‘From pancakes to ninja stars’: Josh has 130 hailstones in his collection

3 weeks ago 11

“Just like a snowflake, every hailstone is unique.”

So says Dr Joshua Solderholm, research scientist at the Bureau of Meteorology and honorary senior research fellow at the University of Queensland.

“You can get all sorts of shapes, from pancakes to ninja stars, and hailstones as large as lawn bowls in some cases,” Soderholm says. “You would never find one exactly the same as another.”

Bureau of Meteorology thunderstorm research scientist Dr Joshua Soderholm with a 3D replica of the world’s largest hailstone, which measured more than 20 centimetres in diameter.

Growing up in Brisbane, Soderholm has witnessed his fair share of thunderstorms spitting out ice rocks in spring and summer. His lifelong fascination with the weather phenomenon led to a career studying thunderstorms and the development of a “hail library”.

The project to catalogue hail started in January 2020 and now contains images of 130 different hailstones, mostly focusing on large hailstones over five centimetres in diameter. The largest in the collection, from Boronia Heights in Queensland in October 2021, measures 14.5 centimetres.

The research is crucial because hailstorms cause more insured losses than any other type of severe weather event in Australia, and they are expected to become more frequent with bigger hailstones in several Australian cities, as the climate warms over coming years.

The largest known hailstone in the world was recorded in Vivian, South Dakota in 2010. It was 20.3 centimetres in diameter and weighed 878.83 grams, despite having melted significantly between impact and measurement.

Over the past five years, technology has made it possible to scan hailstones and accurately measure their complex shapes.

While the hail library is digital, Soderholm also creates plastic replicas of some of the hailstones using a 3D printer. These are used as teaching tools and also to study the effects of their shape and size on the way they tumble and how fast they do so.

Hail is formed when tiny snow and ice particles in clouds called “graupel” come into contact with “supercooled water” – pure water that is still in liquid form below zero degrees.

“When you bring pure water below freezing, it doesn’t freeze instantaneously,” Soderholm says. “It remains liquid until it touches something, and then it’ll touch these little particles of snow and ice.”

In a heavenly version of the snowball effect, the ice particles grow as they collected the droplets, Soderholm says. The longer the ice balls spend in the cloud, and the stronger the wind is in the cloud to hold them up, the bigger they grow.

Strong thunderstorms produce the largest hailstones because they have an updraft – vertically moving air – that stops the hailstone falling. The warm, humid air mass at the surface and cold air aloft that creates the strongest thunderstorms is most common in late spring.

However, the science of how hailstones form is an area of active research. One school of thought, Soderholm says, is that the hailstones cycle up and down repeatedly to grow inside a thunderstorm, while the other theory is that they go up once, then get blown horizontally, then fall to the ground.

A thunderstorm will produce billions of hailstones, and each one will vary because of slight differences in its formation and the path it takes to the ground. Larger hailstones tend to be flatter and more elongated, while smaller hailstones tend to be more symmetrical, Soderholm says.

Hailstones also differ from each other on the inside. Scientists have long been able to cut them open with simple tools such as specialised saws or hot wires to reveal geode-type structures within. The difference now is that researchers can now take detailed photos and measurements of the patterns.

“We also essentially slice them down the middle and take out these thin sections, and we take photographs of them to understand how they grew,” Soderholm says.

“You’ll see the rings in them, just like geode rings or onion rings, and changes in the colour of the ice. From that, we can extrapolate what the temperature and the humidity was when it was growing in the storm.”

Get to the heart of what’s happening with climate change and the environment. Sign up for our fortnightly Environment newsletter.