In 1951, the International Commission on Snow and Ice, which is now the International Commission on Snow and Ice Hydrology, came up with the aptly-named International Classification System for Solid Precipitation. Many later physicists and meteorologists thought their system was not complex enough and came up with their own, but for practical purposes, the International Classification System for Solid Precipitation has stood the test of time.
ICSI decided that there were seven basic types of snowflake: plates, stellar crystals, columns, needles, spatial dendrites, capped columns, and irregular forms. To that classification, ICSI also added three other forms of solid precipitation: hail, ice pellets, and graupel.
Ice pellets form when a snowflake melts completely, then refreezes on the way down. When this type of precipitation hits the ground, it’s usually called freezing rain or sleet.
Hail is a form of frozen precipitation which only falls in summer, because it needs the forces of a thundercloud to produce it. Regardless of the season, in the upper atmosphere, it is always freezing cold. The beginning of hail is exactly the same as the beginning of a snowflake, but then it is bounced up and down in the thundercloud a few times, each time adding a new layer of ice. By the time it finally falls, hail can be as large as a softball.
Graupel is rime which has built up around a snowflake. Graupel is also sometimes called ‘soft hail’ because the ice builds up on something delicate instead of something hard, without destroying that centre. The result is something that looks like a fuzzy snowflake.
Snowflake plates are thin, transluscent crystals of snow which only grow when the supercooled water temperature is close to -5 degrees Celsius. Later classification systems divided plates into hezagonal, stellar, and sectored types. Hollow plates and double plates are also possible. A rare form of plate is the triangular crystal.
The two most familiar types of snowflake are those which look like six-rayed stars or filled-in six-rayed stars. These are the stellar crystals and spatial dendrites. They’re really very similar, except that spatial dendrites have a more fernlike structure, with many delicate fronds filling the space between the main branches. (Dendrite means “treelike.”) These are the best-known type of snowflake because these are the largest crystals. When they land on your coat, you can clearly see their structure just by looking at them.
Some snowflakes crystallize into tiny, frozen needles, which form only when the supercooled water temperature is close to -2 degrees Celsius. When they land on your coat, they look like thin hairs of snow.
Slightly thicker snowflakes form columns or capped columns. It’s thought that capped columns are columns which have been blown into a part of the cloud where conditions are right for building plates. Some columns are hollow, while others are solid. If the snowflake is blown back and forth, the column can end up with a whole string of plates strung along its length.
Finally, the irregular forms are those snowflakes which don’t fit into any other category. The Committee left a lot of shapes for this catchall category, which other meteorologists have divided into new categories such as prisms and cups. Of course, the odds are hugely against any two snowflakes being absolutely identical, so there’s probably as many categories there as people want to make.
In the 1980s, ICSI went a bit further and also divided up the types of snow on the ground, based on the grain shape and size, density, hardness, temperature, liquid water content, and so on. Although snow on the ground isn’t precipitation anymore, being able to classify these kinds of snow is useful to predict the future behaviour of the snowpack. One of the most practical outcomes of this research is to be able to identify the kind of weakly bonded snow which leads to avalanches.