The Influence of Environmental Variables on Hailstone Material Properties

Hailstorms across the United States cause more than $10 billion annually in property losses with major events exceeding $1 billion in losses. The increase in hailstorm losses has outpaced advances in detection, forecasting, and damage mitigation. The ability to forecast with any detail the maximum hail size, concentration, and the true damage potential remains a challenge. Until recently, there were limited quantitative data on hailstone strength. Hailstones were qualitatively referred to as “hard”, “soft”, or “slushy”. The strength of hail influences the amount of damage a given hailstone can produce, at a given impact kinetic energy. The strength of hail is likely function of several factors: the temperatures at which the ice grains were produced, residence time within the hail growth zone, and any imperfections that can lead to crack propagation when under compression (i.e. the interface between layered structures from alternating growth regimes, expansion cracks, large bubble features, etc.)

The Insurance Institute for Business & Home Safety has performed compressive strength tests and detailed physical measurements on over 2,000 hailstones during their annual field research program from 2012-2016. This study examines thermodynamic and kinematic variables and their influence on hailstone mass-diameter relationships, axis ratios, and compressive strength. Twenty thunderstorms cases, the majority of which are supercells that contained sufficient sample sizes of hailstone strength from several locations across the hail swath were analyzed. Proximity soundings from the Rapid Refresh (RAP) archived model analysis fields were reconstructed to represent the storm inflow environment. In general, kinematic variables (i.e. shear vector, storm-relative flow) exhibited a greater correlation to mean and maximum hailstone strength than thermodynamic proxies for updraft strength.