Graupel is precipitation that looks like pith balls. It is caused by snowflakes falling through moist air, such as water-droplet fog that is below freezing or super-cooled* larger drops of water. The water collects on the flakes creating a coating of rime ice. Graupel forms in convection, a condition that occurs when air is unstable. Convection is rapidly rising and sinking air currents. In the warm season convection is what occurs in showers and thunderstorms.
While it might not seem possible, convection occurs cold winter air under the right conditions. It has to do with vertical temperature differences that place heavier air next to lighter air. The heavier air must sink while the lighter air rises, much like a hot air balloon. In summer, temperatures are above freezing at altitudes extending at least several thousand feet above the surface, and often to 10,000 to 14,000 feet high. Precipitation falls as rain. In winter the entire air column is often below freezing so the tiny balls do not melt. The key is not the actual temperature. Convection occurs due to the difference in temperatures. The result is graupel instead of snowflakes.
The balls, about the size of BBs, are easily crushed by squeezing your fingers around them. Pith, is a spongy white material found inside the skin of an orange or lemon and also certain plant stems. While they look like pith balls, graupel is their name. According to Merriam-Webster, the term graupel was first used in an 1889 weather report. The term is Germanic in origin and is the diminutive of Graupe, meaning “pearl barley.” It may look like pith but it is really a form of snow.
Take a look at the photos below showing graupel that fell in Cedar Falls, Iowa on December 4th.
* Supercooled water is water that remains liquid even though temperatures are below freezing. Clouds that are below freezing usually contain liquid water droplets or a mixture of ice crystals and water. The percentage of ice to water changes with temperature. At around -40 degrees clouds are usually all ice crystals.