No humidor for Diamondbacks in 2017

A humidor was supposed to be installed at Chase Field this summer but Major League Baseball has told the Arizona Diamondbacks to wait until next season.

The Arizona Diamondbacks‘ plan to install a humidor at Chase Field this season has been put on hold by Major League Baseball. According to Steve Gilbert, MLB told Diamondbacks GM Mike Hazen to delay the humidor until next year. It makes sense to wait until the offseason. The Arizona players and their opponents have spent half the season playing in typical Arizona conditions and the team is having a great year. To change the way the ball feels and reacts mid-season would be a bad move.

It would be especially bad because of the success of the Diamondbacks this season. They weren’t expected to be contenders to win their division, but they’ve surprised many in baseball with their stellar first-half play. At the halfway point, they are just 2.5 games out of first place in the NL West and are the top team in the Wild Card race. Their success is backed by the second-best run-differential in the NL (+108, behind only the Dodgers).

When the Diamondbacks announced they would add a humidor mid-season, the team’s CEO, Derrick Hall, said they had discussed the idea for five or six years and talked to many former pitchers. The grip on the baseball was a main topic. The dry, thin air in Arizona, especially in the summer, makes the ball more difficult to grip.

One of the Diamondbacks’ current pitchers, Jorge De La Rosa, played for the Rockies previously. In Coors Field in Colorado, he pitched with balls that were kept in a humidor. About the possibility of a humidor at Chase Field, he said, “You’re really going to feel the difference when you pitch. The grip is going to be better.”

Along with a better grip for the pitchers, the humidor was expected by physicists to decrease offense. Notably, Alan Nathan wrote an article six years ago for Baseball Prospectus that analyzed the effect of the humidor at Coors Field. He also looked at Chase Field at that time because there were rumors even back then of a humidor coming to Arizona. Nathan predicted a 37 percent reduction in home runs at Chase Field, which is even greater than the actual reduction in home runs at Coors Field produced by the humidor.

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When the announcement about the humidor coming to Arizona was made this spring, Nathan updated his methods and wrote an article at The Hardball Times explaining how the humidor would affect offense at Chase Field. Storing baseballs in a more humid environment creates two main differences. The humidity decreases the coefficient of restitution (COR) and increases the weight. These changes lead to a reduction in exit speed. When a player hits a ball with a high exit speed and at the right angle, home runs result. Anything that lowers the exit speed will decrease home runs.

Nathan’s new prediction about a humidor’s effect at Chase Field involved using publicly available data from Statcast for the last two seasons. This included exit speeds, launch angles and the outcome of the bat on ball. If you are interested in his methods, please read the article. Nathan’s final prediction is illuminating:

"So what do we conclude? I am very comfortable saying that, with the humidor running at 50 percent and 700F, there will be a reduction in home run production at Chase by 25-50 percent. While it would be nice to come up with a more precise prediction, we should not lose sight of the principal takeaway that the installation of a humidor will reduce the number of home runs substantially."

So the humidor is likely to reduce the number of home runs substantially. Would this help or hurt the Diamondbacks? Consider how the team’s hitters and pitchers have done in the home run category so far this year.

104 HR (13^th)—Diamondbacks hitters, overall (98 wRC+)

63 HR (5^th)—Diamondbacks hitters, at home (112 wRC+).

41 HR (26^th)—Diamondbacks hitters, on the road (81 wRC+).

The hitters have been one of the top teams in hitting home runs at home and their offense has been 12 percent better than average when league and ballpark effects are taken into account. On the road, they are in the bottom five in home runs and have been 19 percent worse than league average on offense.

79 HR (30^th)—Diamondbacks pitchers, overall (3.40 ERA)

49 HR (16^th)—Diamondbacks pitchers, at home (3.50 ERA).

30 HR (30^th)—Diamondbacks pitchers, on the road (3.29 ERA).

The pitchers are in the middle of the pack in home runs allowed at home and the best in baseball in home runs allowed on the road, yet their ERA at home and away are very close.

Introducing a humidor to Chase Field would reduce home runs by the Diamondbacks hitters and home runs allowed by the Diamondbacks pitchers. The pitchers have already done a good job of keeping runs off the board, but will the hitters be as effective with a reduction in home runs? Applying Nathan’s low-end estimate of a 25 percent reduction in home runs to the first half of the season would move the Diamondbacks from 14^th in MLB in home runs to tied for 24^th. Their pitchers have allowed the fewest home runs overall, so they don’t appear to need the help in reducing home runs allowed.

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Considering that the Diamondbacks are 30-12 at home this year, they should probably leave well enough alone. Their 30 home wins are the second-most in baseball and they have a real shot at making the playoffs for the first time since 2011. Given their overall success, and in particular their success at home, it doesn’t make much sense to change the way the ball feels and reacts mid-season.