1 Observations of Hailstone Characteristics in Multicell and Supercell Thunderstorms Ian M. Giammanco Insurance Institute for Business & Home Safety Tanya M. Brown Insurance Institute for Business & Home Safety 1. INTRODUCTION Severe hail events are responsible for nearly $1 billion dollars in annual insured property losses in the United States (Changnon et al. 2009). Despite a general negative trend in population growth across the Great Plains of the United States, an increasing trend in hail-related losses has been observed over the past decade (MunichRe 2013). Despite a decreasing trend in number of hail days per year shown by Changnon et al. (2009) the increase in property losses, has generated a renewed interest in understanding how the characteristics of hail may influence damage associated with existing building stock and new construction. Characteristics of hailstones, such as their size, mass, embryo type, and growth processes have been well documented in the historical literature (Browning 1977; Macklin 1977; Foote and Knight 1977; Knight and Knight 2001). There has been little effort to account for hailstone characteristics other than diameter and mass within engineering applications. It is often assumed that damage states will scale with impact kinetic energy. This is reflected in standardized test methodologies utilizing a steel ball to represent a hailstone (UL 2218). Other test practices use a sphere of clear ice (FM 4473). It is acknowledged that the degradation of common building materials (e.g. asphalt shingles, vinyl siding, wood shake etc.) with time and environmental exposure likely reduces the ability of a given material to withstand hail impacts. *Corresponding Author Address: Ian M. Giammanco, PhD. Insurance Institute for Business & Home Safety Research Center, 5335 Richburg Rd, Richburg SC 29729. igiammanco@ibhs.org Koontz and White (2012) have provided evidence of degraded shingle performance of aged products when subjected to ice sphere impacts. Questions remain regarding the representativeness of laboratory test methodologies to effectively represent the properties of natural hailstones. Differences in observed damage patterns warrant examination of existing methodologies and the exploration of new techniques to adequately represent the characteristics of natural hailstones. In 2011, the Insurance Institute for Business & Home Safety (IBHS) began a comprehensive research program focused on understanding the damage potential of hail, improving laboratory test methodologies, developing damage functions for a variety of new and aged building components, and evaluating construction practices which may help mitigate losses. A key component to this program has been a field phase in which teams collect in-situ measurements of the characteristics of hail (e.g. primary dimension, secondary dimension, mass, peak compressive force at fracture, photographic catalog; Brown et al. 2012). In addition to improving engineering approaches to hail impact testing, the program seeks to understand if the synoptic and mesoscale environments as well as convective mode play a role in the type of hailstone which may be produced (e.g. soft, hard, slushy). The environmental conditions conducive for hail production are well documented (List 1985; Rogers and Yau 1989; Thompson et al. 2012);. Examining conditions which yield a harder type of hailstone may help asses which conditions lead to a more damaging hail event.