The Hailpad: Construction and Materials, Data Reduction, and Calibration

This paper examines in detail the materials from which hailpads are constructed, the procedures for reducing hailpad data, and the methods used to calibrate these instruments. A recommendation is made for a pad constructed of 2.5 cm thick type SI Styrofoam (manufactured by Dow Chemical USA) and sprayed with a 25-50 ��m coating of white latex paint for protection from the deteriorating effects of sunlight. This pad is believed somewhat superior to one made of type IB Styrofoam and covered with 25 ��m aluminum foil. A supporting stand or frame is recommended for the hailpad. Experience in handling the pad in the field, in having to read all the dents, and in storing pads suggests it should have a horizontal surface area of about 0.1 m��. The minor axis of a hailstone dent in a hailpad appears to be the best parameter to measure. It can be more closely related to the diameter of the hailstone making the dent than can the depth of a dent or its internal radius of curvature. It is also found that the length of the minor axis will be less subject than the major axis to undesirable variations caused by wind accompanying a hailfall. It is recommended that measurements of the minor axis be categorized in size intervals no wider than 2 mm. An analysis of the variations between different persons measuring the dents on a hailpad shows it to be necessary in some cases to have each pad "read" two or more times by different individuals. A training program for and close supervision of hailpad readers is essential. Calibration of the hailpad provides a relation between the minor axis of a dent in the pad and the dimensions of the stone producing the dent. The laboratory technique recommended for calibrating the hailpad is to simulate a hailstone impact by dropping a steel sphere onto the pad from a height such that the impact kinetic energy achieved by the sphere equals that of a hailstone of equal diameter falling onto the pad at terminal velocity. Because of assumptions made in the calibration technique about the sphericity, rigidity, density, drag coefficient, and fall direction of hailstones it is suggested that a direct calibration of the pad in the field be attempted. This would involve identifying with each dent on a hailpad some hailstone that has been collected and that can be measured, weighed, and otherwise studied. An expression is developed that explicitly relates the minor axis of a dent to the diameter of the stone producing the dent. Two controlling parameters in this expression are the impact kinetic energy of the stone and a factor p, with dimensions of pressure, which quantitatively embodies the response of a pad to a stone impact. The effect of variations in p on the stone diameter derived from dent minor axis and information supplied by Dow Chemical USA on possible variability in the mechanical properties of Styrofoam between manufacturing batches together suggest that any investigator of hail who is using hailpads obtain at one time all the foam he may need for his work. It is found that a second-degree polynomial adequately describes the calibration relation between dent minor axis D' and stone diameter D. For a pad made of 5 cm thick type IB Styrofoam and covered with 25 ��m aluminum foil the calibration for northeast Colorado, based on 17 different diameter spheres dropped 20 times each onto a set of pads, with dents read by 6 different individuals, is D' = 0.445D�� + 0.146D , for D and D' in centimeters. This relation is valid when there is no wind accompanying a hailfall. Use of this relation in the case of a wind will lead to an overestimate of hailstone diameter of approximately 1% per meter per second of wind speed.