D-backs' humidor already making impact

Home runs per game decrease from 2.6 last April to 1.9

April 30th, 2018

"I'm just fortunate to get the barrel on the ball," D-backs outfielder A.J. Pollock said after hitting a home run at Chase Field earlier this month, "and defeat that humidor."
Ah, the famed humidor. After considering it for some time, the D-backs announced in February that they would proceed with installing a humidor at Chase Field for the 2018 season, becoming the second ballpark to do so after Colorado's Coors Field, where it was installed in 2002. 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.
Chase Field has also long been one of baseball's most famously hitter-friendly parks, generally understood to be behind only Coors. What everyone really wanted to know was, what would this do to offense? Would there be fewer home runs? Could Chase actually become pitcher-friendly?
It's too early to say with total certainty. We haven't made it to the hot summer months yet. But the D-backs have played about 15 percent of their home schedule, and based on the first month of the season, the initial returns seem to be: Yes. According to the very early numbers, the difference is already noticeable.

Though the science can be complicated, the idea is simple. The dry desert air in Arizona dries the ball out, making it slicker to grip and easier to hit hard. A humidor helps the ball retain more water, making it slightly heavier and softer. It doesn't make the ball go less far so much as it does reduce the exit velocity, which then in turn would reduce distances.
We have two things working in our favor as we attempt to break down the impact of the humidor. First, as a southwestern stadium with a roof, Chase Field has been relatively inoculated from the terrible weather that has affected much of baseball this month. Second, we already had a baseline expectation. We have results from 15 years of a humidor in Colorado, and the baseball community is fortunate enough to have a Ph.D. in physics, Dr. Alan Nathan of the University of Illinois, applying his considerable talents to the question.
"I am very comfortable saying that, with the humidor running at 50 percent and 70 degrees," wrote Nathan last year, "there will [be] a reduction in home-run production at Chase by 25-50 percent." Furthermore, he noted that in 2015-16, D-backs hitters away from home had exit velocities 2 mph lower than they did in Arizona, and that a purely physics-based calculation suggested exit velocities might drop by as much as 4 mph.
So that's our baseline, based on science and observation: Home runs down by 25-50 percent, and exit velocity down by approximately 2-4 mph. What's actually happened?
If we compare the first 12 games of this year in Arizona (both teams) to the first 12 games of the previous two seasons there, we see a clear change. We see almost exactly what was predicted.
Exit velocity has dropped by 2.7 mph, which fits perfectly. Exit velocity only on fly balls and liners has dropped by a full 3 mph. Home runs per game are down from 2.6 to 1.9, a drop of nearly 30 percent. Runs per game are down, considerably. Hard-hit rate, defined as balls hit 95 mph or harder, has dropped by six points. The ball is simply not being hit as hard this April in Arizona as compared to either of the last two.

While we're not accounting for player talent in terms of which teams have come through town, that's a fairly sizable drop, particularly when a lot of those metrics were pretty consistent in 2016 and '17. That's especially looking at runs per game, which have dropped by nearly half so far.
Now, the D-backs actually insisted it was more about pitch grips than anything to do with offense. (Which would, in theory, also then serve to depress offense, if the pitchers had better control and command.)
"The ability of the pitchers to grip the baseballs [is affected], especially through the summer months when it gets extremely hot and dry," GM Mike Hazen said in 2017. "No matter what we've done in terms of rubbing them up the right way, it seemed to be a challenge."
It does seem to be working, at least according to some Arizona players.
"Maybe the only thing [now] is that the ball feels similar at home as it does in other cities," pitcher Robbie Ray said. "You can feel the difference between last year's not-humidor and this year's humidor. The only way I can really describe it is it's now similar to a city like in the Midwest."
"It's tough to tell because we're just a month in, but you can say the ball feels a little different," infielder/outfielder Chris Owings said. "I'm not a pitcher, I'm not handling the ball every time, but maybe the ball feels a little bigger after being in the humidor.
"Usually in April and May at home, the balls carry pretty well. I don't know what the numbers show, but I would probably say it's probably the least amount of home runs hit in the first month."
While it's clear to see that offensive performance is down in Arizona, it only matters within the context of the rest of the sport. If we saw the exact same drops in Chicago and Miami and Seattle, then it wouldn't tell you that much about Phoenix. This only matters if we're seeing offense drop more in Chase Field than we are in other places.
As it turns out: We are, almost across the board. No park has seen a bigger drop in hard-hit rate than Arizona, or a bigger drop in fly ball/line-drive exit velocity, or in weighted on-base average.

It's still very early, obviously. Weather in other areas has been a factor. Even so, Chase Field's decline in hard-hit rate and exit velocity on liners and flies isn't the most by a little, it's the most by a huge gap -- and it fits in nearly exactly with what the physics model would have projected.
You can see it in the performance of just the D-backs' hitters at home and away, as well. Comparing this year's first month to last year's, there's a pretty glaring change:
April 2017, D-backs exit velocity
Home: 89.5 mph
Road: 87.7 mph
April 2018, D-backs exit velocity
Home: 87.7 mph
Road: 88.2 mph
Last year, they hit harder at home. This year, it's the opposite. If you go back to April 2016, the D-backs had an 89.1-mph exit velocity at home, and 86.6 mph on the road. Last year, they had 19 more home runs at home in April, and 24 more home runs at home for the entire season. This year, they have more than twice as many road homers, 21 to 9. This all fits.
While we'll need to see what this means as the season progresses, this could potentially be significant.
, for example, hit .321/.443/.639 last year at home, along with 20 of his 36 home runs. He's yet to hit a homer in the home whites this year and has four away from home. Entering the year, he'd long been better at home (.299/.411/.548) in his career than on the road (.298/.388/.516). If that's no longer true, because he's no longer in a "hitters' park," then his overall line may suffer.
It's too soon to make any grand proclamations about that, however. It's still so early, and the humidor didn't stop San Diego's from blasting a 489-foot homer last week, the longest of 2018. We'll need more than a dozen games to know how this is really going to play out. It's not so early, however, that you can't reasonably look at the numbers. The physics predicted that the Chase Field you once knew is gone. The data, early on, appears to back that up.