Dating Hail: Determining When Hail Impact Damage Occurred

This paper was published at the Forensic Engineering 9th Congress, held November 4 - 7, 2022 in Denver, Colorado. The published version of this paper is available for purchase and download at: https://ascelibrary.org/doi/book/10.1061/forensicengineering2022 Dating Hail: Investigation and Determination of the Date of Occurrence of Hail Impact Damage Stewart M. Verhulst, M.S., P.E., RRC, M. ASCE1 J. Daniel Bosley, M.S., P.E., M. ASCE2 Justin Donaldson, P.E., M. ASCE3 V.P. and Executive Technical Director, Nelson Forensics, 9701 Brodie Lane, Suite 201, Austin, Texas 78748; email: [email protected]; phone: 877-850-8765 Regional Manager, Nelson Forensics, 9701 Brodie Lane, Suite 201, Austin, Texas 78748; email: [email protected]; phone: 877-850-8765 Technical Director and Discovery Lab Director, Nelson Forensics, 2740 Dallas Parkway, Suite 220, Plano, Texas 75093; email: [email protected]; phone: 877-850-8765 ABSTRACT Hail impact can damage roof coverings, building exteriors, and exterior equipment and appurtenances. Following a hail event, evaluations to determine the extent of hail impact distress are common. However, many buildings experience multiple hail events over their service life and, in some cases, it is necessary to delineate the extent of damage from different hail events. Weather data resources are useful to determine the dates of reported hail occurrences and the reported or estimated hail sizes in the vicinity of a given site. However, data collected during a site investigation provides valuable, site-specific information, and such an investigation is necessary to evaluate a building for damage related to hail impact. Determination of damages caused by a specific hail event (or events) requires even more extensive site investigation. This paper discusses data collection and site investigation methodologies utilized to determine the date of occurrence for hail impact damage. Case studies are presented to illustrate useful investigative techniques and to provide examples of data collection used to determine the date of hail occurrence. INTRODUCTION The scope of most hail damage investigations is to determine if hail caused damage to a building or building system. However, in some cases it is important to determine the date of occurrence for hail damage or for potentially damaging hail. Many buildings and roofing systems experience multiple hailstorms during their service lives. Determining the date on which hail damage occurred, or delineating between hail damages related to different hail storm events, can be important for building owners and can help insurance companies determine the proper coverage period for insurance claims related to hail. Damages from hail impact result from the transfer of energy from the hailstone to the impacted material or surface. Factors such as the size, density, and shape of a hailstone affect the damage potential. Determining the date on which hail damage occurred often requires the investigator to consider multiple data sources, including weather data and data collected from an on-site investigation. DATA COLLECTION Data useful to an investigator to aid in the determining the date on which hail damage occurred at a given site includes weather data and site investigation data, potentially in combination with historical imagery of the site. Weather Data. Weather data related to hail includes free public data available from governmental entities and paid-for data provided by third-party weather analysis services or professionals. Common sources of the free governmental data are the storm event databases maintained by the National Oceanic and Atmospheric Administration (NOAA) through their National Centers for Environmental Information (NCEI) and Storm Prediction Center (SPC). These databases provide reports of hail size and the approximate respective location (latitude/longitude). The reports may also include a narrative description of the storm event or report, which can include more precise information about where the hail report was made. While, in the authors’ experience, this data is valuable, these are hail reports by weather “spotters” and the potential for inaccuracies in hail size or location should be considered. It is also rare for a spotter report to be located at the specific site being investigated for hail damage. Third-party forensic weather services generally utilize human observation data combined with radar data and proprietary algorithms to provide an estimate of hail size at a given location during a hail event. This weather data can be useful, but is an estimate of hail rather than a confirmation of the actual occurrence of hail at the site. Additionally, third-party weather reports employ varying algorithms with varying inputs into the algorithms, such that different results may be obtained from different vendors. Thus, determining the date on which hail damage occurred at a site is not as simple as looking at the weather reporting and estimate data. Rather, site-specific investigation is important to determine the characteristics of hail that actually occurred at a given site, and this site investigation data is combined with the available weather data to determine the most likely date of hail damage. Site Investigation Data. Data collected directly from a given site is the most reliable data when considering the occurrence, sizing, characteristics, and damage potential for hail. The appearance of burnish marks (also commonly called “splash” or “spatter” marks) and indentations on various surfaces typically provide the most useful clues for determining the timing of hail occurrences at a site.