When a thunderstorm rolls off the mountains and drops icy pellets on our car, we assume this is hail.
That is not always the case.
There are different forms of frozen precipitation, which include: hail, graupel, sleet, and snow. In Colorado, our summer thunderstorms can be just as dramatic as our winter storms.
The big difference between these different types of precipitation is how they grow and form.
Hail forms in strong thunderstorms that contain an updraft and downdraft. The raindrops in these thunderstorms will freeze due to the colder temperatures. These small hailstones will move up and down within the storm cloud, gathering more raindrops and freezing. This will cause the hail to grow until it is too heavy for the updrafts. This is a good indicator of how strong these thunderstorms are, so the bigger the hail, the stronger the thunderstorm.
The other type of frozen precipitation that falls from thunderstorms is graupel. You may even see graupel falling on Pikes Peak Highway up to the top of the mountain. Grapuel starts as a snowflake and will catch super-cooled water droplets. This is usually smaller than hail and more fragile. Think of it like a mini snowball.
Sleet is the other type that gets confused with hail. Sleet will start off as a snowflake, just like graupel, but it hits a warmer layer in the atmosphere, making it melt again. Just before it hits the ground, it refreezes, making little ice pellets.
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