These 9 MLB ballparks are using humidors. What does it mean for baseball's offensive downturn?

To watch baseball in 2021 is to wring your hands about the historically meager and one-dimensional state of offense, and to argue about the frantically assembled laundry list of potential remedies being tested in lesser leagues or bandied about by fans, broadcasters and MLB-appointed savior Theo Epstein.

There are no main characters. There are simply main ingredients in the confluence of trends holding the hitters back.

As the league batting average slumps to .237 — tied for the lowest ever with 1968, the so-called Year of the Pitcher — baseball is reckoning with the need for massive changes to rebalance the sport. There are interconnected discussions of how to reduce or mitigate the constant uptick in velocity, how to induce more contact, and how to stop pitchers’ longstanding but increasingly scientific practice of using sticky substances to get ever more devastating, bat-eluding spin on pitches.

Yet there’s still a nagging feeling that we often aren’t working with all the information needed to understand what’s happening.

In a memo circulated among teams and reported by The Athletic prior to this season, the league said it had deadened the baseball slightly — counteracting the transformative home run surge that skeptical public analysts traced back to changes in the ball, a finding eventually confirmed by an MLB-commissioned panel of experts.

The same reports added that five new ballparks would install humidors to bring the league total to 10. But the locales of the new humidors — which had huge ramifications for offense when they were very publicly installed in Colorado and Arizona — were left unidentified. ( amusingly aggregated the news of the memo, citing AP reporting about the new but unspecified humidors.)

Word of three new teams employing humidors in their home parks — the Boston Red Sox, Seattle Mariners and New York Mets — trickled out in 2020, but your best hope of getting the full list of humidors in use in 2021 was catching a snippet of an April Chicago Cubs broadcast when broadcaster Boog Sciambi rattled off the new adopters.

Last week, MLB provided Yahoo Sports the full list of nine teams currently using humidors. The previously known Rockies, Diamondbacks, Red Sox, Mariners and Mets have been joined by the Houston Astros, Miami Marlins, St. Louis Cardinals and Texas Rangers. The Toronto Blue Jays — who Sciambi also identified as the 10th humidor team — are planning to utilize one when they return to Toronto, but have so far played home games at their Florida spring training site and now in Buffalo.

The full accounting of the new humidors represents a potentially significant piece of the puzzle that has been a source of mystery or uncertainty for even close observers, and a key factor to study and untangle as the sport races to gain an accurate understanding of the state of play.

CHICAGO, IL - APRIL 12:  President Theo Epstein of the Chicago Cubs
is congratulated by Rob Manfred, comissioner of Major League baseball, during a ring ceremony before a game against the Los Angeles Dodgers at Wrigley Field on April 12, 2017 in Chicago, Illinois.  (Photo by Jonathan Daniel/Getty Images)
As former Cubs and Red Sox GM Theo Epstein (L) works to reshape the game for more action, commissioner Rob Manfred is still overseeing a range of tinkering with the major-league product that can make it difficult to untangle changes. (Photo by Jonathan Daniel/Getty Images)

A whole different humidor effect

The humidor entered the baseball lexicon in 2002, when a sharp-minded engineer working for the Colorado Rockies devised a way to tamp down scoring at Coors Field’s high altitude following an outlandish hitter’s feast of a season in 2001. Mirroring the boxes used to preserve cigars, a humidor is a small temperature- and humidity-controlled room where baseballs are stored at the MLB-recommended 70 degrees and 50 percent humidity to regulate conditions before entering games.

In Colorado, using a humidor means boosting the humidity around balls that otherwise dry out in the mountain air, making it harder to belt them over the fence and taking offense down a notch. The move was made in Arizona prior to 2018 for similar, if less dramatic, reasons.

The effects are well-documented and baked into the broader understanding of statistics and results in those parks.

The seven humidors installed since the end of 2019 are notable for appearing in cities where they will usually provide lower-humidity storage, the opposite effect from Colorado and Arizona. But that does not necessarily translate to opposite on-field results.

According to research by Dr. Alan Nathan, the noted expert on physics in baseball, boosting the baseball’s storage humidity, as the Rockies and D-backs do, adds weight but decreases the bounciness of the ball. That means the ball actually carries better once in the air, but leaves the bat at significantly lower exit velocities.

In humid cities near sea level like Houston, Miami or St. Louis, the humidors push the levers in opposite ways. A humidor ball would be easier to hit hard, but wouldn’t fly as far because lighter balls have increased drag, or air resistance. Think about a Wiffle ball — leaves the bat very fast, then slows down a lot.

Those changes are hard to isolate, though, because they are coinciding with the deadened ball implemented around the league, which MLB said is less bouncy and also lighter because of a loosened wool layer. Most humidor parks are getting lighter baseballs from the league, and then storing them in different conditions than 2019, which are also more conducive to lighter balls.

