Pace - much of the post is irrelevant and to do with the different shapes snowflakes can come in with variations of humidity and temperature. I didn't post all the Meteorolgy 101 diatribe because I assumed as a pilot you knew it anyway.
Your definition of sleet, as I said before, is the US one not the UK Met Office one.
There is no such thing as a snow crystal - it is an ice crystal and when they increase in size and complexity (as water evaporates from the supercooled water droplets) they are called snowflakes.
I didn't say snowflakes are frozen raindrops - both water droplets and ice crystals in the atmosphere form the same way in that water vapour changes state and either condenses or deposits onto microscopic nuclei in the atmosphere. The only thing the jury is out on is exactly which types of nuclei trigger ice formation and which trigger condensation into water droplets.
There is some evidence to suggest that airborne bacteria may be a major contributor to ice crystal formation and subsequently snow formation.
I'm sure we'll get back to a 'pure snow' argument again soon.