Graupel is a form of precipitation that falls as soft, opaque ice pellets, often misidentified within a wintery mix. This soft, frozen droplet is sometimes referred to as “soft hail” or “snow pellets.” Graupel requires specific atmospheric conditions for its creation, distinguishing it from other frozen forms like sleet or true hail.
Defining Graupel: Appearance and Characteristics
Graupel pellets are identifiable by their physical traits. They possess a characteristic white or cloudy appearance, contrasting sharply with the translucent nature of frozen raindrops. These particles often look like tiny, round beads of foam or small, squishy snowballs.
The size of graupel typically ranges between 2 and 5 millimeters in diameter. The texture is soft and fragile, causing the pellets to disintegrate easily upon impact or when touched. This fragility is why “soft hail” is a common alternative name.
The Formation Process
The creation of graupel begins with an ordinary snow crystal high in the atmosphere. As the crystal descends, it encounters a layer of supercooled water droplets. Supercooled droplets are liquid water that remains below the freezing point but has not yet turned to ice.
The snow crystal acts as a nucleus. When supercooled water droplets collide with its surface, they instantly freeze upon contact, a process known as accretion. The resulting coating is called rime.
Riming continues as the particle falls, adding layers of ice that completely obscure the original crystalline structure. This transforms the particle into an opaque, ball-like pellet. Atmospheric conditions require sufficient liquid water content and temperatures cold enough to maintain the supercooled state, often involving active vertical air movement. This riming mechanism creates the soft, crushable texture characteristic of graupel.
Distinguishing Graupel from Other Ice Precipitation
Graupel is frequently confused with three other forms of frozen precipitation: sleet, hail, and snow, but each has a distinct formation and appearance. Sleet, also known as ice pellets, is clear and hard, forming through a different vertical temperature profile. Sleet originates when a snowflake melts completely into rain as it passes through a warm layer, then refreezes into a small, solid ice pellet in a layer of freezing air near the ground.
Hail, in contrast, is much harder, denser, and generally larger, with stones needing to be at least 5 millimeters in diameter. Hailstones form in severe thunderstorms through an intense cycle of being carried upward by strong updrafts and falling repeatedly, which creates visible layers of ice. Graupel, however, is soft, formed by a single process of riming, and lacks the layered internal structure of hail.
Snow crystals are the initial building blocks of graupel, but they maintain their distinct, often elaborate, six-sided crystalline shape. The characteristic difference is that a true snowflake retains its delicate structure, whereas the graupel particle is a snowflake whose intricate arms have been entirely encased and hidden by the thick, opaque layer of rime ice.