During a winter storm, a type of frozen precipitation often falls that is much harder to identify than typical snow or rain. These small, white, opaque pellets are frequently mistaken for tiny hail or a different kind of snow. Understanding this unique precipitation requires knowing its scientific name and the cloud physics that bring it to the ground.
Graupel: The Scientific Name for Snow Pellets
The official term for this frozen precipitation is graupel, also known as snow pellets or soft hail. These particles are characterized by their milky white color and opaque appearance, making them distinctly different from clear ice. The World Meteorological Organization (WMO) formally classifies them as “Snow Pellets.”
Graupel pellets are relatively small, typically measuring between two and five millimeters in diameter. Unlike hard ice, they are quite fragile and easily crushed between the fingers. When they strike a hard surface, they tend to bounce briefly before disintegrating. This soft, opaque quality results directly from their unique formation process high in the atmosphere.
The Process of Formation
The creation of graupel begins when an ice crystal or snowflake falls through a cloud layer containing abundant supercooled water droplets. Supercooled water is liquid water that remains unfrozen below \(0^\circ \text{C}\). The ice crystal acts as a nucleus, and when it collides with these droplets, they instantly freeze onto its surface.
This process is known as riming or accretion. As the initial ice crystal continues its descent, it collects more supercooled water, causing a rapid accumulation of tiny ice particles. This buildup eventually encases the original crystal completely, obscuring its delicate, six-sided structure. The final product is a soft, low-density pellet full of trapped air bubbles, which gives it the characteristic opaque, white appearance.
Distinguishing Graupel from Sleet and Hail
Graupel is often confused with sleet and hail, but they form under fundamentally different conditions. Sleet, scientifically known as ice pellets, is transparent, translucent, and hard. It forms when snowflakes melt into raindrops in a warm layer of air, then refreeze into solid ice pellets when passing through a deep, freezing layer near the ground.
Hail forms primarily in the powerful updrafts of intense thunderstorms, often during warmer months. Hailstones are much larger and denser, growing by cycling repeatedly within the cloud and adding successive layers of clear ice. Graupel is soft and opaque, while true hail is hard and features layers of clear ice. Graupel can sometimes serve as the initial embryo for a hailstone, but the rimed pellet is distinct from the larger, multi-layered ice ball.