The Direct Measurement of the Sizes, Shapes and Kinematics of Falling Hailstones

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Abstract
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 VT where (±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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