The Impact of the Humidor on Busch’s Park Factor

Grab a carpet square because it’s storytime.

The longest home run I ever hit came in 1997. I spent that summer working at Yellowstone National Park cleaning hotel rooms. (I apologize deeply if you stayed in the nice rooms on the first floor of the Old Faithful Inn that season.) The job sucked and so did the living arrangements, but the scenery couldn’t be beaten.

I made friends with wolves. I got lost a few times. I nearly died. Twice. I rescued some people. I didn’t clean many rooms. It was a great way for a 19-year-old to live. It was also my first and last summer without baseball. I spent the entire summer with no TVs, radios, internets, or newspapers.

Since we couldn’t watch the grand ol’ game, the best we could do was play it ourselves. Someone in our group had stumbled on an overgrown old ballfield hidden in the “employees only” section of the park out past the cabins. We dug around the employee Canteen, found some gloves, bats, and balls, and went out for a pickup game.

We were terrible. Just want to make that clear. But it was baseball, so, it was awesome.

In my first at-bat in my first game out there, the pitcher left a (not very) fastball middle/middle. It was pure batting practice stuff. I threw all of my 145-pound middle infielder’s frame into that pitch and smacked it to straight-away center.

The ball carried over the wall, over the hill behind the wall, and then past some trees before landing near some smoking geysers.

I’m guessing I Tyler O’Neill’ed … no, I Mark Whiten’ed that thing 500 feet.

Am I bragging? No! I already told you we were terrible!

Old Faithful just happens to sit at 7349 feet above sea level. That elevation is nearly a half-mile ABOVE Mile High Stadium in Denver. We were playing ball a skip and a jump from the Continental Divide. The air is so thin up there you get out of breath just thinking about breathing.

If I hit that same pitch with that same power here on the banks of the Mississippi the center fielder would have to come in to catch it.

Altitude, air density, humidity – all of these things factor into how far a baseball will travel.

We learned this when baseball expanded into Colorado. Suddenly a team just a few years from expansion was among the league leaders in offense. Rather pedestrian players like Charlie Hayes (one of the great in-season fantasy pickups I have ever made) and Dante Bichette became All-Star players. Larry Walker, already a star in Montreal, threatened to hit .400 and win Triple Crowns on an annual basis.

The Rockies tried all kinds of things to better curb the impact of their climate on baseballs, including failed experiments with curveballs (Darryl Kile) and changeups (Mike Hampton), before turning to science. Or, well, hunting.

Tony Cowell, the stadium engineer at Coors Field back in the mid-90s, was out elk hunting high in the mountains when his leather boots got soaked. When they dried out, he noticed that the leather shrunk in the thin air of high elevation. It was a light-bulb moment.

The dry air pulled moisture from the leather and the boots compressed. What if the same thing happened to leather baseballs when stored in the high-elevation, low-humidity climate around Denver?

The idea for the humidor was born. Crowell tested his theory with a homemade unit and found that balls kept in a wetter environment didn’t bounce as high when dropped from a distance. He took his concept to the suits in the front office of the Rockies and the rest is baseball history. The club installed a humidor in the stadium and stored their game balls there. Offense remains up in Denver but the playing environment is not quite as insane as it was the first few years of the club’s existence.

Fast forward one score and a few more years and humidor usage has expanded. Multiple ballparks are using them now, and it’s not limited to high elevation locations. The Cardinals were among several teams who installed a humidor this offseason.

At first thought, that might seem a little odd for those who haven’t followed this story. Busch Stadium has the reputation for being a pitcher’s park. Why would the club want to further suppress offense with a humidor?

The answer is that humidity works both ways on a baseball. In a low humidity, low air density environment like Coors Field, adding humidity to a baseball will cause it to be less bouncy and not travel as far. A humidor in that climate cuts a little from the offense.

In a high humidity, high air density environment like Busch Stadium and St. Louis, a humidor wouldn’t add water to the balls. It would pull humidity out of the ball, thereby increasing its bounce and overall offense.

