Every now and then, we get precipitation that looks like snowy hail. What is it?
This precipitation is actually called graupel. It forms when water droplets freeze onto snowflakes in a process called riming.
It requires below freezing temperatures at cloud level, along with moisture, to create snowflakes. Then, warm water droplets are brought upwards into these snowflakes, freezing onto them.
In short, it takes cold air producing snowflakes to meet up with warmer droplets, which freeze onto the flakes.
The best time of year for this to happen is in the early spring or late fall, when we can have cold air above but some warmth lower down.