Hail develops in most mid-latitude thunderstorms, often melting before reaching the ground. Severe hail is officially defined as being .75 inch or more in diameter -- or bigger than a dime. [A dime is slightly less than .75 inch across.] But even most hail which is technically "severe" measures an inch or less across -- too small to cause personal injury or serious property damage, except to crops (which can be destroyed even by very small, wind-driven hail).
On the other hand, extremely severe hailstones, like the ones below and at left, can total cars, ruin roofs, break windows, kill animals and seriously hurt or kill humans. The costliest thunderstorms in history have been supercells producing very large hail over metropolitan areas -- such as Fort Worth, TX, in 1995 and Sydney, Australia, in 1999. A hailstone of about this size killed a man in northern Ft. Worth, in the same thunderstorm which produced the downtown tornado in March 2000.
Such giant hail falls from supercell thunderstorms. The rotation of a supercell's updraft dynamically strengthens the upward wind motion a great deal, helping to hold larger ice particles aloft for a longer time. The longer a hailstone can remain suspended in a growth region -- where supercooled liquid raindrops hit it and freeze on impact -- the bigger the hailstone will grow.
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