Baseball’s Newest Pitcher-Friendly Park

After years of false starts and uncertainty, we’re finally here — the Diamondbacks are going to start storing their baseballs in a Chase Field humidor. The idea itself isn’t particularly new, and of course the Rockies beat the Diamondbacks to the punch by more than a decade and a half. But, well, this isn’t a race. Arizona took its time to get to this point, and Chase does happen to be the second-most hitter-friendly ballpark around. It’s behind only…Coors Field, which the humidor didn’t render neutral, but the humidor there is still serving a purpose.

When the Diamondbacks have talked about this installation, they haven’t so much indicated a desire to drive offense down. Rather, the goal seems to be to make the baseballs more “grippy.” Pitchers have complained about the balls in Arizona being slick, and that has a lot to do with the low relative humidity of the climate. Now, you can’t exactly help pitchers without hurting hitters as a consequence. But there’s also just more going on. A humidor wouldn’t change only the surface of the baseballs. Moving forward, Chase Field might no longer be a hitter-friendly environment.

The function of a humidor is fairly simple. Dry baseballs stored in a humidor absorb water. So they get heavier, and the water also reduces each ball’s coefficient of restitution. That essentially reduces exit velocities. You might be able to tell from the context that I’m about to link to an Alan Nathan article. Here it is! That’s Nathan, talking about a potential humidor in Chase Field, in 2017. The article remains perfectly relevant, and you should give it a read. I’ll come back to it in a bit.

The Coors Field humidor stores baseballs at 70 degrees, and 50% relative humidity. The Chase Field humidor will be similar, if not identical. Let’s assume the procedures will be effectively the same. Same temperatures, same relative humidities, same storage times. What’s the best way to understand what’ll happen in Arizona? By looking at what happened in Colorado. I can’t say anything about the grip. I can just present you with numbers.

So here’s a plot. I think it should be fairly simple to understand. I looked at the five years pre-Coors Field humidor, from 1997 – 2001. Then I looked at the five years immediately post-humidor, from 2002 – 2006. I looked at all Coors Field numbers over road-game numbers, with the results expressed in percentages. This doesn’t include only Rockies players; it also includes their opponents.

Before the humidor, over five years, Coors Field increased runs per game by 53%. It increased home runs — per plate appearance — by 45%. It increased BABIP by 16%, while reducing strikeouts by 15%. Then after the humidor, over five years, Coors Field increased runs per game by 30%. It increased home runs by 23%, and BABIP by 11%, while reducing strikeouts by 12%. In other words, Coors Field remained extremely hitter-friendly. There’s not really a reasonable alternative. But offense was effectively reduced. Compared to a regular ballpark, Coors baseball is still bananas, but at least it’s more recognizable as baseball. There’s only so much you can do when you’re that high in the sky.

So now let’s do something very simple. Maybe *too* simple. Let’s apply all the same exact humidor effects to the 2013 – 2017 Chase Field numbers. For example, between five-year windows, the Coors Field humidor reduced the runs-per-game effect by 15%. It also reduced the home-run effect by 15%. I know there are ways to do this more scientifically, but, look at that, I’ve already gone and inserted the image.

The asterisk up there indicates that this is only a guess. Over the past five years, though, Chase Field has increased runs per game by 12%, compared to Diamondbacks games on the road. Were the humidor to do the same thing, then Chase Field would reduce runs per game by 4%. It would go from increasing home runs by 9% to decreasing home runs by 8%. You can look at all the different bars, keeping in mind that a bar at 100% would be exactly neutral. I’d advise taking the triples-rate bars with some grains of salt; triples are infrequent, so the numbers are noisy.

Baseball tends to be more complicated than you think it’ll be. I don’t actually know what the humidor will do in Arizona. There’s just the one ballpark to compare it to. Yet that one ballpark saw its offense reduced rather significantly. And there’s something else, too, which Nathan touched on in his piece. The average relative humidity in Denver is about 30%. But, as Nathan writes, in Phoenix it’s more like 20%. So if the humidors are set the same, then, in Chase Field, the difference would be even more dramatic, because the baseballs would be absorbing more water. They’d gain more weight. They’d be less bouncy. It’s nothing you’d notice if you just looked at two baseballs sitting in front of you, but the numbers could be in line for some wild swings. The humidor could do *more* in Arizona, and while that could certainly help with pitcher grips, it would take a big chunk out of exit velocities.

I’m not going to pretend like the Diamondbacks haven’t examined this. Everything I looked at, I calculated in 20 minutes. And I’m sure they’ve talked to a number of specialists, while I just read one article by one scientist. I can’t imagine the Diamondbacks are just flying blind, here. It’s just that the consequences might be enormous, as far as park factors go. Chase Field has long been kind to hitters. It might leave that behind entirely, with the balls no longer coming off the bat with such force. If you’re familiar with our own FanGraphs park factors, then, in 2001, Coors Field was way out there, with a run effect of 120. It had a home-run effect of 125. Chase Field, meanwhile, has had a run effect of 105, and a home-run effect of 104. Coors Field was pleading for something to be done. Chase Field, less so. But, pitchers want better grips. What can you do? Someone has to lose.

Based on our most recent park factors, the NL West has had three of baseball’s four most pitcher-friendly environments, and the two most hitter-friendly environments. One of those two is now going to change, and it could be poised for the change to be dramatic. As if Rockies hitters didn’t already have it hard enough on the road. Chase Field isn’t going to be what it was.