There’s Definitely Something Strange About Citi Field
The other day, I was reading something written by Tom Verducci over at Sports Illustrated. Verducci was ultimately making an argument about Jacob deGrom and his Cy Young candidacy, but on the way there, he talked about what’s apparently been dubbed the Mystery of Flushing. The mystery concerns why the Mets can’t hit at home. More specifically, it’s about why the Mets’ team BABIP consistently suffers at home. Citi Field has been modified multiple times, but it was modified most dramatically before the 2012 regular season. Since 2012, the Mets rank last in the majors in runs scored at home. They rank seventh in runs scored on the road. And, since 2012, the Mets rank last in the majors in BABIP at home. They rank third in BABIP on the road. There’s an existing BABIP gap of 30 points. This is spanning the better part of a decade. That’s big, and that’s weird. It’s worthy of some kind of investigation.
Verducci’s article, to be clear, was missing something. He analyzed the Mets’ hitters, but he didn’t analyze the Mets’ pitchers. Since 2012, they’ve allowed the eighth-fewest runs at home. They’re in 17th in runs allowed on the road. And, since 2012, they’re 12th in BABIP allowed at home. They’re 28th in BABIP allowed on the road. Run scoring in general is harder at Citi Field. Turning batted balls into hits in general is harder at Citi Field. Mets hitters are hurt, and Mets pitchers get to benefit. But a question remains: why? Why is Citi Field so strange?
I’m going to tell you right now, I don’t have an answer. All I have is evidence that something is up. Let’s stick with that existing 2012 – 2018 data window. I hope you’re okay with that, because I’ve already assembled my spreadsheets. Over that span, relative to games on the road, Citi Field has reduced runs by 14%. It’s been the biggest run-suppressor in baseball. Interestingly, the home-run park factor is almost neutral. That was the intent when the fences were moved. But Citi Field has reduced BABIP by 8%. At home, Mets hitters have run a BABIP 30 points lower than on the road. That’s the biggest such difference in the league. At home, Mets pitchers have allowed a BABIP 21 points lower than on the road. That’s also the biggest such difference in the league. Citi Field doesn’t like allowing hits in play. Here’s all the proof you need:
Mets home games have had a BABIP about 25 points lower than Mets road games. That’s more than twice as large as the next-greatest negative effect. The other side isn’t too surprising — Coors Field is huge and it’s set at elevation, and Fenway Park has that ridiculous giant wall. You can talk yourself into understanding those. Not so much Citi Field. It’s not immediately apparent why what’s happening is happening.
This is our page for park factors. One contributing variable I can highlight: No ballpark increases infield flies more than Citi Field does. An infield fly is more or less an automatic out, which means infield flies are bad for BABIP. That helps to get us somewhere, but it still doesn’t give us a solution. Why not break down the batted balls that we’re dealing with? Above, you see the home/road BABIP split for all balls in play. Here’s the split for just grounders:
Citi Field reduces hits on ground balls, more than any other ballpark. How about now looking at just flies and liners? Remember that home runs are excluded.
Citi Field reduces hits on flies and liners in play, more than any other ballpark. Again, the other side makes sense. Coors Field allows a lot more air balls to drop in, while providing only a modest boost to grounders. Fenway Park doesn’t boost grounders at all; all the extra hits in play come in the air. Citi Field is just unusual across the board. It doesn’t matter how you hit the ball. The stadium doesn’t want you to reach.
Park factors are complicated. To really understand them, park factors are complicated. You’re dealing with wind, you’re dealing with temperature, you’re dealing with humidity, you’re dealing with batter’s eyes. I don’t know what it’s like to hit in Citi Field. I don’t know how it compares to other ballparks. Maybe it’s just tougher for hitters to pick up the spin, or something. I really don’t know, but I can point to something else. Not long ago, I wrote about Chase Field, which installed a humidor before the regular season. The main intended effect was to reduce how hard batted balls came off the bat. Based on the early evidence, mission accomplished: Chase Field doesn’t have much of a 2018 exit-velocity park factor. But let’s look at the same information again, highlighting a different ballpark. There’s a pattern here, something I initially regarded as a probable calibration error. Perhaps it’s not so.
