You must have heard of Snow, Freezing rain, Sleet or Hail but in all likelihood, haven’t heard much of graupel. So what on Earth is Graupel? Graupel, also known as snow pellets, soft hail or rimed snow is a kind of precipitation.
How exactly does it form?
Graupel forms when extremely cold air is present at the cloud level. This leads to the formation of supercooled water (water that remains in its liquid state way below the freezing point). These droplets come in contact with snowflakes and begin to freeze. The snowflakes now have a rim – hence the name “rimed snow”. With more and more droplets freezing and attaching to snowflakes, the original shape of the snowflake is gone, resulting in the formation of Graupel.
See the series of images below (from Wikimedia Commons) on how a Graupel forms:
How is it different from Sleet, Freezing Rain and Hail?
Graupel is generally white and cloudy, unlike Sleet. The pellets are fragile – they may disintegrate if you touch them or if they make contact with the surface. Sleet, on the other hand, is a lot sturdier and bounces when it hits the surface. It typically forms when the snow melts due to a warm surface and then refreezes before hitting the surface. Freezing rain takes a similar route – the only difference being it passes through a thicker layer of warm air & freezes only after hitting the surface.
While Sleet, Freezing rain and Graupel occur during Wintery conditions, Hail occurs mostly during warmer conditions. It forms when updraft of a thunderstorm carries the water droplets into the colder part of the atmosphere, resulting in water freezing and falling as Hail. It is a chunk of ice that falls during thunderstorms. The larger the chunk, the greater the damage to vehicles, roofs, crops, and humans.
So the next time someone asks you about what a Graupel is, tell them it’s not a dish or an animal, but a kind of precipitation that you know all about!
Human and the Climate 