There's more to rain and snow than just water falling from the sky.
Hailstones are the result of an ice pellet or graupel being caught in a thunderstorm updraft and cycled through the cloud for several minutes, accreting multiple layers of ice in the process.
These are the conditions most likely to lead to measurable snowfall. Like liquid droplets, crystals can collide, resulting in multiplication as the crystals shatter (normally in cold air) or growth as they combine (common in warmer air).
Very large snowflakes occur in this manner. They have generally fallen through a region of above freezing air. Frequently, a snowflake will fall through a region occupied by freezing rain. In these situations, the liquid may accrete, or rime, onto the crystal-freezing on contact, producing what is commonly known as graupel.
However, if the crystal falls through a deep enough superfreezing layer in the lower atmosphere, it will melt partially or entirely-and, depending on the temperature in the layer beneath that, may refreeze partially or entirely before reaching the surface.
If the crystal melts entirely, it will likely reach the surface as either rain or freezing rain. If it partially melts and then refreezes, ice pellets or sleet are normally the result. Often, snow grains or snow pellets fall from the sky.
These are forms of graupel and are generated in the same manner. Snow grains tend to fall from low stratus clouds as small ice crystals develop some rime accretion. The lack of sufficient vertical air currents permits them to fall at such small sizes.
Snow pellets, on the other hand, tend to be larger in size (up to about 5 mm diameter), and require thicker, moisture-laden cumulus clouds to form. The minor convection beneath these clouds allows the snow pellets to become larger than snow grains, but not quite as large as graupel before they precipitate.
A final type of frozen precipitation of which pilots should be aware is hail. This is normally the product of ice pellets or graupel that have been caught in the updraft of a cumulonimbus cloud. As the embryo travels upward through thousands of feet of supercooled rain droplets, it develops a mixed rime/glaze coat.
Subsequent cycling through warm and cold regions of the cloud deposits additional glaze coats, enlarging the stone. Eventually the stone becomes heavy enough to counter the updrafts, or more commonly, is ejected from the storm by either the updraft or a downdraft.
While pilots are aware of many of the generalizations about precipitation types and formation, it always helps to understand some of the details behind why we get the type of weather we must fly through.
It is that extra level of information that gives us a better insight into the atmosphere-insight that may allow us to make better decisions about the weather we face.
Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University and was a scientist with NASA's Global Change Master Directory. Shein holds a commercial license with instrument rating.