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Five Questions About Hail, Answered

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How do you turn a simple family drive from dinner into a harrowing 1 minute as you approach your home? The answer is to add a severe thunderstorm with sustained golf and tennis ball-sized hailstones. This is exactly what my family encountered in suburban Atlanta, Georgia on July 21st, 2018. As a meteorologist, there was a level of excitement because I had never experienced hail like that. However, the concerned husband and father immediately kicked in because I wanted to get my family home safely and protect our car. I even shared some video of the tremendous "hail fall" on my Twitter page at this link. Once the "ice settled," many people in my community were buzzing about what happened, sending me pictures, or asking questions. I have synthesized some of those queries into 5 questions or a "hail 101."

Jason Shelton

How Does Hail Form? It is useful to start with this question. Hail is found in cumulonimbus clouds or thunderstorms. I cannot tell you how many times during winter sleet events someone has messaged me saying that it is "hailing." The National Oceanic and Atmospheric Administration National Severe Storms Laboratory website has a succinct explanation of hail formation:

Hailstones grow by colliding with supercooled water (exists as a liquid at temperatures below 32 deg F) drops. Supercooled water will freeze on contact with ice crystals, frozen raindrops, dust or some other nuclei. Thunderstorms that have a strong updraft (strong upward moving air) keep lifting the hailstones up to the top of the cloud where they encounter more supercooled water and continue to grow. The hail falls when the thunderstorm's updraft can no longer support the weight of the ice or the updraft weakens. The stronger the updraft the larger the hailstone can grow.

NWS Burlington

Why does the hailstone look like an onion when I cut it? As noted, the multiple trips in the updraft cause the hailstone to take on a layered appearance like an onion. You can basically tell how many times the storm traveled upward by the number of layers in the hailstone.

Marshall Shepherd

Why all of the different shapes and sizes? This was overwhelmingly the question that I received most frequently from friends after our jarring storm. If you are reading this in the Great Plains with a slight eye roll, please keep in mind that Atlanta, Georgia doesn't typically experience a hail storm of this magnitude. As the hailstones take their rollercoaster ride up and down in the cumulonimbus cloud, some melting and re-freeze happens as it encounters different temperature layers within the cloud. This causes the hailstone to take on various irregularities and form appendages. Logically, you might conclude that a more rounded hailstone like this one below on my deck (above) had a smoother ride with less of the melt-refreeze cycle.

Marshall Shepherd/Radarscope

What determines how big a hailstone can get? What was so fascinating about the Georgia storms yesterday is that we had sustained periods of golfball and tennis ball-sized hailstones. That just doesn't happen here that often and it tells me something about the strength of the updrafts of these storms on the radar (above). The largest hailstone recovered in the United States was a hailstone found in Vivian, South Dakota measuring 8 inches  in diameter. It also had a circumference of 18.62 inches and weighed 1 lb 15 oz. Imagine that falling from the sky and hitting your head or car. Exactly, so stay inside during hailstorms. As a rule of thumb, golf ball-sized hailstones would require at least a 64 mph updraft speed. The chart at this link provides some other useful comparisons.

John Trostel

Who gets the most hail in the United States? For the answer to this question, I turn back to the NOAA NSSL website:

Though Florida has the most thunderstorms, Nebraska, Colorado, and Wyoming usually have the most hail storms. The area where these three states meet – “hail alley,” averages seven to nine hail days per year. The reason why this area gets so much hail is that the freezing levels (the area of the atmosphere at 32 degrees or less) in the high plains are much closer to the ground than they are at sea level, where hail has plenty of time to melt before reaching the ground. Other parts of the world that have damaging hailstorms include China, Russia, India and northern Italy.

As usual, kudos to the National Weather Service who was on top of the storms in the area with appropriate warnings. Please take severe storm warnings as seriously as you would a tornado warning. If that warning is issued it means the potential for winds 58 mph or stronger and/or hail 1 inch in diameter or larger.

 

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