What makes hail clear, not cloudy?
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On May 15, we posted a story on hail that included a photo and video of hailstones that fell from a severe thunderstorm the previous day, on May 14. The photo featured in our post showed a hailstone that was fairly clear and irregularly shaped compared to the other spherical hailstones. I also heard a report of a clear and flat-shaped hailstone that fell from the same storm a few miles south of Manassas. I decided to research how these clear and irregularly shaped hailstones might have formed.
Keep reading for more on how hailstones are formed and why some turn out clear while others grow to be cloudy or white softball-sized bombs.
I already knew that most hailstones move up and down within a thunderstorm's updraft and grow layers of ice as they circulate through the storm -- these are the "textbook" hailstones that usually fall as round, cloudy balls of ice. But how does a thunderstorm create a clear chunk of ice that is not round? I found a report by Aurthur Few of Rice University that explained the growth of hailstones. I later exchanged emails with Dr. Few, and after more study I can now explain how clear and irregularly shaped hailstones can fall from the sky.
First, I'll explain how clear ice grows on a hailstone versus cloudy or white ice. Clear ice is formed when the hailstone makes contact with super-cooled water droplets within the thunderstorm cloud, which freeze onto the surface of the hailstone. This is termed "wet growth." Cloudy- or white-ice growth occurs when a hailstone collides or aggregates with ice particles or snowflakes in the colder region of the cloud. This is termed "collision coalescence and aggregation." Most hailstones move up and down multiple times through the thunderstorm and form alternating layers of clear ice and cloudy ice. The overall appearance of the hailstone will often be cloudy or white and the shape will be fairly round.
So, how does an irregular-shaped (non-round) chunk of clear ice form within a thunderstorm? One theory is that a hailstone can become removed from the thunderstorm's main updraft winds and can stay suspended on the edge of the updraft in a region of wet growth for an extended period of time. Think of how eddies form in rivers along the edge of strong currents of water. If a hailstone stays suspended in an eddy of wind on the edge of the updraft, in a region of wet growth, it can accumulate a large layer of clear ice that may not be symmetrical in shape. If this hailstone then falls out of the eddy and the updraft, it will hit the ground as a chunk of clear ice. That's what might have happened with the unusual hailstones from the storm on May 14.
OK, if that's how clear-ice hailstones form, then how do the massive, cloudy or white softball-sized hailstones form within a thunderstorm? As I mentioned before, most hailstones circulate in a thunderstorm's updraft many times, moving up and down through the storm cloud growing layers of ice. If the thunderstorm's updraft becomes particularly strong, hailstones can grow large and still remain suspended in the cloud without falling to the ground. Occasionally, the larger hailstones can aggregate and freeze together to form massive balls of ice the size of a softball, or larger. Once the large hailstones aggregate, the spherical shape of a single hailstone is lost and the hailstone becomes a conglomerate of many other hailstones. These aggregate hailstones can grow into huge ice bombs that fall from the sky causing major damage to cars, homes or even people.
I have one other hailstone type and related story worth mentioning here. Eight years ago, I was talking to a resident of La Plata, Md., who had just experienced the major thunderstorm of April 28, 2002, that produced the F4 tornado that devastated the town. He described an utterly amazing hailstone that crashed through the front windshield of his car and rested on the car's dashboard. He said it resembled a big zucchini or squash. It was about 10 inches long and 4 inches wide. He also said it was fairly "dirty," which may have been caused by dirt or debris sucked up into the thunderstorm by the tornado.
Anyway, for a hailstone to grow that large, what probably happened was that two or three baseball-sized hailstones froze together to form a super-aggregate hailstone. Many other baseball- and softball-sized hailstones fell from that storm. A beautiful photo of that storm is included below.
The next time hail falls from a thunderstorm, take some time to analyze the hailstones. You may be able to see the layers and get an idea of how it was formed. And, just like with snowflakes, no two hailstones are exactly identical.
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