The Physics of Humidors: A Second Case Study at Chase Field

In its first year, the humidor at Chase Field proved to be effective. (via Cygnusloop99)

The first case study on the effect of humidors on baseballs goes back to 2002. The vagaries of mile-high baseball at Coors Field drove the decision to control the temperature and humidity in which the balls were stored. The typical humidity in Denver is about 30 percent, while humidors maintain 50 percent humidity at 70˚F.

Balls stored in the humidor have a higher water content than they would if they were stored in the Rockies dugout. The higher water content means the balls are “mushier.” In addition, they weight a bit more. As a result, such a ball will come off the bat with a lower exit velocity and thus won’t travel as far.

While home run numbers in the mountain air of Colorado remain high compared to other parks, they dropped about 13 percent from 2001 to 2002, demonstrably due to the humidor. The reason the home run numbers remain stubbornly above MLB averages is due to the thin air, which is a more difficult challenge to address. The Coors humidor remains in place to this day.

The second case study was conducted in Phoenix last season when the Diamondbacks introduced their own humidor to Chase Field. The typical outdoor humidity there is around 20 percent – even lower than in Denver. So one might expect the change in homers could be even larger.

Alan Nathan has written about the connection between home runs and humidors. Perhaps the most complete description is here. That article includes a preliminary prediction of the effect of a humidor at Chase; a more complete prediction can be found here. For more technical readers, there are additional details about the effect of humidors here.

The humidor affects three properties of the ball: its bounciness, its mass, and its size. These changes drive the basic physics of humidors. You can probably imagine a crisp dry ball will bounce off the bat faster than a waterlogged ball. As a result, a ball stored in the ambient dry air of Denver or Phoenix will bounce off the bat better than a ball stored in the damper air of the humidor.

The technical term for the bounciness of the ball is the “coefficient of restitution,” or COR. The COR for a ball stored in the ambient humidity at Coors drops from around 0.537 to 0.513 when stored in the humidor. That’s a 4.5 percent drop in COR. If we compare a ball from the humidor with an exit velocity of 100 mph that would travel 400 feet to a ball with the reduced COR, the exit velocity would drop by 3.6 mph, and the distance it would travel falls by 15 feet.

The additional water in the ball increases its weight. This also decreases the exit velocity of a well hit ball. Using the Colorado situation, there is a 3.5 percent increase in the weight of the ball. This will reduce the exit velocity by 1.2 mph and the distance traveled by 5.6 feet compared to the humidor ball with a launch velocity of 100 mph, which would travel 400 feet. This is roughly one-third of the change due to the COR decrease.

The flight of the ball is not affected much because the added weight actually makes the ball harder to slow down, increasing the distance. Meanwhile, the swelling of the ball due to the added water causes the ball to interact with more air, slowing it down and reducing the distance. The two effects tend to cancel each other.

Enough about Colorado — let’s look at Arizona. Last year in the Diamondbacks’ 81 home games, there were 171 homers hit by both teams compared to 215 the previous year. This is a 20 percent decrease, as you might have predicted understanding the effects of the humidor from Coors.

Naysayers would point to the fact that Arizona parted with J.D. Martinez prior to the 2018 season. He hit 46 homers in 2017, although only 16 of those dingers were at Chase. However, the table below shows the Rattlers hit essentially the same number of homers away from Chase Field in both 2017 and 2018. Meanwhile, their production at home dropped dramatically, consistent with the Coors experience.

Chase Field Home Runs
Year Away Home Runs Chase Home Runs
2017 98 122
2018 94 80

Granted, this is only one year of data, but since the Coors home run reduction has sustained over time, one would suspect no less from Chase. However, before we wipe our hands and say we’re done, take a look at the plot below of home versus away round-trippers for the Diamondbacks the last few years.

If it was the case that 2018 was the first year the Snakes had more dingers on the road than at home, then it would nearly clinch the case for the home run decrease at Chase in 2018 was due to the humidor. Alas, 2015 shows the same away versus home split. So more investigation might be warranted.

I tried to find any changes to Chase in 2015, such as a higher yellow line in center field or a taller fence somewhere. The only thing I could find that happened before the 2015 campaign was new sod in the outfield – hardly a cause for the D-backs’ power outage at home.

However, in the Report of the Committee Studying Home Run Rates in Major League Baseball, Chase Field was one of only five ballparks that experienced a drop in home run rate between the beginning of the 2015 season and its end, suggesting there was something a bit out of the ordinary at Chase that year. By the way, the conclusion of the report was that the drag on the baseballs was less for some unexplained reason. Thus, the balls traveled farther than would be otherwise expected. It’s possible Arizona didn’t get any of these baseballs for some reason.

In any case, instead of looking at homers, let’s examine exit velocity. That way, we don’t need to worry about the trajectory of the ball. Below is a plot of the number of balls hit with an exit velocity over 105 mph by year at Chase Field.

