Observations of Hailstone Characteristics Utilizing a New Instrumentation Platform

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. In existing studies, hailstones are often referred to as “soft”, “hard”, 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. To study this property of natural hailstones, a unique, rugged and portable, custom-designed instrument was developed at the Insurance Institute for Business and Home Safety (IBHS) Research Center. The instrument package utilizes load cell technology to determine 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.

A pilot field study was conducted for two weeks beginning in late May, 2012. The primary objective of this pilot field study was to collect compressive force measurements of natural hailstones. In addition to the compressive force, each stone was measured, weighed, and photographically cataloged. GPS information was also used to document the location of each measurement. These data will provide a guide for artificial laboratory hailstone production for use in impact testing of roofing and other building materials, to determine their susceptibility to hail damage. The field project also provided a test-bed for the compressive force instrument, associated software, and the experimental plans for use in developing a larger-scale hail field program. This paper presents the findings from the pilot field project regarding the characteristics of measured hailstones, as well as the future research objectives through continued field measurements.