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Richard J. Matson
Arlen W. Huggins


The kinematic behavior of hailstones falling in their natural environment near the surface was studied using stroboscopic photography in a mobile van. The experimental results permitted determination of the shape and dimensions, velocity, and in a few cases the rotation rate, of hailstones failing into the van. Hailstones were sampled in southeast Wyoming, southwest Nebraska and northeast Colorado. About 84% of the hailstones photographed were classified spheroidal, the remainder being roughly conical. Change in orientation of stones is observed in most photographs, though only ∼6% of the hailstones could be assigned a rotation rate. Velocity data were obtained for more than 600 hailstones in the diameter range 5-25 mm. It is shown that the vertical velocity component of hailstones near the surface (air density =9.93×10−4 g cm−3) can be predicted by the expression V T where De 0.50 (±m s−1), Deis the equivalent volume diameter of a spherical hailstone in centimeters. Fall-speed and hailstone oblateness are shown to be slightly negatively correlated. Hailstone drag co-efficients, as inferred from the measured vertical velocities, are found to be higher than the values most frequently quoted in the literature. A mean drag coefficient of 0.87 is found over a range of Reynolds numbers from 1032×104 with a tendency for the drag coefficient to decrease with increasing Reynolds number. Implications of the fallspeed and drag coefficient results on hailstone growth and hail instrument calibration are discussed. The time dependence of hailstone size is presented for two storms. A comparison of hailstone size versus arrival time indicates, at least for one of the storms, that the stones may have been size sorted.

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Alexis B. Long
Richard J. Matson
, and
Edwin L. Crow


This paper reports on work carried out in the National Hail Research Experiment (NHRE) on hailpad materials, on procedures for reducing hailpad data, and on hailpad calibration. 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 μm coating of white latex paint for protection from the deteriorating effects of sun-light. 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. It is recommended that measurements of the minor axis be categorized in size intervals no wider than 4 mm.

The NHRE laboratory technique for calibrating hailpads involves simulating a hailstone impact by dropping a steel sphere onto a 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 in an environment with known horizontal wind. The pad is tilted to preserve the stone impact angle found in nature. A second-degree polynomial in sphere diameter D satisfactorily describes the calibration relation between D and the dent minor axis. Application of the calibration relation developed for the particular case of no wind to hailpads which have been hit by hail falling in a wind leads to an overestimate of hailstone diameter of approximately 0.5–1% per meter per second of wind speed. This effect of the wind is about twice as large as that found by others.

A theoretical expression is developed that explicitly relates the minor axis of a dent produced by a sphere to the diameter of the sphere. Two controlling parameters in this expression are the impact kinetic energy of the sphere and a factor p, with dimensions of pressure, which quantitatively embodies the response of a pad to a sphere impact. The effect of variations in p on the sphere diameter derived from dent minor axis and information supplied by Dow Chemical USA on possible variability in the compressive modulus of Styrofoam between manufacturing batches together suggest that the user of hailpads obtains a one time all the foam he may need for his work.

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