When asked about the installation of the humidor, John Mozeliak explained this way, as reported by Derrick Goold at the Post-Dispatch.

“We see (it) as working both ways. When we are in a drier time of year, it will work as a humidor. When we are in a humid time of year, then it will work as a dehumidifier. So, in the end we are just trying to have a consistent baseball throughout the season.”

I chatted with Jeff Jones of the Belleville News-Democrat about this, hunting for the specific humidity settings at Busch. He confirmed that MLB has established 70 degrees and 50% relative humidity as the recommended storage environment for game balls. He could not confirm if the humidor at Busch was still set according to those guidelines but since the club is going for consistency and not to correct some dramatic offensive outlier, it’s probably safe to run with league standards for the purpose of this article.

If Busch’s balls are stored at 50% humidity, how does that compare to a typical day of baseball in St. Louis?

Throughout a baseball season, the humidity at Busch ranges from an average low of 63.5% in April to a high of 71.6% in September. The chart is a little backward from what I expected, but I’m no meteorologist. (You can find the information I used here.)

The Busch Stadium humidor is dehumidifying the ball between 14-22 percentage points on average. That’s not a huge rate but it might be just enough to notice a difference in the way the ball plays.

So, is the humidor having an impact on offense?

You wouldn’t think so with the way the club has hit lately, but glancing at runs scored by team or collective batting averages isn’t how we measure the influence of a ballpark.

To do that we use a stat called park factor. Park factors are used in sabermetric stats to negate the influence of ballpark on individual and team performance. They show up in the guts of offensive stats like wRC+ or wOBA and pitching stats like FIP.

Because park factors are mostly used as a stat to create other stats, they’ve been pretty hard to dig up. They’re also not consistent across all platforms.

MLB analytics sought to normalize that. Baseball Savant – baseball nerd heaven – used their massive collection of Statcast data to produce a tool earlier this season that lays out all the various, uh, factors that go into their park factors. They then used their typical color-coding system to show how each MLB stadium affects a variety of offensive events – from singles and walks to triples and homers. You can check out that info here.

For 2021, with the humidor in effect, Busch Stadium currently has a park factor of 98 for the season – just two points below average. It is 18^th in the league. It is currently playing as a strong park for singles and triples but a poor place to hit doubles and home runs.

Now let’s track backward to pre-humidor years to see how the humidor is impacting play, skipping 2020 because of the tiny home sample size. Here are the park factors and league ranks for Busch going back to 2015:

2021 – 98, 18^th
2019 – 95, 26^th
2018 – 95, 26^th
2017 – 98, 18^th
2016 – 96, 22^nd
2015 – 99, 17^th

Has the humidor had an effect on offense at Busch Stadium? Compared to the past two full seasons, the answer is yes. Offensive production at Busch for all teams is up 3 percentage points relative to 2018 and 2019. A 3 pp change is not dramatic, but neither is the change in the city’s average relative humidity to a constant 50%. The arrangement of the ballpark itself still limits offense – suppressing doubles and homers – in ways for which the humidor can’t fully compensate.

If the Cardinals’ goal was to create a “fair” park with consistent ball performance, they’re getting pretty close.

At the same time, the influence of the humidor may be negligible here. The data from 2017 and 2015 is right in line with this season. So, the answer to this question depends on how much we want to weigh recent history – with new additions like Ballpark Village Phase 1 & 2 in play – versus older data.

Personally, I would give more weight to 2018 and 2019 than I would to 2015. There will always be variance but my hunch is that Busch is playing a bit up and I would expect that to continue. It will take a larger sample size to lock in that data, and create a new narrative about the way Busch plays.

The humidor is working. It’s no Yellowstone. Its effect is very subtle. But it might be just enough that we’ll have to stop saying Busch is a “pitcher’s park” in a few years.

Too bad its impact isn’t showing up in the team’s offensive performance right now.