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Posted at 11:15 AM ET, 06/11/2010

What makes hail clear, not cloudy?

By Kevin Ambrose

* Hot & humid coming back; t-storms? CWG's Full Forecast *

Baseball- and softball-sized hail that fell near Hughesville, Md., during the storm of April 28, 2002. Hailstones of this size are usually aggregates of smaller, spherical hailstones that have frozen together to create a large mass of ice. Source: The book "Washington Weather."

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.

Hailstones that fell with a thunderstorm on May 14. Five of the hailstones in this photo are spherical but the one at the bottom-center contains a large amount of clear ice and is irregularly shaped. Source: CWG visitor brucebogtrotter.

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.

A thin section of a hailstone that contains a large portion of clear ice around a spherical hailstone nucleus. Source: Authur Few (

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.

A cross-section of a hailstone that shows alternating layers of clear ice and cloudy ice.

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.

Aggregate hailstones approach the size of a CD-ROM in Hughesville, Md., after the storm of April 28, 2002. Source: The book "Washington Weather"

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.

A very large aggregate hailstone composed of many smaller hailstones fused together. At one time, this hailstone may have been two baseball-sized aggregates frozen together. This hailstone has experienced significant melting, which has altered its appearance and size. Source: NWS, Sioux Falls, S.D.

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 supercell thunderstorm of April 28, 2002, that produced the La Plata tornado and large hail. This storm had a tremendous updraft that helped to keep large hail aloft in the sky. Note the overshooting top of the storm, which is indicative of extremely strong updrafts. This photo was taken from a jet after the storm had moved east of La Plata. Source: Steve Maciejewski

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.

Walking barefoot on hailstones in Burke, Va., after the storm of April 23, 1999. The hail produced widespread roof and car damage in central Fairfax County. These are spherical hailstones that range in size from marbles to golfballs. Source: John DiCarlo

Spherical hailstones that fell on Washington, D.C., May 3, 1948. Source: The book "Washington Weather."

Spherical hailstones cover the ground near Poolesville, Md., on July 10, 1975. Snowplows were used to clear roads that had up to five inches of hail accumulation. Source: The book "Washington Weather."

The light helps show the structure of an aggregate hailstone that is about the size of a baseball. The hailstone is composed of smaller stones frozen together. Source: NOAA.

By Kevin Ambrose  | June 11, 2010; 11:15 AM ET
Categories:  Photography  
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Great post with interesting photos. .
I would be very, very unhappy to see ice bombs like that coming down. But if I ever do I will look them over carefully, unless I'm on the way to the emergency room with a concussion.

Posted by: FIREDRAGON47 | June 11, 2010 11:41 AM | Report abuse

Terrifying weather! I hope never to see that sort of bombardment in person.

Posted by: Langway4Eva | June 11, 2010 12:43 PM | Report abuse

Kevin, very nice! I agree with FIREDRAGON47 :-) and man oh man those are impressive stones. jeeze!!
Langway4Eva, yeah I hope I don't see anything like it either--a serious threat to life & property!

Posted by: Camden-CapitalWeatherGang | June 11, 2010 1:25 PM | Report abuse

very interesting! I always want to get a better look at hail, but it always melts so fast! (thankfully because it's so small where i'm at). Those photos were great!

Posted by: pgcoresident | June 11, 2010 4:31 PM | Report abuse

Wonder who the guy is in the 1948 photo...he bears some resemblance to Doug Hill who was probably about that age in 1948.

Posted by: Bombo47jea | June 11, 2010 5:40 PM | Report abuse

Great article - thanks for covering these amazing natural events. -Doug

Posted by: | June 12, 2010 6:00 PM | Report abuse

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