What role does baseball storage play in home run likelihood? For the first time, MLB is paying attention.

MATT MORET: Welcome to Field Study, a podcast about sports and science from the Pittsburgh Post-Gazette. I’m Matt Moret.

Many sports organizations like the National Football League have strict mandates for the storage and transportation of balls. Ask Tom Brady and the New England Patriots how much air a football should hold and you may end up fleeing for your life.

That’s not the case everywhere, though. Major League Baseball has traditionally issued very little guidance about how balls should be handled. Until now.

In February 2018, MLB announced that, for the first time in its 116 year history, it would be monitoring the climate in which every team stores baseballs. Some believe the edict has to do with an almost unprecedented boom in home runs. Others argue that pitchers mystified by slippery baseballs are the culprits.

Our question this week is, what scientific factors is MLB looking for when evaluating ball storage and why? What does a freezer in Denver have to do with the future of big league equipment standards?

**Note: The following interviews were recorded prior to the start of the 2019 MLB season.**

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MORET: In 1996, Tony Cowell was an electrician who’d just been hired by the Colorado Rockies -- still a relatively new team, having just joined MLB three years earlier. He’d grown up as a fan of the Los Angeles Dodgers but quickly converted when Denver got a team of its own.

Prior to joining the Rockies’ engineering department, Cowell was part of the crew that built the team’s home.

COWELL: I’ve sorta been at Coors Field for a long time. I got to see it from the dirt, growing up with steel and deck work and all that. I knew once the stadium was being built that it was a special place and that I really wanted to somehow figure out a way to stay here.

I was lucky enough that they started needed help with the engineering department. A gentleman by the name of Jim Weiner, who was our Senior Director of Engineering and who has since retired, offered me a position as an electrician. So I was hired on March 1st of 1996.

MORET: For his first few years with the Rockies, Cowell remembers things at the park going pretty smoothly. Most of the issues he faced were maintenance problems that any stadium faces.

COWELL: Back in the day, this ballpark had thousands and thousands of incandescent light bulbs that were everywhere, so we had a lot to take care of. Field lights were a lot of work to take care of, just to keep them going. There were a lot of metal headlights at the time. That’s about it. We had plumbing issues and stuff like that. Nothing out of the ordinary as far as other ballparks.

MORET: The Rockies as a whole, though, had a huge problem on their hands and they knew it. For that matter, so did everyone else in baseball.

Regardless of the team, hitters were blasting home runs out of Coors Field with shocking regularity. Pitchers moved in and out of the lineup, coaches tried to instil new techniques, but try as they might, it seemed like there was nothing the Rockies could do to stop the avalanche of scoring.

COWELL: It was known as a hitter’s park, for sure. Just look at the scores, we would have crazy scores here. From what I knew of baseball being a Dodgers fan, when you see a score like 8 to 12, it didn’t feel like real baseball to me. It was kind of something different.

Games went 3 hours and 20 minutes. Games that were really high scoring compared to other ballparks or games, you’d see on TV where the score would 2-5 or 3-7 at the most, that’s a high scoring game. But we would do that regularly, no problem.

MORET: Not everyone was bothered by the high scores. Many fans found them thrilling.

Imagine a brand new team dropping into your baseball-starved city and suddenly racking up huge numbers. You’d probably be hyped up, too.

COWELL: The fans liked it. I, myself personally, I like well-pitched games and prefer the lower scoring, defensive games. I know the fans really liked it. At the time we had what was called the “Blake Street Bombers” with Ellis Burks, Larry Walker, Dante Bichette. It was really exciting. It drew a lot of fans to the ballpark and we hit a ton of home runs, which was great.

The whole city, counties and state of Colorado were pulling for the Rockies.

MORET: But as the years wore on and home runs continued to spike in Denver, the electricity surrounding Coors Field started to give way to narratives about the talent of Rockies players themselves. Maybe if the team’s pitchers were just, you know, “better,” there wouldn’t be so many runs … and losses.

By 2001, the Rockies had only recorded four winning years. Like many in the organization, Cowell had heard the commentary and didn’t take kindly to it.

COWELL: This was starting to become a national reputation for Coors Field that our talent wasn’t really as good as it appeared to be because of the high scoring and high batting averages that we had here at Coors Field and then not so much on the road.

