The precipitation can be broken into two categories, frozen, and freezing or unfrozen precipitation. In the frozen category we have snow, graupel, ice pellets (sleet), and snow pellets. In the unfrozen or freezing category we have freezing rain or drizzle, and "plain" rain or drizzle. Let's look at these one at a time.
Snow is precipitation of ice crystals, mostly branched in the form of six-pointed stars. The shape and size of snowflakes can vary, from large "fluffy" flakes to smaller ice crystals. The type of snow and the snow density is dependent on many complex factors. Among these are are the temperature and water vapor within the dendritic growth zone in the clouds (where the temperature is -12°C to -18°C), and the depth of the dendritic zone. Dendrites are beautiful, ornate snow crystals that most people are familiar with. Additional factors affecting snow density include the shape of the snow crystals, vertical motion in the clouds, the amount of liquid water in the cloud,and the thermal profile of the layers from cloud level to the surface. The large "silver dollar" snowflakes tend to occur when the air is warmer and moisture is plentiful. What appear to be huge snowflakes are at times multiple snowflakes that have collided and stuck together. Go here for more information on snow density and the snow-to-water ratio.
|Rime on tree branches|
Snow grains are very small, white, and opaque grains of ice, the solid equivalent of drizzle (or as I like to call it, "snizzle"). They are flat and elongated, with very small diameters. They fall in small quantities, and never in the form of a shower. When these grains hit the surface, they neither bounce or shatter.
Commonly called sleet, ice pellets are transparent or translucent pellets of ice which are round or irregular. Typically they are hard grains of ice consisting of frozen raindrops or largely melted and refrozen snowflakes. It often looks like it's raining, but the ice pellets often bounce when they hit a flat surface and they accumulate. Driving on a a few inches of sleet is like driving in sand or on tiny ball bearings.
All of the above precipitation types fall as frozen precipitation and are reported as "new snowfall" on the CoCoRaHS Daily Precipitation report.
Freezing Rain and Freezing Drizzle
|The challenge of freezing rain.|
Since freezing rain falls as liquid, it is reported the same as normal rain after melting the ice on the inside of the gauge and measuring the liquid.