Process

Graupel

Ice crystals fall through a cloud of super-cooled droplets minute cloud droplets that have fallen below freezing temperature but have not frozen. The ice crystal plows into the super-cooled droplets and they immediately freeze to it. This process forms graupel, or snow pellets, as the droplet continue to accumulate on the crystal. The pellets bounce when they hit the ground.

Graupel Snow

Graupel 

Ice pellets

Out ahead of the passage of a warm front, falling snow may partially melt and refreeze into a frozen rain drop before it reaches the ground. These ice pellets are called sleet. Because it is easily seen and does not accumulate ice, it is not as dangerous as freezing rain.

We were severely pelted with granular ice pellets

W underground ice pellets

Dry ice pellets

Dry ice can be used to flash freeze food but does not alter the taste

Rime

Rime is a milky white accumulation of super-cooled cloud or fog droplets that freeze when they strike an object that has a temperature of 32 °F (0 °C), the freezing point of water. The process is called riming when super-cooled cloud droplets attach to ice crystals in the formation of graupel. Rime ice can pose a hazard to an airliner when it forms on a wing as an aircraft flies through a cloud of super-cooled droplets.

Rime Ice (Snow Feathers)

The ground just the trees covered in rime ice.

Rime ice grows when droplets rapidly freeze upon striking an aircraft.