is more than offset by gravity, so that the crystal (or multiple individual snow crystals stuck together in the aggregate form we usually see as "snowflakes) falls as snow. This snow may end up reaching the ground as snow, rain, sleet or freezing rain, depending on the particulars of the temperature profiles it travels through on the way to the surface.