I started looking at it differently, thinking our guys are not getting the accolades or national recognition that they should be compared to other players because they would say, “That guy can play at Coors Field, so anybody can hit there.” Well, that’s not true. It still takes a tremendous amount of work to play the game of baseball. That’s definitely not true.

MORET: Frustrated by public perception of his team, Cowell wanted to help.

COWELL: In the summer of 2001, our president at the time, Keli McGregor, was very inclusive. He was a great man. He challenged the entire staff to break your boundaries. Just because you’re in ticketing doesn’t mean you can’t make a suggestion as far as how the ballpark is kept up. Or just because you’re on the baseball side, you can’t make suggestions on the ticketing side.

If you come up with a good idea, then you should share your idea. Don’t feel that you have to stay in your area and only think about your area. If there is anything you think we could do to preserve and help the ballpark, that would be welcome.

MORET: Cowell, and most of the baseball media, believed that the root issue was Denver’s high altitude. Coors Field sits 5,200 feet above sea level.

COWELL: That was the story line. Everyone was saying it’s the altitude. They’re playing a mile above sea level and that’s why the ball carries so far. Or the air is thinner and that’s why the ball won’t break as much. That was in all the papers and stories on TV and everything. I was thinking, there has to be a way to help that.

Originally, I started off thinking maybe I can come up with a better baseball. Something that would make a baseball behave closer to one that would behave at sea level versus at altitude.

I took a baseball and went down to the woodshop. And on a bandsaw, I cut the ball in half and I would sit there and look at it and think, what can I do different. I wanted to come up with something that I knew wouldn’t be too radical because I didn’t believe baseball or MLB would go for a substantial change to the ball. I just couldn’t come up with anything.

MORET: A few months went by without much progress. Cowell was stumped. It wasn’t until the fall of 2001 that he made a breakthrough.

COWELL: I went hunting and had a pair of leather honey boots that I had put away. Last time I wore them, they were walking through streams and soaking wet. I put them away and sat there through the summer and fall. I didn’t put them on before I went up the mountains. The first day I get up on the mountains, I put my boots on and these things were killing my feet. I was hiking and I sat down and that’s when I had my aha moment and thought, “Wait a second, my boots are made of leather and a baseball is made of leather …”

My boots had shrunk, because once leather dries out, it shrinks. And I was like, “No way, it can’t be that easy.” I thought maybe the baseballs are doing the same thing.

MORET: He knew he was on to something. Cowell figured out that if the outer covering of the baseball was shrinking, then that would make the baseballs harder. That means home runs would be easier to hit, and pitches would be harder to throw.

It would, in effect, explain both theories behind the Coors Field offensive boom.

COWELL: I get back from the mountains. I start really diving into how baseballs are made with the pill in the middle, the rubber core, and then it’s wrapped with wool. So I start looking at the properties of wool and come to find out that wool, when it is dry, like a wool sweater, when it comes out you can’t hardly fold it and it springs back out. So wool gets more springy and has more rebound the dryer it is. The dryer it is, the more it pushes back.

In addition, leather, when it dries out it becomes harder, slicker, and it shrinks. So I got some new baseballs and compared it to balls that had been around Coors Field and you can actually see the difference. You can see the seams pulling tight and it was hard. You can actually feel the difference. So I thought, that makes sense, that’s probably why even the pitchers had trouble doing breaking balls, which you have to really grip the ball to get it to spin. And it’s hard to do if you can’t squeeze it with your fingers and get that rotation.

MORET: Cowell built a small container with a source of moisture in half of it. He took a dozen baseballs and split them into two groups. The first group would be a control group exposed to no additional moisture. The second would sit in the more humid chamber for a week.

COWELL: Me and one of my coworkers went up to the top deck of the stadium and I had him down below. I said, “I’m going to drop these balls off the stadium down to the sidewalk and I want you to measure how high they bounce.” Of course, the balls that were dry bounced considerably higher than the balls that had been exposed to the moisture.

At that time, I went to Keli McGregor, who was gracious enough to give me an audience, and I said, “Hey, I think I got something here.”

I told him what I did and he goes, “That’s a great idea, let’s try it!”

MORET: With that, the Coors Field baseball humidor was born.

COWELL: I actually didn’t call it a humidor, I called it an “environmental chamber” because we wanted it to have the ability to replicate weather conditions in other cities. I want to be able to set this thing so I’m indoors in Atlanta or L.A. or New York. You can pull up their weather data and say, “Well this is what their conditions are, I want to be able to do that and see if it makes a difference, and it did.”

