Following the powerful cold front over the weekend, early Sunday morning featured a different kind of precipitation we don’t usually see in this area: Graupel.
Snow forms within a cloud in what we call the ‘snow growth region’ where ice crystals can grow into snowflakes; they then fall to Earth as snow if the temperature is below freezing from the cloud to the ground.
Sleet actually starts out as snow higher in the atmosphere but encounters a warm layer above our head and turns into rain. Before it hits the ground it falls into a cold layer and refreezes into a ball of ice which hits the ground as: sleet.
Freezing rain can start out as snow or rain high in the atmosphere but sometimes there’s a warm layer wedged above us. Down at the surface the cold layer is below freezing and very, very shallow. If the cold layer of air at the surface went up high enough we would actually see sleet at the surface; but often, it’s too shallow and doesn’t have time to refreeze until it hits the surface so it freezes on contact with the surface as: freezing rain.
So what is “graupel”?
Graupel starts out as a snowflake. But as the snowflake falls down towards the surface it encounters a layer of air rich with humidity. The moisture is in liquid form but it’s supercooled below freezing. Supercooled water droplets can stay liquid as cold as -20 or even -40 degrees. The moisture starts to accrete on the snowflake; a process sometimes known as “riming”. It’s similar to a process that can cause hailstones but hail forms from convection or rising air within thunderstorms.
Rime is also an aviation hazard which can be deadly to pilots who fly into it. If a plane flies into a supercooled cloud without the right deicing equipment the ice can accumulate on the wings and tail causing loss of control in flight.
Early Sunday morning, we had snowflakes falling into the supercooled layer full of rich moisture around 10,000 feet above us. (700-500mb). As the snowflakes fell they accumulated the supercooled water droplets and fell to the ground as: graupel.