Of Humidors and Humidity

Thanks everyone for providing feedback on my recent article discussing the humidor and the leveling of the playing field in Denver this season. There are a couple of points I'd like to clarify, however.

In the article I said the following:

As mentioned in the previous column, it has been reported that the Rockies are now simply using balls that have been in the humidor longer. The effect of that practice is ostensibly to have the baseballs absorb more moisture, making them heavier with a lower coefficient of restitution (COR).

COR is a measure of the "bounciness" of the ball, or more precisely, it’s the ratio of the velocity of the ball when rebounding from a hard surface with its initial velocity. Major League Baseball specifies that the COR must be between .514 and .578, as measured by firing balls at 85 feet per second at a wall of ash.

As any kid can tell you after they leave their baseball in the yard all night in a rainstorm, combining these effects should serve to decrease the distance balls can be hit.

Several readers piggy-backed on this passage to ask whether a heavier ball would lead to arm injuries for Rockies pitchers or whether it would affect velocity or movement.

Well, as some of you know In the majors the ball has to weigh between 5.0 and 5.25 ounces and be between 9.0 and 9.25 inches in circumference. Without the humidor I would imagine that balls were weighing around 5 ounces with a circumference of 9 inches or even slightly less. With the humidor last year it was somewhere around 5.12 ounces and I don't know how many inches. One would assume with leaving the balls in there longer they are closer to 5.2 ounces and probably in the 9.15 inch range.

In retrospect the analogy I gave of leaving a ball out in a rainstorm wasn't great since it overstates the actual effect by a large amount. When you leave a ball out in the rain it probably increases the weight by 50% or more. A ball left in the humidor would feel and weigh very much like any baseball you might pick up so I couldn't conceive of it having any effect on injuries and I've never heard that mentioned in connection with the humidor before. After all, we're really talking about getting the ball to be more like those at other parks, not less.

One of the things pitchers have been saying in recent years with the humidor and more so this year is that the ball is easier to grip and the seams don't feel as tight which makes it easier to throw breaking balls. But of course the light air will always make it more difficult to get the same break at Coors than at sea level.

Other readers asked whether the Rockies could use the humidor to their advantage by, for example, soaking the ball a bit more with Brandon Webb in town or using a drier ball against a flyball pitcher when sinkerballer Jason Jennings is on the mound. That makes a certain amount of sense and I suppose we'll have to wait and see how opposing teams react.
Finally, I also mentioned in the article the following:

The following table shows the number of non-bunt ground balls hit at Coors Field over the past few years and the number and percentage that went for hits.

        GB  GBH    PCT
2003  2100  474   22.6%
2004  2037  468   23.0%
2005  2108  497   23.6%
2006   828  158   19.1%

So again it would appear that ground balls are not making it through the infield quite as frequently and in fact the difference between 2006 and the previous three seasons is statistically significant at the 10% level (that is, there is a greater than 90% chance that the difference we're seeing reflects an underlying reality). One might then postulate that a key contributing factor is the length of the infield grass.

Of course my assumption here was that fewer groundball hits could be directly attributed to longer grass in the Coors field infield. That's not necessarily the case, however, and fewer groundballs going for hits could also be laid at the feet of heavier baseballs. Since both factors appear to be different this year, it's probably impossible to determine which has had the greater effect.