All Statcast information comes from Baseball Savant. As you know, information exists going back to 2015. Let’s start there. For every team, here’s the difference between their home average exit velocities and their road average exit velocities, both for and against:
Citi Field, second to last. An average reduction of 1.2 miles per hour. Here’s the same plot for 2016:
Citi Field, tied for last place. An average reduction of 1.4 miles per hour. Here’s the same plot for 2017:
Citi Field, last place. An average reduction of 1.0 miles per hour. Here’s the same plot for 2018:
Citi Field, second to last. An average reduction of 1.0 miles per hour. At last, here’s the overall plot for 2015 – 2018:
Citi Field, last place. An average reduction of 1.1 miles per hour. At first I didn’t think much of it, because I couldn’t think of a good explanation. I still don’t really have a good explanation, but this lines up well with all the other information. Why might Citi Field act as such a hit-suppressor? Because it looks like it might well be an exit-velocity-suppressor. For some reason, batted balls in Citi Field don’t come off the bat as hard as they do in Mets road games. I know it seems like one mile per hour or so isn’t significant, but it can make a pretty huge difference over the course of a year. Just look at this year’s change in Arizona. Maybe it’s something about the baseballs, or maybe it’s something about the batter’s eye. Could be something else. But Citi Field has been working against the offense. Hasn’t mattered whether it’s been the Mets or their opponents.
I think I can mostly understand why the Mets’ hitters have felt it worse than the others: The Mets, over this span of time, have had a fly-ball-oriented offense, which has left them a little more vulnerable to the park effects. So it goes. Such things tend to be cyclical. What I’m most interested in understanding is why Citi Field seems to reduce batted-ball speed. When the Mets brought in the fences, they increased homers at the expense of other hits. That makes sense — the surface area of the field was reduced. But why does BABIP suffer so? Why does Citi Field look like such an anomaly? What’s happening in this part of New York that isn’t happening in the other part of New York? Again, I don’t have the answer. I’m not certain that anyone does.
But the Mets would be well-suited to figure something out. I’ll give you one more image before I go. Since 2012, here’s the home/road split in team winning percentage:
I mostly just put that in for a laugh. But either the Mets need to build a better team for the ballpark, or they need to understand what the ballpark is doing. This has all been going on long enough, and this is literally the place the Mets call home. One’s home oughtn’t be so mysterious.
Jeff made Lookout Landing a thing, but he does not still write there about the Mariners. He does write here, sometimes about the Mariners, but usually not.
Been there as fan dozens of times and I don’t have any theories, or even heard any theories after they moved the walls in from the original configuration.
The old Shea used to have a weird vortex wind that would knock back high fly balls. I haven’t noticed that at Citi. There doesn’t seem to be anything in center that would distract a hitter. The weather couldn’t be much different than the Bronx and not at all compared to Shea.
Citi Field is surrounded by a lot more water than Yankee Stadium is, and there’s a lot of weird wind patterns at Willet’s Point. Jets players used to complain about it a lot because the wind would mess with their throws. But Shea was open in the outfield, whereas Citi has a lot of stuff back there. I do remember hearing rumors that the Mets were too cheap to do a wind study before building Citi which sounds plausible but I can’t find anything about from a quick google search (but no shortage of articles about the Yankees doing a wind study). I do know that they planned the orientation of the ballpark so that it would fit in the Shea parking lot without taking up too much parking while being built (they also failed to take advantage of the beautiful sunsets over the Manhattan skyline, but I digress). So I am guessing that the wind has a lot to do with it. Maybe they need an anti-humidifier. A dryer?
Ahhhhh, here is the article I was looking for:
https://www.nytimes.com/2009/02/07/sports/baseball/07field.html
“Striking a balance between a hitters’ haven and a pitchers’ delight is a difficult task, one made more challenging by wind patterns. Other teams, including Philadelphia and Cincinnati, commissioned wind studies before completing the outfield dimensions of their stadiums.
Jeff Wilpon, the Mets’ chief operating officer, said the team elected not to because they felt that there were too many variables — the three-tiered stands in left field, the administration building beyond the concourse in right-center and the right-field overhang, for starters — that could diminish the value of the findings.”
Maybe they uhh, should have spent the money on that wind study?
Wind patterns definitely seem like the only obvious factor that could be significantly different from Shea (hard to imagine humidity and so on changing much when you move the park by only 500 feet, though of course Shea was also a significant pitchers’ park). Citi is oriented differently from Shea, too — about 90 degrees off — so you could easily imagine that the prevailing wind coming off Flushing Bay might do more to cut down fly balls.
But on further thought, there’s another possibility — the humidity of the room where the balls are stored could easily be different. Maybe it’s in the basement and it just doesn’t drain as well as the equipment room at Shea did. Maybe the answer here is as simple as Citi Field being built with an effective, but unintentional, humidor.