That would, in theory, mean a lot of fly balls dying sooner than we have been conditioned to expect.

Why the humidors could make a difference

But none of these changes happens in theory.

Rob Arthur has tracked the moving target that is the ball for Baseball Prospectus and The Ringer alongside Ben Lindbergh — often proving to be several steps ahead of MLB. He has noted that the 2021 version is not behaving quite as the league prognosticated, with the weight change looming larger than the supposed toned down bounciness. Among his major findings in recent years has been the potential for the ball to change notably from month to month, or from the regular season to the postseason.

What we can see right now, then, isn't sure to last. But the search for understanding is beginning.

First, the loud but not significant anecdotal evidence: Yes, four of the six no-hitters in MLB’s early-season spree occurred in humidor parks — two in Texas, two in Seattle. Bad offenses were probably the main culprit, but it’s possible the new ball and cool temperatures in Seattle contributed.

A broader look shows that the league at large is making less contact, which is the predominant source of the offensive nosedive, though Arthur has shown the lighter ball could be hastening that trend by gifting already dominant pitchers more movement.

Hitters are also getting less out of good contact. Statcast data shows the slugging percentage on fly balls with exit velocities of at least 95 mph is down to 1.685 in 2021 from 1.906 in games played through June 7 in 2019. (Because the shortened 2020 season began in the hot, humid summer months, it does not make for a useful comparison.)

Where 43 percent of those balls went for home runs up to this point in 2019, only 36.8 percent have left the yard in 2021. Again, it’s nearly impossible to separate out the causes, but purely by 2019 conditions, there could have been about 296 more homers.

Houston’s Minute Maid Park and New York’s Citi Field have been markedly less friendly to those fly balls than they were in the early months of 2019, even compared to other parks around the league. St. Louis’ Busch Stadium and Miami’s loanDepot Park are among the least hitter-friendly parks, but that was also true prior to the humidor installation. Texas has switched to a new ballpark since 2019, which makes this particular comparison impossible, but it is clear the new Globe Life Field is worse for offense than its predecessor.

All of this could morph as the season goes on. Red Sox officials explained to The Athletic last season how Fenway Park's ball storage conditions varied from the cooler, drier beginning of the season to the summer months prior to installing the humidor.

That unknown weather factor is one of many reasons Arthur is not ready to make grand pronouncements. He and Dr. Meredith Wills, an astrophysicist who has studied the composition of the baseball, articulated a number of questions and avenues of interest in a SportTechie panel last week. Wills has said she is using homemade humidors to drill down on potentially larger than expected reactions to changing atmospheric conditions.

So a whole host of factors are muddying this picture, including whether a stock of baseballs from the shortened 2020 season is still being used up and how baseballs were previously stored at the humidor parks.

An MLB spokesman said teams without humidors are instructed to store balls in a temperature-controlled environment at 70 degrees. That guidance has remained largely unchanged since it was issued prior to the 2018 season.

PHOENIX, ARIZONA - MAY 31: Starting pitcher Jacob deGrom #48 of the New York Mets pitches against the Arizona Diamondbacks during the MLB game at Chase Field on May 31, 2021 in Phoenix, Arizona. (Photo by Christian Petersen/Getty Images)
The conditions at Citi Field are important to understand as we seek to contextualize Jacob deGrom's historic performance. (Photo by Christian Petersen/Getty Images)

What we don’t know can hurt us

These changed dynamics have the potential to affect just about everything about how we understand the game. As MLB embraces sports betting for the first time through a partnership with BetMGM, details about the ball and its reaction to conditions at individual parks — especially compared to expectations built upon years and years of past results — are vital to betting on run totals or the home run race, and to forecasting team performance. (BetMGM is also a Yahoo Sports partner.)

Citi Field’s tendencies, for instance, could shift our interpretation of Mets ace Jacob deGrom’s historic season, and decide whether he’s an NL MVP contender (+700 at BetMGM, the third-best odds right now) or simply the prohibitive Cy Young favorite (-165).

Grasping the reality of what players are doing vs. what their environment is doing to them affects the sacred statistical record of the game and, perhaps most crucially at this moment in history, the decisions about how to recalibrate the batter-pitcher relationship for a more action-packed future.

Humidors actually could and should be a steadying force. The panel commissioned over the home run surge recommended studying the idea of putting one in every park. That tact, presuming the conditions are identical and publicly announced, could get the sport much closer to playing with a consistent central object.

What Arthur is most interested in is simply a more transparent, more forthcoming detailing of the specifications around the league so that when the data is robust enough to analyze, he can be more precise in locating and attributing shifts.

“It just makes it a lot more complicated knowing what's going on,” Arthur said, “because of this scattershot implementation of the humidors and lack of transparency with how they’re being implemented.”

Hannah Keyser contributed reporting.

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