In 1951, the body of scientists called the “International Commission on Snow and Ice” produced a system of classification for solid precipitation, which quickly became the standard system used in the field (though there have been other classification systems before and after that also have their proponents).
The Commission identified ten total types of solid precipitation—seven types of snowflakes, plus hail, ice pellets, and graupel.
Ice pellets are what’s often referred to as “sleet.” Graupel, also called “soft hail” is a combination of hail and snow where a snowflake forms the nucleus around which supercooled water solidifies as a hail stone.
The Commission’s seven types of snowflake are as follows:
1. Capped column
Combination of a column and plates (see below). A column that has plates at both ends, resembling a thread spool.
2. Column
Long and thin, same as a needle (see below), but hollow at both ends.
3. Needle
Long, thin shape.
4. Plate
Flat, symmetrical design, sometimes with elaborate, intricate patterns.
5. Spatial dendrite
Like a plate, only three dimensional.
6. Stellar crystal
The kind of aesthetically pleasing designs that show up on Christmas cards, with tree-like branching from the center.
7. Irregular forms
All other snow flakes that don’t fit neatly into any of the above categories. This, in fact, is the most common type of snowflake.
In the 1980s, and updated multiple times since then, the Commission (now renamed the “International Commission on Snow and Ice Hydrology”) produced additional means of classifying snow in its “International Classification for Seasonal Snow on the Ground.” Snow can now be distinguished according to multiple factors, which are:
* Grain shape
* Grain size
* Snow density
* Snow hardness
* Liquid water content
* Snow temperature
* Impurities
* Layer thickness
Scientists go out into the field armed with tape measures, thermometers, magnifying glasses, etc. and measure all these factors so that they can have a better picture of precisely what kind of snow with what properties is present. The measurements all change quite rapidly the longer the snow sits on the ground. Pressure and temperature, through a process called metamorphism, gradually rounds the snow crystals, altering the texture, structural strength, permeability, thermal conductivity, and density of the layer of snow.
As one can imagine, information like this can be highly valuable to many people. Understanding more about the nature of the snow on the ground is relevant to those who run ski resorts, those charged with snow removal from roads, and those concerned about the likelihood of an avalanche, among many others.
Sources:
Travis Wampler, “What are the Different Types of Snow Crystals?” eHow.
“The International Classification for Seasonal Snow on the Ground.” Hydrology.nl.
“Snow on the Ground.” Blue Ice Online.
