Meet the New Chase Field

Prior to the season, they installed a humidor in Arizona, much like they did several years ago in Colorado. The underlying ideas were similar: rein in the offense, which was increasingly out of control. Chase Field was never as hitter-friendly as Coors, and nothing will be as hitter-friendly as old Coors until there’s a big-league team in Mexico City, but there’s value in trying to make the game more neutral. The perception was that play in Chase was too lopsided. Those in control wanted to balance things out.

I wrote about the possible consequences of the humidor in February. Even better than that, Alan Nathan wrote about the possible consequences of the humidor the previous April. The potential existed for a dramatic effect. While part of the stated goal was to just make the baseballs more grippy — thereby benefiting the pitchers — the humidor would also decrease each baseball’s coefficient of restitution. Put another way, in theory, the ball wouldn’t come off the bat quite so fast. Now that we’re three-quarters of the way into the season, it’s possible to take a look at how things have actually gone. If you’re in a rush, let me give you the conclusion right here: Chase has turned into what was expected. It does seem to have become more neutral, indeed. Not in so much of a rush? Below, I’ll present the basic evidence.

You don’t even need to look that far, if you don’t want to. For example, last year, in Diamondbacks home games, there were 215 homers. Meanwhile, in Diamondbacks road games, there were 176 homers. This year, in Diamondbacks home games, there have been 130 homers. Meanwhile, in Diamondbacks road games, there have been 134 homers. Look at that! Lopsided home-run factor erased.

That’s also far too simple. Numbers like that can lead you in a direction, but they can’t really prove much of anything. Let’s take things to another level, now. At FanGraphs, we have splits information stretching back to 2002. So let’s check out almost two decades of Chase Field year-to-year park factors. The plot below includes wOBA, K-BB%, and average runs per game. For each measure, I’ve taken Diamondbacks home-game results, and divided by Diamondbacks road-game results. For anyone confused, this includes both the Diamondbacks and their opponents.

A neutral park factor would come out to 1.0. What you can see here is a bunch of variation. That’s just the nature of the thing, when you’re dealing with single-season samples. But, this includes information for 17 years. This year’s wOBA park factor is the second-lowest. This year’s runs-per-game park factor is the third-lowest. Interestingly, there hasn’t been positive movement in K-BB% — that’s where you’d think better grippiness might show up. No such gain is apparent. That doesn’t mean the baseballs themselves don’t feel a little more tacky. That doesn’t mean pitchers in Arizona don’t feel more comfortable pitching these days. There’s just nothing to see in terms of walks or strikeouts. We’ll see how that plays out over a greater number of seasons.

Now, from 2002 – 2017, the Chase Field wOBA factor was 1.06. So far this year, it’s 1.02. And, from 2002 – 2017, the Chase Field runs-per-game factor was 1.13. So far this year, it’s 1.05. That’s all neat enough, and it does point us somewhere, but of course, this is the Statcast era. So let’s dig into that information, with the help of Baseball Savant.

As mentioned, one of the theorized effects of the humidor was that it would reduce overall exit velocity. Here’s how all the ballparks played between 2015 – 2017. This shows the difference between overall average home exit velocity and overall average road exit velocity.

Chase Field shows up all the way to the left — on average, balls in Arizona were being hit harder by more than one full mile per hour. One thing we can’t easily account for is calibration error. It’s possible that, in some ballparks, exit velocities were being over- or under-reported. But at least the Arizona figure makes sense, given the area’s low average humidity. The baseballs would’ve been lighter and bouncier. Hence the humidor. Here’s the same plot now, but showing only data for 2018:

All of a sudden, the ballpark effect is gone. Balls in Arizona are being hit as hard as they’re being hit not in Arizona. This is exactly what was predicted to happen, with the Chase Field factor dropping from first to 17th. The balls are heavier and less bouncy, and while they’re not too heavy or too dead, the low area humidity has successfully been negated. Chase Field has apparently become exit velocity-neutral. That’s not the kind of thing you’d be likely to notice on any given ball in play, but for fun, here’s a home run by David Peralta from 2017:

Here’s a remarkably similar batted ball from Peralta in 2018, but with a slightly lower exit velocity:

I’m not saying the humidor is what turned that from a home run into an in-play fly ball, but those are the kinds of batted balls that would be affected the most. When a hitter hits the ball square, it’ll still get out. But those just-barely home runs from before might now be likely to turn into different outcomes. It’s something that would feel most obvious to long-time Diamondbacks hitters in batting practice. They know the ball doesn’t scoot exactly like it used to. They can see the increased number of batted balls that end up on the warning track.

Just what does a subtle difference in exit velocity mean? We can turn now to Statcast’s expected wOBA metric. Here’s another plot for 2015 – 2017, showing the difference between home xwOBA and road xwOBA:

Chase Field again, all the way to the left. Here’s the plot for 2018 alone:

…and there’s Chase Field in 14th place. But we can go one level deeper. Instead of just looking at overall xwOBA, why don’t we look at the same plots, but for just xwOBA on contact? The plot for 2015 – 2017:

Chase Field, left-most bar. A three-year xwOBA-on-contact boost of almost 30 points. And now for our last image, the information for only this season:

A difference of one point in the other direction. Chase Field here ranks in 19th place. It’s crucial to understand that we still do need more data, because we have only four and a half months, and this stuff can take a while to find its level. We might not know the real, exact truth about the new Chase Field until 2020 or 2021 or whenever. But based on the early evidence, at least, the humidor has functioned just about as it was intended to. I don’t know if the balls themselves are now sufficiently grippy, but the batted balls aren’t coming off so hot. Through this point of the season, Chase Field seems to be more or less batted ball-neutral. It’s a significant change for what had been one of baseball’s most hitter-friendly environments.

For outside observer purposes, I don’t know how much this really matters. In the short-term, because our FanGraphs park factors take a while to update, know that Diamondbacks hitters are being underrated by wRC+, and Diamondbacks pitchers are being overrated by ERA-. That’s not too terribly important. Maybe the bigger takeaway is that, where there are two functioning humidors, there might soon be more. There might soon be 30. I don’t know what the argument would be against storing balls under league-wide consistent conditions. The ballparks are never going to all be the same. The baseballs, however, probably should be. We can talk about that some other time.