Graupel Isn't Snow, Nor Sleet, Nor Hail, So What the Heck Is It?

But what happens when Mother Nature serves up a precipitation combo of supercooled water droplets we don't know what to call? Enter: graupel, which The Washington Post calls "the wintry precipitation you've never heard of" and Merriam-Webster calls "pearl barley."

Graupel is actually an interesting mix of snow crystals and ice, no matter what Merriam-Webster calls it. Graupel should not be confused with sleet, which is sturdier and more frozen; graupel occurs when a snow pellet falls and is encapsulated by ice. Sounds like hail, right? Well, it isn't hail. Not exactly.

Hail is formed from raindrops that are lifted upward into freezing air by wind drafts. Frozen hailstones can start as small as single raindrops that fall, but as the process continues, they grow in size and dimension as more and more rain freezes to the hailstone. Once the frozen raindrop becomes too heavy for the updrafts moving in the freezing upper atmosphere, soft hail is created and the hailstone falls earthward. Hail usually occurs during severe weather.

Graupel, on the other hand, can be — but doesn't have to be — associated with severe weather. While graupel can be seen in weather that creates supercooled water droplets, graupel doesn't necessarily mean nasty atmospheric conditions. In order to form, all graupel needs is cold, winter-like temperatures.

Graupel begins as individual snowflakes formed in the upper atmosphere. The snowflakes then fall through a layer of supercooled liquid droplets, which causes the raindrops to "rime," or instantly freeze onto the snow crystals at temperatures of 32 degrees Fahrenheit (0 degrees Celsius) or lower. The end result is graupel: Graupel is tiny, white pellets that resemble small hail but which, unlike hail, remain soft and crushable.

Graupel, in general, has the appearance of riced cauliflower and this gives us a clue as to the origin of the precipitation's name, "graupel." The word graupel first appeared in the Germanic languages and is derived from "graupe," which is the word for pearl barley. The word graupel's first known association with grainy falling snowflakes, pellets or soft hail, was in 1889 when a weather report used it to describe rimed snow crystal pellets. In modern use, the word "graupel" has become a synonym for "soft hail."

The most important criteria of rime is that it is comprised of supercooled rain that freezes and attaches to an exposed surface during winter storms. When that exposed surface is a snowflake, this rime creates graupel, but that's not the only condition in which raindrops freeze around an object at its core, creating ice pellets. This type of freezing process can encapsulate many objects, including trees and branches, resulting in a winter wonderland full of glittery surfaces.

Depending on how these frozen surfaces are coated, however, it may not be rime that's responsible at all — but the hoar frost that forms. Hoar frost has a similar look to rime, but skips the supercooled droplets stage of formation. It goes straight to crystallizing into ice pellets, forming fragile frozen droplets on just about anything it encounters during low-to-the-ground freezing temperatures: grasses, leaves, branches and even the occasional unfortunate spider.