杏吧原创

A team of storm chasers spent years tracking down terrible weather across the US to establish this simple fact: hailstones aren鈥檛 round. They have more complex shapes than previously thought and this knowledge could help meteorologists understand their formation and better predict the dynamics of hailstorms on weather radar.

鈥淚t鈥檚 along the same lines as 鈥榥o two snowflakes are alike鈥� 鈥� we can say the same thing about hailstones,鈥� says Matthew Kumjian at Pennsylvania State University.

He and his colleagues carried out a hailstone survey over the course of six years. The team literally chased hailstorms across the Great Plains of the US, positioning themselves in the likely paths of storms and using weather radar data to rush to the scene of each hail fall before it melted.

The researchers collected and measured more than 3600 hailstones from 42 storms. They determined the shape of the larger hailstones with a 3D laser scanner. Hailstones are generally assumed to be spherical, but the team found that the typical stone is technically a shape known as a triaxial ellipsoid or scalene oblate spheroid.

鈥淭he hailstone is only about half as thick as its maximum dimension, and only about 80 per cent of the maximum dimension across in the third axis,鈥� says Kumjian. He likens the shape to a flattened American football or rugby ball.

The region of the US stretching from Montana in the north to Texas in the south is known as Hail Alley for its powerful and frequent hailstorms, and the majority of stones the team measured there were between 10 and 30 millimetres long. The biggest were more than 12 centimetres across, with the shape becoming less regular at larger sizes.

鈥淭he larger hailstones tend to develop protuberances or lobes, which can give them very irregular or spiky shapes,鈥� says Kumjian. The shape is determined by the growth process in which ice accumulates around a nucleus. Their irregularity suggests that hailstones don鈥檛 tumble symmetrically during growth as previously assumed, but more randomly.

Weather radar may overestimate the size of the stones in hailstorms with irregular shapes, and common weather models have been programmed assuming hailstones are simple spheres of ice. The new finding could help meteorologists better estimate the force and path of hailstorms. The shape of hailstones also affects the speed at which they fall, so a better understanding could help predict the damage they might do.