MORET: Cowell’s design was pretty simple.

COWELL: I’m sure you’ve seen a walk-in cooler they have in restaurants or ballparks and stuff like that. Aluminum skin with foam core walls. So we bought one of those and mounted the heating and cooling equipment and the humidifier on top of it, sealed it all up so it was tight. You can’t keep it completely airtight, moisture just always finds a way out no matter what. We just used a cooler and put some temperature controls on it. It was right over $16,000.

MORET: For the Rockies, that was a low cost to revive their reputation. The team set the humidor to simulate 70 degrees fahrenheit at 50% relative humidity for the 2002 season. There were positive results almost immediately.

Four years later, in a 2006 interview with The New York Times, Eric Byrnes of the Arizona Diamondbacks vented about the dramatic change at Coors Field.

“All I heard for so many years was that this was the greatest place in the world to hit. I want to know what happened. I want to see the conspiracy theories. I’ll believe them.”

There was no secret conspiracy at play, though. Just some physics.

Dr. Alan Nathan is a professor emeritus of physics at the University of Illinois. He spent a long career doing experimental nuclear physics and has now transitioned into studying the physics of baseball.

According to Dr. Nathan, if you look at home run rates from before the humidor and compare them to rates from the first eight seasons it was in service, there was a roughly 25% decrease in home runs at Coors Field.

For the first seven seasons without a humidor, there were 3.20 home runs per game at Coors Field. That’s compared to 1.92 home runs per Rockies game on the road.

But from 2002 to 2010, the Coors home run rate decreased to 2.39, while the away game rate stayed essentially constant at 1.86.

There’s a big margin of error on that 25% figure because there are obviously different people playing in different baseball games. But even accounting for that, the Rockies humidor has been a massive success.

The question is: Why? Here’s Dr. Nathan.

NATHAN: Typically in MLB, different clubs get shipments of balls maybe once a month throughout the season and they are stored somewhere. Now, if they are stored long-term, for days or a week or more, in a humid environment, the following things happen.

The ball absorbs water. That’s the basic of what happens. So it gets a little bit heavier and it might get a little bit bigger, but that’s been shown to be a really, really small effect so we can ignore that.

And it makes the ball less bouncy. This is the major effect. There is a quantity called the “coefficient of restitution” of the ball. And it has to do with how bouncy it is. So a super ball has a high coefficient of restitution, it is very bouncy. A lump of clay has essentially 0 COR. You drop it on the ground and it doesn’t bounce at all.

A baseball is sort of halfway in-between — you bounce it on the ground and it falls somewhere halfway between a lump of clay and a super ball. And when the ball absorbs water it gets less bouncy.

MORET: This is a little complicated, so stay with me here.

According to Dr. Nathan, a humidified baseball will have a lower coefficient of restitution, or COR, and will thus be less bouncy. It will also be heavier and a little larger. These factors interact in different ways.

The increase in the weight and the decrease in bounciness means the ball isn’t going to be hit as hard. But, once the ball is hit, the weight is actually going to have a positive effect.

The baseball, as it travels through the air, slows down due to air drag, which plays a huge role in its flight.

NATHAN: So a typical 400 ft. home run would travel probably more than 700 ft. if it wasn’t for air drag. But a heavier ball is less affected by the drag.

You can even see that with your own eyes. You could throw a baseball and then compare it with throwing a styrofoam ball, and you’ll see a styrofoam ball just gets buffeted around by the air while the heavier baseball isn’t as much.

The fact that the ball absorbs some water means that not only is it not hit as hard, the air drag will actually be a little bit less. That is, the decreased velocity of the ball coming off the bat is more or less canceled out by the increased distance you get because of the reduced effect of the drag.

MORET: That means that the primary reason for any change in a humidified ball’s flight distance is due to the coefficient of restitution. This is the major thing that would have an effect when you store the ball in a humidor.

By simply making their baseballs more moist and less bouncy, the Rockies effectively reduced the chance that a hit would result in a home run by nearly a third. And it turns out that the thinness of Denver air during a game actually had little to no role in the Coors Field home run boom.

Dr. Nathan says humidity would take days, if not weeks, to fully affect a baseball, so the storage conditions prior to a game are what really matter.

