WRAL WeatherCenter Blog

Hail Salad...

Posted May 22, 2006

Not literally, of course, but Shannon Hill of Clayton sent in the photos below, first of an unusual variety of hail types that all collected on her deck in Clayton during a severe thunderstorm on Saturday, May 20th. She posed the question "why are some hailstones solid white, others are clear, and still others have white centers and clear outsides?" If you look closely you can see each of those varieties within the pile of stones in the image. Better yet, in the second photo is a hailstone that looks like a "lifesaver!" How did THAT happen? See below for some ideas...

First, it's worth noting that hail in general forms when strong updrafts and downdrafts in thunderstorms carry initially small ice crystals or rain droplets up and down through portions of the cloud having varying temperatures and liquid water contents, or suspend growing hailstones at various altitudes depending on the size of the stones and the varying speed of the updrafts. The basic idea is that once a small crystal or frozen droplet forms, it is carried through parts of the storm in which additional moisture freezes onto it, causing the hailstone to grow until it is either ejected from an updraft, or until it becomes too large for the updraft to keep it suspended (for a golf ball sized stone, this requires about a 50 mph updraft, whereas baseball sized stones need about 90-100 mph).

The terrific variety of stone types that can be formed is due to the permutations and combinations of temperature, droplet size and vertical motion that can be encountered by any given group of hailstones during their life cycle. Most of the water that freezes onto a growing hailstone, for example, is "supercooled," meaning it's temperature is below freezing but it remains in liquid form until it has a solid surface to freeze onto. Also, the stones may drop to levels in the storm where the water is a little above freezing, collect water on the outside (or have the stone melt somewhat on the outside) and then be carried to much colder levels where that outer layer freezes again.

When a hailstone spend most of its time at temperatures well below freezing and encounters many much smaller supercooled droplets that freeze almost instantly on contact, the very small droplets (called "rime" once they freeze into ice particles) will trap tiny pockets and bubbles of air within the stone. Just as with air bubbles trapped in water or bubbles in shaken soda water, the many interfaces between air and ice will scatter and reflect light rays in such a way as to create a milky or sometimes solid white appearance.

At the other extreme, a stone that spends most of its time in the presence of larger drops that are only slightly supercooled, or that partially melts and then moves into colder air above, may have the smaller droplets spread out more or less smoothly, then freeze more slowly, trapping little or no air and creating a clear, glazed appearance. Both of these types of stones appear to have fallen on the deck above.

One can finally imagine that a storm producing both of those types of stones, might occasionally generate a stone that is white by way or riming up to a certain size, that is then thrown into a different environment and subjected to "clear" ice buildup for a while. This leads to the stones (a few of which can be made out in the photo) that have a white center and a clear outer shell.

Another possibility is that these two processes alternate, which can produce stones that have an "onionlike" appearance if sliced open, due to alternating clear and rimed layers. These did not seem to occur much among the stones on Shannon's deck. Finally, some of the largest hail on record (the grapefruit size hail and so on) is often of an "aggregate" composition, in which a number of smaller hailstones have reached the proper temperature to clump together and freeze into a larger mass, again not the case in these photos.

Finally, what about the "life saver" above? I'm speculating some here, but my guess would be a hailstone initially of the "white rime" center and "clear ice" outer shell type, perhaps with the rime portion offset a bit from the center of the sphere. If a stone like this landed on the deck and began to absorb heat from the deck and the surrounding, both well above freezing, it is likely that as soon as the rime portion of the stone, which is less dense and contains tiny air channels, is exposed to air or comes in contact with the deck, it will melt more rapidly than the remaining, denser clear ice, at which point a ring is left behind. Thus, this stone was probably laying around for a few minutes before it was found, and happened to land in such a way as to encourage melting of the white portion first.

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