“I read your article about the biggest snowflakes, and was wondering what causes the difference in size of the flakes. We get “snow pellets” here along with what a co-worker called “cotton ball flakes”, and every other size. Is it the amount of moisture falling thru the air, or the speed in which it falls, or something else?”

Actually, some or all of the above. The shape and size of a snowflake can tell you a lot about its brief history. Take the snow pellet example. Technically called “graupel”, snow pellets form when a small snowflake falls through super cooled liquid water, similar to how hail forms.

The size and shape of snowflakes is determined by a combination of the temperature profile of the atmosphere, the amount of moisture throughout that profile, and the presence or absence of vertical winds. The example of graupel (snow pellets) above usually indicates you have considerable updrafts in your snow storm… a process not unlike that which forms hail in thunderstorms. Large fluffy snowflakes can form in a moist and relatively warm snowstorm. Smaller, intricately shaped six-sided snowflakes will occur in a colder and drier airmass, which allows the crystals to form more slowly, and keeps them from joining up with others.