NATHAN: There’s the humidity in the environment where you’re storing the balls, but there’s also simply the humidity in the air on the day the game is played. And that has a really, really small effect on the flight of the ball. Humid air is less dense than dry air, so when it’s humid, the ball actually travels a little bit further.

MORET: Seeing the impact the humidor has made at Coors Field, the Arizona Diamondbacks decided to build one of their own ahead of the 2018 season. It probably goes without saying that the air at Chase Field in Phoenix is dry. It is a desert, after all.

The Diamondbacks have seen a similarly high rate of home runs at the park since it opened in 1998. Chase Field has a retractable roof that covers the field during inclement weather, but that level of climate control does nothing to prevent the team’s baseballs from drying out. This makes the ball feel slick and hard to grip, just like the dryness of Denver air.

Here’s Mike Petriello, a columnist at MLB.com and Statcast.

MIKE PETRIELLO: Chase Field has always been considered a hitter’s park, you know, not quite Coors Field but still one of the two or three most favorable parks to hit in.

For example, if you were to look at Arizona hitters in 2017, they had a pretty huge advantage hitting at home, just looking at slugging percentage. They had a .492 slugging percentage at home and a .398 slugging percentage on the road. That’s almost 100 points, so it’s a pretty big deal.

MORET: And just as the Rockies saw at Coors Field, the Chase Field humidor showed an immediate and significant impact on offensive production.

PETRIELLO: So, keep in mind that I just said they had a .398 percentage on the road and then, this past season, .397 on the road. That’s great, that’s almost no change whatsoever. Except at home, they had a .398 slugging percentage.

Basically what that means is that instead of having a 100 point advantage at home, there was no advantage at all. It didn’t turn into a pitcher’s park, necessarily, but it did seem like the humidor turned from a very advantageous park for hitters into sort of a neutral park, which is really interesting.

MORET: Petriello says it will take a few years before we can draw any firm conclusions about the new humidor, but he points to the performance of Paul Goldschmidt, who now plays in St. Louis but who spent the last several years with the Diamondbacks, including the first humidor season.

PETRIELLO: If you look at his home-road numbers last year, at home he had a .420 slugging percentage and on the road he had a.638 slugging. That’s huge, and it’s certainly not what you expect from a guy who plays half his games at Arizona.

MORET: And when there are huge divides like this between a player’s road performance and their home field performance, it complicates basically every evaluation metric major league teams rely on to sign players.

PETRIELLO: Which version of that player is the player you’re trading for? For Rockies players it’s been an issue for years.

MORET: DJ LeMahieu, for instance, played second base for the Rockies before signing with the New York Yankees in free agency this season.

PETRIELLO: If you look at the surface-level stats, they’re pretty good. He’s a strong defender with a high batting average, a decent amount of power.

But then if you look a little deeper, at his splits between home and road last year, they’re enormous. He hit .317 at home and hit .229 on the road. His on-base percentage was .360 at home and .277 on the road. So if you’re looking at signing LeMahieu and taking him out of Colorado and putting him in Los Angeles or Chicago or Detroit or wherever, you can’t just expect that you’re going to get the same guy. He’s not going to have the same boost that his home field offered.

What’s even more interesting is that, if you go even deeper, there’s some evidence that guys who leave Colorado will get worse at home, clearly, but they may get slightly better on the road. It almost seems like there’s an adjustment factor back to the way things are different on the road and the way that pitches move a little differently. It seems like they have a little trouble adjusting to things on the road.

I’ve looked into this in the past and when Rockies guys leave, yeah, they don’t do as well at home but they get somewhat of a boost away from home. Not enough to counteract, their stats usually go down overall, but it just means it’s tempting to simply look at a guy’s lousy stats on the road and say, “Oh that’s the player he really is, he’s a Coors Field invention.” But even that may not be true.

MORET: This is really the root issue for this entire conversation about park-specific conditions. Most of baseball is not uniform.

Some outfields are significantly larger than others. Some fields have roofs that alter variables like wind. Historically, baseball storage has fallen into the same open book. That may be changing, though.

NATHAN: Up until this current year, the way that baseballs were stored by major league clubs was totally uncontrolled, unregulated and mostly unknown — except for Coors Field. We knew what was happening at Coors Field, and of course now we also know what’s going on in Phoenix.

But MLB put out a decree during the offseason this past year that all major league clubs need to monitor and report the temperature and humidity in which their balls are being stored, more or less continually. It’s all done electronically. So now we have a lot of information about how the different clubs store their balls. And at some point in the future, MLB might decide to standardize the way the baseballs are stored.

