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It's not the Heat, It's the Humidity

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The Colorado Rockies are once again making a September rally towards the playoffs. Pitchers usually cringe when they have to play at Coors Field due to its past reputation as a hitter’s ballpark. Many people concentrate on the effects of the mile high altitude, which can add 20 feet to a ball hit 400 feet at sea level, as the main factor influencing better offensive statistics. Perhaps one should also be concerned about the low humidity in Denver and its effect on the baseball?

If a baseball is stored in a high humidity environment for several weeks, it absorbs moisture and becomes heavier, reducing its coefficient of restitution - or bounce off the bat. If a ball is stored in a dry environment, it will dry out, lose some weight and have a higher coefficient of restitution - and gain bounce off the bat. Demver's humidity averages 35% during the baseball season - only Phoemix is lower (18%) among major league cities.

Since 2002, the Rockies have been trying to address the home run happy nature of their home park by keeping the baseball used during the game in a temperature and humidity controlled humidor.

Why? In an excellent interview with Nick Piecoro of the Arizona Republic, University of Illinois physics professor Alan Nathan and Washington State mechanical engineering professor Lloyd Smith stated that….

“(b)aseballs stored in a humidor at a higher relative humidity exhibited significantly decreased speeds off the bat. The humidor's biggest effect, the researchers say, is in how it changes a ball's "coefficient of restitution," i.e., the bounciness of the ball. Nathan said they found that a ball's coefficient of restitution decreases by 12 percent when it goes from 0 to 100 percent humidity. Nathan said that when baseballs that had been at 30 percent relative humidity were stored for two weeks in a humidor at 50 percent, the balls' speed off the bat decreased by about 2.5 mph, which they say comes out to a 14 foot drop-off on a typical long fly ball. "Our rough statistical estimate was that (14 feet) will decrease the number of home runs by about 25 percent," Nathan said. As it turns out, that's exactly the reduction seen in home runs at Denver's Coors Field since they began using a humidor in 2002.”

The temperature and moisture controlled room keeps the Rockies' baseballs in a state within their allowed “bounciness” parameters. But this accepted use of the humidor by a team - not the league - to deaden the baseballs brings about some potential risks. Who monitors the chain of custody of the baseballs once they leave the humidor? Is it possible for non-humidor influenced baseballs to be introduced into the game and given to the umpires when the Rockies are at bat? Accusations that this may be happening were made by San Francisco Giants broadcaster Jon Miller.

The chain of custody of the baseballs allows for the rumors of "gamesmanship" to flourish. The umpires do not rub up the baseballs that come out of the humidor, an assistant to the umpires (a Colorado Rockies employee) does this, and team ballboys are in charge of delivering the baseballs to the home plate umpire when asked. Nowhere in the chain is a neutral major league baseball employee involved. This must be addressed at the Winter meetings before other teams like the Arizona Diamondbacks considering adding a humidor to Chase Field in order to reduce home runs.

For examining the physics of baseball, I highly recommend the work of Professor Alan Nathan and some of the work done by Professor David Kagan.

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