Observations of Hailstone Sizes and Shapes from the IBHS Hail Measurement Program: 2012–2014

There exists a wealth of discussion throughout the historical literature regarding the general characteristics of hailstones (e.g. diameter, mass, density, and growth processes). However detailed in-situ measurements of hailstones are confined to a select few individual cases. With increasing property losses as a result of severe hail events, the need for improved hail parameterizations in convection resolving numerical weather prediction models, and the dual-polarization upgrade to the WSR-88D network, there has been a renewed interest in measurements of hailstone characteristics. These observations are useful in appropriately tuning polarimetric radar-based hail detection algorithms, improving hail parameterizations within numerical models, and providing accurate aerodynamic drag and terminal fall velocity approximations.

The Insurance Institute for Business & Home Safety (IBHS) began a field measurement program in 2012 focused on evaluating the compressive strength of hailstones (Brown et al. 2012). Through this program, detailed physical measurements of the major and minor axes and the mass were made for over 2500 hailstones from nearly 30 different parent thunderstorms in the Great Plains of the United States over the past 3 years. In addition, a compressive strength test was performed to evaluate the relative strength of hail. This study leverages this relatively large observational dataset to examine hailstone sizes and shapes to provide a comparison with historical literature. The distribution of measured hailstones ranged from a minimum diameter of 3 mm to a maximum of 106 mm. A subset of over 500 hailstones collected during the 2014 field phase featured measurements of the major and minor diameters as well as a third intermediate dimension. This information is used to further characterize the shapes of measured hailstones beyond the axis ratio.