MORET: For some fans, that may strip away part of the game’s identity. Here’s Petriello’s take.

PETRIELLO: There’s certainly an argument for standardization, right? Same kind of ball that plays the same kind of way across all 30 parks. On the other hand, you have 30 different parks that are not standardized. You know, this isn’t the NFL, where everyone plays on the same sized field and there’s not a huge home field advantage anywhere outside of crowd noise or whatever.

And in baseball, the parks aren’t the same size and they’re not the same shape. For a lot of people, I think that adds to the charm of the game. So I don’t necessarily mind that there are slight differences just based on the atmosphere and all that.

I do think once you get into the super extremes like pre-humidor Coors Field and maybe even Arizona, there’s an argument to be made for trying to make it look a little bit more like “regular baseball.”

So I think you could really go either way. I don’t see a solid case that it has to be one way or the other. You could see what happened in Arizona last year and say, maybe it makes it a little more fair, but maybe it also makes it a little less interesting. Now Chase Field is just a “regular park” instead of an “extreme park” in that way.

MORET: There are several reasons MLB may ultimately take the standardization plunge. The first is that we are in the midst of a historic boom in home runs across the league.

PETRIELLO: The offensive environment in baseball always goes through these ebbs and flows. You know, like in the ’60s there was the year of the pitcher and they actually had to change the mound, and obviously in the ’90s there was an enormous home run boom. And then over the last few years, it’s really coming back. I actually remember in 2014, people were all decrying the “death of offense” and how there’s too much pitching and how we need to help hitters, but then home runs started coming back.

The last two seasons, maybe three seasons, have all set home run records. In 2017 we had over 6,000 homers, which was the most in baseball history. And I think this last season there were a few hundred less, so it seems like it may be going back the other way, but clearly there are more homers in baseball than ever.

MORET: Some observers, including Petriello, have also tied the increase in home runs to a possible change in how MLB balls are manufactured. MLB has confirmed that the ball has undergone some sort of unspecified change that results in it being more slippery.

It’s unclear what exactly that change is. It could be related to stitching or the wool inside every baseball. And if material differences are actually at play, then humidors at every park would certainly have an impact on offensive production.

Remember: A more moist baseball will be less slick and less bouncy, meaning better pitches and fewer home runs.

That opens up an additional question, which we alluded to at the beginning of the show. If the point of MLB is to put on an entertaining product, wouldn’t the thrill of home runs be something to fuel, not deflate? Petriello doesn’t necessarily think so.

PETRIELLO: Home runs by themselves are exciting, I don’t know anyone who doesn’t like seeing home runs, so they’re cool. But that’s not necessarily saying the same thing as “more offense.”

We’re in the midst of something else called the “Three True Outcomes Boom.” And that means that we’re seeing strikeout records every single year, a lot of walks and a ton of home runs. What that means is that we end up with fewer balls in play, fewer base hits and singles. This is the first year that there were more strikeouts than hits, which isn’t really great.

So I think that’s not necessarily the path the game wants to go down, especially if you try to forecast the future and you think that this is a trend that’s going to keep going on. It’s not necessarily a great experience when there’s not a lot of action and it’s just home runs and strikeouts.

I think that’s a trend the sport is grappling with and trying to find the right balance, both for the on-field way the game is played, but also for the entertainment value for the sport.

MORET: There’s been no official word from MLB about the prospect of standardized baseball storage. The league did issue a “recommendation” that teams store baseballs at 70 degrees Fahrenheit and 50% relative humidity.

If that sounds familiar, it’s because that’s the same setting used with the two humidors we know about, but the league stopped short of calling it a new rule.

For any organizations that are interested in a humidor of their own, though, Tony Cowell is your hook up.

COWELL: We have come up with a handout. We email the teams stating this is how we built it, this what the equipment consists of, this is what we paid for it, this is where we got it. Feel free to take that and change it and do whatever you want.

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MORET: Hopefully you enjoyed this episode of Field Study. I’d like to thank all of the guests for appearing. I produced this episode, and Ryan Winn helped with scripts.

You can find full transcripts and photos for every Field Study episode on our website, which is in the show notes. Next week is the final episode of the season. It’s about the decorated career of runner Caster Semenya and the debate over how international sports classify female athletes.

Thanks for listening.