10 Meters Per Second

The Hail and the Tortoise

Your average hail usually ranges in radius from 0.1 to 0.8 cm. A 1 cm stone has terminal velocities around an ouchy 10 m/s, while to a run-for-your-life or new-car-denting 7-cm radius stone arrives at about 50 m/s (aka 110 mph).

We are indeed lucky that hail doesn't fall according to Mr. Stokes' Law! Stokes law would demand a 10^10 m/s terminal velocity for our 7 cm hailstone. It is generally thought that hail fall-velocities must occur in the upward direction to get the hail to grow-up to this stout, fast-falling size in the first place. If you get a chance, tell the pilot of your next commuter plane to avoid flying through hail-producing thunderstorms!

So where is the tortoise in all this hail? In an article you may have missed [(1894) Monthly Weather Review 22:215] we find a report of a hailstone that came to earth with a gopher turtle at its core. There were numerous thunderstorms around Vicksburg, Mississippi on the day of the turtle fall: May 11, 1984. The turtle was 6 by 8 inches and entirely encased in the hail stone. In 1970 a 1.67 lb. hailstone 17.5 inches across was found, kept frozen and sent to the National Center for Atmospheric Research (NCAR). I have little trouble with a turtle in a thunderstorm. Temperatures in the upper reaches of a large thunderstorm gets mighty low and a turtle could serve as a giant ice nucleus (the larger the radius of the ice collecting surface the faster it collects vapor from the air) and super-cooled water in such a cloud would freeze nicely on the gopher turtle. Hail-speed updrafts could carry the little fellow to great heights collecting layer after layer of ice. So how do you get a tortoise airborne? Well, a shrew will tumble along the ground with Beaufort wind force of only 4 and a barney-loving child will tumble along at Beaufort force 9. Somewhere in the middle you could get a turtle airborne. If there were a tornado associated with the thunderstorm you could make a hailstone out of Toto!


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