Monday, June 27, 2011

High-Altitude Tennis Balls

The ITF and USTA standards for high altitude play specify a ball that rebounds from 48 to 53 inches when dropped from a height of 100 inches at sea level.  The standard for normal balls, that is low-altitude balls, is for them to rebound from 53 to 58 inches when dropped from 100 inches at sea level.

Since the high-altitude balls have the same mass and volume, I assume that this means that pressurized high-altitude balls are simply pressurized less so that they rebound less.   The standard calls for a high-altitude ball to rebound roughly 91% of the low-altitude ball at sea level.  Since the air at a mile high has roughly 84% of the pressure of air at sea level, we can assume that the high-altitude ball will rebound higher at a mile above sea level than a normal ball will rebound at sea level. [See UPDATE below]

This is moronic. [It would be moronic if true. It's not. See UPDATE below.]

The only accommodation made in the rules for high-altitude play is for a ball that bounces roughly 10% higher than a standard ball does at low altitude.  The ball has the same nap, volume, and mass.  So not only does the ball come off the racquet 10% faster than a ball does at low altitude, it slows down over 16% less slowly.

Oh, and when our rocket ball hits the ground remember it will bounce 10% higher than what our low-altitude counterparts are dealing with.

No wonder we don't develop any good players up here.

Why in the world isn't the standard for the high-altitude ball, when tested at sea level, to rebound to a height that is more like 84% of the sea level ball?  That ball, when brought to Denver, for example, would at least bounce the same as a ball at sea level (the pressure differential between the inside and the outside of the ball being the same).

Too bad the bigger balls failed and disappeared.  Those would be an improvement, though they'd clog up ball machines and tubes.  Probably wouldn't fit through the bottom slats of hoppers, either.  Oh well.

We're screwed up here.

UPDATE. It turns out that there's more, or less, to rebound than the pressure difference between the inside and outside of the ball. Probably the rubber material plays a role, too. So, in testing some balls more recently, I've discovered that the high altitude ball, which rebounds between 48 and 53 inches when dropped from 100 inches at sea level, does in fact rebound between 53 and 58 inches when dropped from 100 inches at 5300 ft above sea level.

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