Often seen on days with many varieties of wintry precipitation, these soft, round pellets of snow may just appear on your windshield. It's a very odd looking form of winter precipitation. With their cloudy, circular appearance, these white pellets take on a form resembling styrofoam or "Dippin Dots" ice cream. It goes by its most common name snow Pellets, but to meteorologists, it is known as graupel.
Graupel is formed by a process called accretion. Accretion occurs within a cloud where ice particles, supercooled water droplets, and snow collide with each other as they get tossed by turbulent air. Turbulent air is the rising and sinking air due to temperature and pressure differences within a cloud. It can either be really strong, like within thunderstorms, or light, like within stratus or nimbus clouds that produce rain and snow. Once the ice and snow particles grow heavy by collision, they become too heavy to be suspended by the turbulent air and drop from the cloud as a form of winter precipitation. Unlike other types of precipitation, graupel sets itself apart in the accretion process. Within a cloud that produces graupel, temperatures need to be just below freezing (32 degrees Fahrenheit) with some portion being below 15 degrees Fahrenheit. This allows both supercooled water droplets and snow to exist in the cloud. Supercooled water droplets are droplets of water that are just below freezing but not cold enough to freeze completely. During the accretion process that forms graupel, the parent particle (snowflake) collides with supercooled water droplets as it gets tumbled by turbulence. When these supercooled droplets attach to the parent snowflake, the droplets immediately freeze on contact covering the snowflake with a thin layer of ice. This is a process called ice riming which gives graupel its circular shape.
Graupel tends to form in the same way that hail does. It is sometimes mistaken for hail, but they are two very different forms of precipitation. For example, they are both formed through the process of ice accretion and ice riming. But for hail, the parent particle isn't a snowflake; it's a particle of ice. Graupel gets its shape from a form of soft ice riming where the supercooled droplets don't freeze into hard ice but freeze as soft ice. This is because the core temperature of a snowflake is much warmer that the core temperature of an ice particle. Since the particle of ice that starts a hailstone is much colder, the supercooled droplets freeze on the hailstone creating a harder surface. Size also plays a large role if differentiating between hail and graupel. Graupel is relatively small in form. It grows from about two to five millimeters in diameter compared to a hailstone which can grow from pea size to softball size. The type of cloud and time of year is also a factor in hail and graupel differences. The reason why hail can grow so large is because it is made within thunderstorm clouds called cumulonimbus. These types of clouds form in the summer and are highly turbulent due to larger changes in pressure and temperature from the surface up to the cloud top. The stronger turbulence in this type of cloud is able to suspend heavier particles such as softball size hail.
Graupel is a very interesting type of wintry precipitation. It's very easy to spot and if you have already seen it, I'm sure it has raised your eyebrows. Next time it snows in your neck of the woods, keep a lookout for some graupel and share the knowledge of what you learned today with a friend. Up next: Sticking to the subject of precipitation, you will get the chance to learn about the meteorology term "Virga".
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