Q: Every so often we hear in the news of abnormally large hailstones that kill or injure people or damage property. Hailstones that fell in Texas recently were described as 鈥渙range-sized鈥�.
The size of hailstones would seem to depend in some way on temperature, humidity, cloud thickness, initial height, air viscosity, acceleration due to gravity, charge and turbulence.
Do any other factors apply? And what is the maximum size a hailstone might attain before impact, what is the likely terminal velocity of such a monster, and which are the geographical locations that should be avoided?
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A: Storms producing large hailstones require an input of very moist, warm air and an unstable atmosphere. The critical feature is that there should be a significant change in wind velocity with increasing height (shear). This shear has to be such that small hail fired out of the top of the storm eventually falls into the strong inflow at the base of the storm to be swept round again, perhaps several times. With each passage through the storm, the size of the stone increases. The strength of the updraught needed to keep the stone aloft also increases.
This type of storm usually ensures that the first hailstones from an approaching storm are the largest, so that the only warning is the sound made by the curtain of precipitation as it comes closer.
The largest hailstones I am aware of 鈥渨ere as large as coconuts and good-sized mangoes鈥� falling near Bellary, India, on 28 March 1867. Some weighed 鈥渟ix seers, or three pounds鈥�. (Meteorological Magazine, December 1993).
Reports of large hailstones (for example 鈥渢wo-thirds of a cubic yard in size鈥� in the same article) should be treated with care as they may well be the result of several stones fusing together after they have hit the ground. Giant stones should leave craters and holes for the survivors of the storm to measure.