The Oddities of Hail Observations: Just How Big is a Golf Ball?

Large hail can produce significant damage and losses to property and agriculture, over nearly all parts of the world. Despite this potential for impact, records of hail occurrences and its dimensions are limited by necessary presence of an in situobserver. However, even in the case that an observer is present on the ground, the observation made can vary remarkably depending on who makes it, and how they make their measurement of a stones size. This can make the application of hail records to a variety of problems challenging (e.g.understanding the influence of climate variability, verifying radar and satellite observations), and can lead to unexpected results if due care is not taken to filter the data appropriately.

In this presentation, we will explore the limitations and quirks of United States hail records, including the temporal trend, approaches to smoothing, and sources contributing to observations. Utilizing the 1955-present record of observations of hail size in the United States, the record is found to be heavily quantized toward fixed diameter reference objects and influenced by spatial and temporal biases similar to those noted for occurrence, with better and longer records available for urban regions compared to rural areas. Comparison is made to three of the other relatively reliable hail observational datasets for Europe, Canada and Australia, to ascertain the influence of using only reference objects to record hail dimensions compared to other approaches. Regional and local influences on hail reporting are identified, stemming from verification procedures and contributions from local officials. The change in the definition of severe hail size from 0.75 in (1.9 cm) to 1.00 in (2.5 cm) in 2010 has a particularly clear signature in the report statistics, and will likely remain as a breakpoint in the temporal record for future climatologies. The contribution of storm chasers and source of report factors beyond population to the hail dataset is also explored, and the difficulty in removing these changes discussed.  Based on these findings, recommendations for improving the existing hail record moving forward will be discussed.