The data show the fewest number hit were indeed in 2018. This is consistent with the humidor theory because the reduced COR and larger ball mass would make it harder to launch a ball at a high exit velocity regardless of the resulting trajectory.

In fact, if we stop obsessing about the abnormal number of away versus home four-baggers in 2015 and just look at balls hit by the Diamondbacks at high exit velocity since 2015, we might further clarify matters. Below is a plot showing these data. The percentage shown is the difference between home and away high exit velocity hits for each year.

You can see the Rattlers consistently hit the dry balls in Arizona at a high exit velocity more frequently than the mushier balls used in the rest of the league. Not only was there a steep drop in the number of high-exit-velocity balls in 2018 at Chase, the decrease between home and away is dramatically smaller after the introduction of the humidor.

In summary then, the humidor in Phoenix has done what humidors do to dry baseballs. This conclusion is based upon the physics of humidors and the following data:

  • The reduction in total home runs hit at Chase Field in 2018.
  • The Diamondbacks, for just the second time since 2014, hit more round-trippers away from home.
  • There were fewer very-hard-hit balls in 2018 than at any time in the Statcast Era.
  • The difference between the number of very-hard-hit balls at home and away is as small as it has ever been.

It still remains a bit of a mystery why Arizona would want to have fewer hard-hit balls and home runs in their ballpark. Conspiracy theorists would probably argue that Martinez left and Paul Goldschmidt is gone, making the humidor something of an equalizer. According to’s Steve Gilbert, the purpose was “improving the grip that pitchers are able to get on the baseball.” Now, does anyone have any data on pitcher grip?

References and Resources

David Kagan is a physics professor at CSU Chico, and the self-proclaimed "Einstein of the National Pastime." Visit his website, Major League Physics, and follow him on Twitter @DrBaseballPhD.
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D.K. Willardsonmember
4 years ago

Very interesting. The ball/humidor/drag thing is very difficult to reconcile. My understanding is that MLB mandated humidors at all stadiums last year – largely as a response to the HR spike and the committee conclusions that it was a drag issue (my assumption on the reason). Since EVs were up slightly last year, this would seem to indicate they would have been up more without the humidor policy. But this creates a question since based on the data, the primary change in 2018 was increased drag – EVs and LAs up slightly and distance and HRs down materially. Given that you say the humidors don’t impact drag, that would seem to indicate a separate drag-related issue with the ball last year? Or, is it possible that the drag impacts of the humidor are not fully cancelling?

Alan Nathanmember
4 years ago
Reply to  David Kagan

If I recall, the measurements by the U of Colorado guys (I forget their names) showed no significant increase in the size of the ball. There is a partial cancellation regarding the increased mass of the ball: A heavier ball will carry better, as David says, but will also not be hit as hard.

D.K. Willardsonmember
4 years ago
Reply to  Alan Nathan

Thanks for both comments. This suggests drag should have decreased slightly vs. the observed increase last year(several articles written on the subject). So likely some other ball change in how they were manufactured. Guess we need another study!

4 years ago

It is incorrect that all teams were using humidors in 2018. MLB required all teams to store balls in an air conditioned room and also to monitor the temperature and humidity of storage. The decision to move to universal humidors would be based on this data for 2019. So far I have not seen that decision reported. It would be useful to know. If every team is now storing balls at the required 70 degree and 50 percent humidity many more venues will be lowering their humidity than raising it at this time of the year. Determining the overall effect could be complicated.

4 years ago

The higher water content means the balls are “mushier.”

I have six post-doctorate degrees in physics and I believe the technical term that you are looking for is ‘smooshier’. Mushier requires the ability to eat the thing in question – eg overripe apples and peaches are mushier. nomnomnom

4 years ago

This article shows that Chase played differently with a humidor, which 99% of baseball fans had expected.

What Nathan suggested, and others claimed even more aggressively, was that not only would it have an effect, but that Chase would play as a significant pitcher’s park, which it did not.

Alan Nathanmember
4 years ago
Reply to  evo34

Excellent article.

Re evo34: Actually I have never claimed..or even suggested..that Chase would play as a “significant pitcher’s park”, or any words to that effect. I only addressed change in home runs. Others may have suggested it based on my home run claims, but I didn’t. Just sayin’

John Autin
4 years ago

Re: the 20% drop in HRs at Chase Field for 2018 vs. 2017:

You don’t mention the overall 8.5% drop in MLB total HRs, as well as major drops at 11 other parks, leaving me skeptical of this whole study.

The vast majority of parks saw some drop in total HRs.
— 6 parks saw % drops at least as big as Chase did: OAK, MIA, DET, CHC, MIN, TBR.
— 5 more saw drops of at least 15%: ATL, BAL, CHW, NYM, PHI.

Whatever else your data suggest, the simple drop in total HRs at Chase Field seems misleading without that appropriate context.

