Observations of Hailstone Characteristics in Multi-cell and Supercell Thunderstorms

The general characteristics of hailstones and the climatological frequency of hail events have been well documented over the past several decades. There is much information regarding size, shape, and density of hailstones, yet little information exists regarding the hardness properties of individual hailstones. Throughout historical literature, hailstones are often referred to as “soft”, “hard”, “spongy”, or “slushy”, providing only a qualitative description. It is hypothesized that the hardness of any individual stone may increase the potential for damage to buildings through the net- force imparted to the impacted material. The Insurance Institute for Business & Home Safety (IBHS) developed a unique, rugged and portable, custom-designed testing device in an effort to measure the hardness property of hailstones in-situ. The device utilized a strain-gage based load cell to measure the compressive force required to fracture a hailstone, where it is understood that more compressive force would be required to fracture a harder stone. The methodology is similar to that used to determine the compressive strength of building materials such as concrete.

Observations were made in two separate field phases (2012, 2013) in order to collect compressive force measurements of natural hailstones. Each stone was also measured, weighed, and photographed with GPS data documented at the location of each measurement. In the two year history of the project, more than 920 individual hailstones have been cataloged, from 21 parent thunderstorms throughout the Central Plains. The complete dataset contains primarily stones with diameters 4 cm or less with 90% found below 3.5 cm in diameter. The largest stone measured was over 10 cm in diameter. Typical shapes where spheroidal with two nearly identical dimensions and a secondary smaller dimension (e.g. disk shapes). However, some conical and irregular shapes were documented as well and represent a small fraction of the dataset.

The compressive force measurements increased with diameter and mass. When scaled by cross-sectional area, however, the peak compressive stress values (>400 kPa) did not correspond to the largest diameter or most massive stones within individual datasets. Compressive stress values were also somewhat clustered by parent thunderstorm. It is noted that a small percentage (< 6%) of the hailstones measured were spongy and deformed when the compressive force test was performed such that they did not fracture.

Quality spatial resolution datasets were collected across the hail swath from several individual supercell thunderstorms. A bi-modal distribution was often observed in the maximum size with the largest peak located to the storm-relative, immediate left of the parent mesocyclone. The compressive stress distribution exhibited little relationship with regards to storm-relative location. These cases also offered an opportunity for comparison with dual-polarization WSR-88D observations.

Data collected during the 2012 and 2013 field phases have shown that the hardness property of hailstones can be quantified through in-situ measurements. These data have also have provided validation and guidance to improve laboratory hailstone impact testing. The development of representative laboratory hailstones will allow for detailed investigation of the damage potential of hard and soft hailstones and how they affect various materials upon impact. Despite the success of the two year study, compressive stress information on stones greater than 3.8 cm (1.5 in.) is relatively lacking and continued field measurements are needed.