What will a humidor do to AT&T Park?

I’d been meaning to write about the topic of a AT&T Park installing a humidor ever since I saw this tweet:

But I got distracted by other things and put the idea on the to do list for later. Then I saw these plays over the weekend in Arizona...

Gorkys Hernandez’s pinch hit in Friday’s game:

Joe Panik’s sacrifice fly in the top of the 3rd on Saturday night:

Belt’s flyout in the top of the 3rd ended with Duane Kuiper remarking, “Humidor.”:

... and was reminded that Arizona had installed a humidor before the start of the season and that I had been meaning to write about the topic of AT&T Park installing a humidor.

A series in Coors Field is the perfect time to revisit the topic anyway, since the installation of a humidor hasn’t stopped the place from remaining the Giants very own Overlook Hotel.

There’s incontrovertible evidence that the humidor has worked as intended: home runs are down, and the park plays a lot less like an introverted auteur’s interpretation of a Stephen King novel. As MLB’s Mike Petriello puts it:

Although Coors is still something of a hitter’s haven, it’s easy to forget how much impact the humidor had. According to research at FanGraphs, in the five years before the humidor, Coors games averaged 53 percent more runs than the league average. In the five years after, that fell to 30 percent above average. It’s still a lot, but less.

Let’s jump back to Chase Field for just a sec. We definitely witnessed the effect of a fully operational humidor this weekend, but to what extent? You might be thinking, “Aww, Bryan, you big dumb idiot, it’s because the Giants are just a bunch of warning track power guys”, to which I will respond: the evidence suggests that is strongly not the case!

Alan Nathan, an actual physicist, wrote on FanGraphs last year:

I am very comfortable saying that, with the humidor running at 50 percent and 70⁰F, there will 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.

The Diamondbacks and their opponents combined to score on average 11.2 runs/game in 2016 and 9.9 runs/game in 2017. Through 42 games this season (more than half the home schedule), teams are combining for 8.07 runs/game. A really simple explanation for this is that the Diamomdbacks pitching has gotten better since the 2016 squad that went 69-93.

That’s a fair point. But going back to 2001 — when Randy Johnson, Curt Schilling and Luis Gonzalez at the height of the PED era led them to a championship — the Chase Field runs per game average was 9.76. And if we jump ahead to the 65-97 2011 team, the runs per game average is still 9.79. So that 8.07 is a noticeable difference.

The average exit velocity at Chase has dropped by 2 mph. Arizona hit 122 home runs in 81 games last season (1.50/game). They’ve hit 43 in 42 home games so far in 2018. That projects out to about 83 Chase Field home runs, a 32% reduction versus last year. You might say, “Bryan, you ignorant garbage troll, they let J.D. Martinez walk. He hit 16 home runs at Chase Field last year.” Well, you may have a point. The humidor alone might not account for such a huge drop in dingers, but it’s still definitely one of the reasons.

I don’t have all the Statcast data the way Petriello does, but that’s an important factor here. The basic premise of Nathan’s paper is that the humidor will make the balls heavier, which lowers their exit velocity upon contact, which reduces the distance they’ll travel.

What does that mean for the Giants?

The average humidity in San Francisco is anywhere from 70-75%, so a humidor would actually make the balls at AT&T Park lighter. That wouldn’t overcome the bay winds and wet air, but it would still change the fly ball dynamics significantly.

Before Alan Nathan wrote about Chase, he wrote about the humidor at Coors, concluding,

... given that the typical humidity in most MLB parks is greater than 50 percent, storing the baseballs at 50 percent in every park would result in an increase in home-run production by an amount that we could predict using the techniques employed here.

That 50 percent humidity doesn’t come out of nowhere.

Major League Baseball guidelines say that baseballs should be stored at 70 degrees with about 50 percent humidity. That isn’t possible in the dry desert air, particularly during the hot summer months, without the help of a humidor.

And it’s not possible to dry out your balls in San Francisco without the help of a humidor. Will this new equipment be enough to make AT&T Park a more desirable place for hitters? Will it give the Giants their first 30-home run hitter since Barry Bonds in 2004?

The principle of air resistance tells us that heavier objects experience less resistance. However, a heavier ball means a slower exit velocity and a momentum more easily slowed by the air resistance force. Therefore, a lighter ball, despite being more susceptible to air resistance force, will travel farther simply because the higher exit velocity provides greater momentum. I think. I’m a liberal arts major.

AT&T Park’s dimensions probably aids certain types of home runs (especially for lefties) and suppresses others (center field and right center field, especially), and so there might always be an unshakable What the hell, AT&T Park? factor, but let’s say that a lighter ball combined with the natural China Basin humidity and air resistance means the average exit velocity increases only 0.5 mph. A 2 mph reduction led to a 30% drop in home runs for the Diamondbacks. Does that mean a 0.5 mph increase will lead to a 7.5% increase in home runs for the Giants?

Probably not, but let’s apply that percentage to recent home run totals:

2016 HR @ AT&T: 55 * .075 = 4 (59)
2017 HR @ AT&T: 48 * .075 = 4 (52)
2018 HR @ AT&T: 42 * .075 = 3 (45)

So, we’re in the 0.3-0.4 win range with just this dumb, context-free adjustment. Of course, visiting teams would gain the same benefit, but I, for one, welcome our new humidor overlord.