Physical Properties of Hailstones Collected during the IBHS Field Project: 2012–17

Every year, on average, hailstorms cause billions of dollars of damage in the United States, including significant damage to buildings and cars. However, anticipating the risk of hail is challenging largely owing to gaps in the understanding of the damage potential of hailstorms. In order to better understand the damage potential that these storms have, the physical properties of hailstones must be understood. Improved knowledge of hailstone physical properties will also aid in understanding scattering properties of hailstones for radar-based detection and sizing algorithms, and can help improve hail hazard modeling.

To this extent, the Insurance Institute for Business and Home Safety (IBHS) has conducted a field experiment every spring since 2012 across the Great Plains. In this study, we analyze measurements from >3500 hailstones collected in the field from 2012-2017. Hailstone property measurements include mass tri-axial dimensions, and, recently, 3-D infrared laser scans of the hailstone shapes. First, we quantify the distribution of hailstone maximum, intermediate, and minimum axes, and the representative aspect ratios. We find that hailstones are ellipsoidal, with a modal minor-to-major axis ratio of 0.63 and a modal intermediate-to-major axis ratio of 0.85. Further, we investigate how these properties vary across the hail swath, and find that aspect ratios tend to decrease with time collected, implying that melting is ongoing. Some calculations produce unrealistic densities over that of solid ice (~0.9 g cm^-3), suggesting biases in the mass measurements and/or poor representation of hailstone shape by ellipsoids. Although 3-D scanning alleviates some of this concern by providing precise volume measurements, further improvement in mass measurements is needed. Our results provide a more realistic constraint on hailstone shapes and sizes across the hail swath, an as such may aid in radar detection and hailstorm modeling.