The midwestern US has been getting ripped up by a massive supercell storm this week, which will continue traveling east throughout the weekend. So far, the weather system has left behind quite an impression in the form of strangely shaped hailstones—some bloated and see-through, some covered in misshapen lumps, some that look like frozen sea anemones, and others more reminiscent of marble mushroom caps. The weird menagerie took Twitter by storm (ha!):
Like a lot of people, we were struck by how much bizarrely shaped hail could fall out of the sky just from one kind of storm. And naturally, we turned to science to tell us how this kind of hailstone formation was possible.
“Just like no two snowflakes are alike, no two hailstones are alike,” says Conrad Ziegler of the National Severe Storms Laboratory in Norman, Oklahoma. Hail formation is a complex process: Tons of variables can dictate the size, shape, color, and density of the icy stones. But Ziegler had some ideas as to why there were some recurring patterns in hail from supercells that don’t show up in other storms.
Hail forms in the clouds under sub-zero conditions, from 0 to -40 degrees Celsius. A tiny seed of ice begins to collect droplets that almost instantly freeze on contact. The more time hailstones spend in the clouds, the more likely they are to collect more droplets and grow. Supercells are special because they exhibit stronger updrafts than normal storms, keeping hail in the air longer and allowing stones to grow bulkier before they fall out of the air.
You’ll notice some of the ice in the hailstones pictured above look clear, while others have a very thick, opaque white color. Ziegler explains that hailstones can form in two different phases that sometimes go back and forth—a dry phase and a wet phase.
The dry phase happens at colder temperatures: When droplets freeze faster on the surface of a stone, they also freeze more compactly, resulting in denser, more opaque ice. As temperatures increase, however, hailstones release heat that’s pushed to the surface, meaning droplets don’t freeze as fast. Air can get captured in between the stone’s surface and the droplet, creating ice that’s more transparent and less dense.
But what’s with the weird shapes of some of these falling balls of ice? Some stones form are shaped like lumps of dirt clumped together, others have icy tentacles stretching out, and others resemble jellyfish heads sent down from the sky like some biblical tale of punishment.
That all has to do with the way hailstones are spun around in the air during formation. “Every stone takes a different path through the cloud,” says Ziegler. Often, storm winds will push stones around and give them a spin—they begin to rotate on an axis that further affects how droplets on the surface will collect and freeze. Irregular shapes or bumps might form on one side or another that build up on one another, creating lumps.
Winds can also change very fast and force abrupt shifts in spin and temperature, creating tentacles or other weird perturbations. And if air is hitting a stone in only a single direction, it might shave off much of the surface of one side until it’s smooth and opaque with round, softer edges, like the ‘shrooms.
So the takeaway is that supercells will generally create bigger hailstones, but any other morphological changes are determined by highly variable temperatures, wind speeds, cloud densities, and other meteorological factors. This supercell storm is making its way to the East coast over the weekend, and chances are others in the Northeast and mid-Atlantic will come across the same kinds of strange hail their neighbors farther West were hit with. Hopefully by then, someone comes up with a catchier hashtag than just #hail.
