(WSYR-TV, Syracuse) Often, during the Fall and Spring, you'll hear us mention the term graupel. No, it's not a made up term. Here's the definition, as stated in the Glossary of Meteorology, put together by the American Meteorological Society:
|graupel—Heavily rimed snow particles, often called snow pellets; often indistinguishable from very small soft hail except for the size convention that hail must have a diameter greater than 5 mm. Sometimes distinguished by shape into conical, hexagonal, and lump (irregular) graupel. |
Okay, what does that mean? In the clouds, when temperatures are below freezing, there is frozen precipitation particles in the cloud. There is also supercooled water, which are tiny droplets of water, in the clouds. (Water can exist as a liquid to temperatures as cold as -40F) All of this "stuff" mixes around within a cloud.
When you get a snowflake, it collides with the supercooled water droplets in the cloud. Those droplets in turn stick to the snowflake branches. This is called riming. As the riming process continues, the snowflake loses its identity as a typical flake and becomes more of a lump of snow. This all depends on the degree of riming.
The reason that this phenomenon is more prevalent in the Fall and Spring is because while we are cold enough aloft for snow, it's not sufficiently cold enough for pure flakes of snow. The "sweet spot" for snowflake production is between 0 and 14 degrees Farenheit.
Graupel pellets remind me of the stuffing inside of a beanbag chair.