John Autin
4 years ago
Reply to  David Kagan

Sorry, I don’t have any EV data. I’m just an old-school numbers guy … and a permanent skeptic. : )

D.K. Willardsonmember
4 years ago
Reply to  John Autin

As noted in my comment above, EVs increased very slightly in 2018 over the prior year on well-hit balls 99.04 to 99.14

4 years ago

This is an interesting start, but why limit the data as you have? Two teams play at Chase for 81 games: the D-Backs and their opponent. If the humidor were having an effect on the D-Backs, it should be having a similar effect on everyone who plays there, but you’ve only looked at the D-Backs’ HR data.

HR totals for all teams playing at Chase (with HR per 100 PAs in parenthesis):

2010: 201 (3.23)
2011: 173 (2.82)
2012: 174 (2.83)
2013: 149 (2.40)
2014: 148 (2.40)
2015: 155 (2.47)
2016: 221 (3.40)
2017: 215 (3.46)
2018: 171 (2.75)

Very noticeable decrease, but still higher than 2013 and 2014, so could be more variability than anything. Worth looking at trends across baseball as a result.

HR per 100 PA across the entire league (at all parks, excluding Chase)

2010: 2.46
2011: 2.44
2012: 2.67
2013: 2.53
2014: 2.27
2015: 2.68
2016: 3.03
2017: 3.29
2018: 3.03

At first glance, that shows a far bigger decrease at Chase (~20%) vs the rest of the league (~8%), which would suggest the humidor had a big impact. But we see this sort of discrepancy in other years. For example, from 2010 to 2011, Chase experienced a drop in HR per 100 PA of about 13% but the league only dropped 1%. From 2013 to 2014, Chase experienced no drop in HR rate, but the league dropped about 10%. So the drop from 2017-2018 might be the humidor, or it might be sheer variability. We need more years of data to know.

As for exit velocity, looking at such a small sample (EV > 105) might skew the data. Absolutely there were fewer super-hard hit balls, but the math would tell us that exit velocity should decrease for all batted balls, not merely the ones hit really hard.

Average exit velocity at Chase for all teams.

2015: 88.0 MPH
2016: 88.7 MPH
2017: 87.9 MPH
2018: 87.9 MPH

Shows no decrease in average exit velocity 2017-2018 for balls hit at Chase. What about if we just look at splits for D-Backs hitters:

Average exit velocity for D-Backs hitters home vs (away)

2015: 88.0 MPH (87.6 MPH)
2016: 88.5 MPH (87.6 MPH)
2017: 87.5 MPH (86.4 MPH)
2018: 88.5 MPH (88.3 MPH)

Each year, D-Backs hitters hit the ball less hard on average on the road than at home, but that difference was smallest last year, suggesting that the humidor might have leveled the playing field, at least somewhat. But given that 2015 also had a pretty narrow split, we cannot say that with full confidence yet.

What shows up pretty clearly is the well-struck ball average exit velocity (barrel or solid contact per Statcast):

2015: 103.1 MPH
2016: 103.4 MPH
2017: 103.2 MPH
2018: 102.6 MPH

and for D-Backs hitters home vs. (away):

2015: 103.2 MPH (102.3 MPH)
2016: 103.7 MPH (102.8 MPH)
2017: 103.4 MPH (101.9 MPH)
2018: 103.0 MPH (103.0 MPH)

The well-struck ball isn’t hit as hard at Chase post-humidor, and D-Backs hitters no longer have a difference home vs. away.

Bottom line: physics is physics, and the humidor changes the baseball in predictable ways. But it’s really hard to tell the in-game effect after just one year, especially when the data is somewhat inconsistent in the story it’s telling us and is still a bit noisy. We should check in at year’s end; should have a better idea by then.

4 years ago

Very interesting discussion. I want to comment on the final paragraph:

“It still remains a bit of a mystery why Arizona would want to have fewer hard-hit balls and home runs in their ballpark. Conspiracy theorists would probably argue that Martinez left and Paul Goldschmidt is gone, making the humidor something of an equalizer.”

I’m not sure if you’re saying that with fewer power hitters on the team, AZ has a greater advantage, or less of an advantage, by reducing EV via the humidor. I’ll just point that we would expect that it would work to the home team’s disadvantage. Reducing the average distance of a FB penalizes weaker hitters more than more powerful hitters, for the same reason that data apparently showed that during the HR spike beginning a couple of years ago, thought to be a product of juiced balls, moderately powerful hitters gained more from power hitters. A power hitter has a higher mean FB distance than a lighter hitter, so increasing the distance a ball will travel (or reducing the distance to the fences) should increase the % of FB clearing the fence by a lesser amount than is the case for weaker hitters. And vice-versa here: if FB distance is reduced, weaker hitters with a lesser mean FB distance, stand to suffer a greater drop in HR than is the case for power hitters.