Hailstone Size Inferred from Dents in Cold-Rolled Aluminum Sheet

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  • 1 National Physical Research Laboratory, C.S.I.R., Pretoria, South Africa
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Abstract

The reflecting properties of microscopic corrugations on the surface of cold-rolled aluminum sheet used in hailpads led to a new technique for determining the size of hailstones. Hailstone models of various shapes and sizes were used to produce dents in the sheet, for normal and oblique impacts. It was found that ϕ, the diameter (or equivalent spherical diameter) of hailstones, was related to d, the diameter of the characteristic shadow inside dents, such that ϕ = 0.6d + 3 (for 4 ≤ ϕ ≤ 40 mm). The standard deviation of ϕ (for normal impacts) was found to be ±8% for spheres of ϕ = 6 mm, ±5% for spheres of ϕ ≥ 11 mm and between approximately ±6 and ±17% for nonspherical objects. Also, the value of d for nonspherical objects that impacted at angles of up to 50° was only about 10% larger than the corresponding value of d if the impact had been normal. An estimate of the kinetic energy of an impacting hailstone may therefore be made from d. This method proved to be particularly suitable for recording actual hailfalls and results thus obtained are discussed.

Abstract

The reflecting properties of microscopic corrugations on the surface of cold-rolled aluminum sheet used in hailpads led to a new technique for determining the size of hailstones. Hailstone models of various shapes and sizes were used to produce dents in the sheet, for normal and oblique impacts. It was found that ϕ, the diameter (or equivalent spherical diameter) of hailstones, was related to d, the diameter of the characteristic shadow inside dents, such that ϕ = 0.6d + 3 (for 4 ≤ ϕ ≤ 40 mm). The standard deviation of ϕ (for normal impacts) was found to be ±8% for spheres of ϕ = 6 mm, ±5% for spheres of ϕ ≥ 11 mm and between approximately ±6 and ±17% for nonspherical objects. Also, the value of d for nonspherical objects that impacted at angles of up to 50° was only about 10% larger than the corresponding value of d if the impact had been normal. An estimate of the kinetic energy of an impacting hailstone may therefore be made from d. This method proved to be particularly suitable for recording actual hailfalls and results thus obtained are discussed.

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