That is a good point as well. My summer experience living in Queens generally ranges from Swampy to Soupy, so there’s a good chance if the baseballs aren’t stored in the right place that they are over hydrated. If the Mets were smart they would look in to that, much easier to fix ball storage than it is to reorient a whole ballpark.
If the Mets were smart? Let’s not raise the bar that high, we’ll only set ourselves up for disappointment.
Humidity can explain the exit velocity drop, but wind can’t…
mmmm, whatever could suppress exit velocity…and save money. Save money…reduce exit velocity…who needs ‘new’ baseballs. haha, just a thought. The Mets are too cheap to continuously replace baseballs so those you see the catcher hand to the umpire that then go to the ballboy get polished and come back out for use again:)
Maybe the mound is not 60/6? Maybe it’s just 60? Maybe the mound height is higher? Batter’s eye being bad is a good idea, but I don’t think it’s bad….If the mound was higher would that result is more pop ups possibly?
What a mystery. Great piece.
I think the size and composition of Mr. Met’s head throws something out of whack.
Could it be related to the batter’s eye? Proximity to an airport? Is there a difference in air density due to population density, pollution, taxi exhaust? So many avenues to explore and all seem fruitless.
Chemtrails!
Citi Field is not far from the Flushing River and right next to a river. Would humidity at sea level next to a body of water have an affect on offense?
Yankee Stadium should be similarly affected. I doubt the humidity or even the thermal layering of the atmosphere above Citi is materially different than above Yankee Stadium. For that matter, just off the top of my head, here’s a list of non-dome parks that are close to significant (where “close” is defined as “within 5-10 blocks” and “significant” is defined as “a lot more than the Platte, which runs by Coors and usually carries less water than the average shower”):
– AT&T (duh)
– Oakland (people forget, but it’s about two home run’s distance from the Bay)
– Yankee Stadium
– Citi Field
– Fenway (depends on your definition of “block”; Bostonians appear to have no idea what one is, so YMMV)
– Cleveland
– Wrigley
– Safeco
– Comerica
– Rogers Center
– Nationals Park
I *believe* that’s all of them. I wasn’t 100% sure about Minute Maid so I didn’t include it.
Anyway, there’s not a lot of correlation here to anything in the article. I don’t think water in close proximity has too much of a consistent impact. Extremely specific localized phenomena (such as the marine layer at Dodger Stadium, AT&T, and Safeco; or the lake effect at Wrigley) definitely has an impact, and maybe that’s part of what’s happening at Citi, but I don’t think it’s being caused by the river alone.
Edit: Oh yeah, nearly all of the above parks are at or within rounding error of sea level (e.g. Wrigley is at 578′ above sea level, the highest of these by a wide margin, but still not particularly high).
There is a difference between the wind off the water of the LI Sound/Flushing bay compared to the fairly small Harlem River. Shea always had a reputation for crazy wind that Yankee Stadium never had – check out this article from 1975 I stumbled across about football players in the wind. They may be in the same city, but the exact location makes a huge difference.
https://www.nytimes.com/1975/12/02/archives/its-an-ill-shea-stadium-wind-that-blows-namaths-passes-jets-are.html
Busch stadium is pretty close to the Mississippi as well.
PNC as well.
“Extremely specific localized phenomena (such as the marine layer at Dodger Stadium, AT&T, and Safeco; or the lake effect at Wrigley) definitely has an impact, and maybe that’s part of what’s happening at Citi, but I don’t think it’s being caused by the river alone.”
Was thinking of Laguardia Airport. So I followed your methodology (Stadium to Airport).
Searched Google: Result: Reddit Article: Which MLB stadiums are the easiest to get to from the airport?
Top Answer was Citi.
I smirked.
And don’t forget about Angel Stadium being next a river: http://upload.wikimedia.org/wikipedia/commons/9/9b/Santa_Ana_River_Anaheim.jpg
(In all serious though Great American in Cincinnati is close to the Ohio River as well).
That river is not so much a river as it is a dry lake bed that serves as a shanty town for the local homeless community. The Pacific Ocean is also really close, though.
Seems like they might have a secret humidor (pure conjecture). Maybe when Harvey-deGrom-Syndergaard-Wheeler-Matz came up, they wanted to inflate their pitchers’ numbers?
Perhaps it’s the same gravitational vortex that causes the Mets’ injury curse?
The Mets got a bunch of little-league baseballs on discount.
My only theory is that maybe the field wasn’t designed to full scale. I could see the Wilpons skimping on the field design and the foul lines aren’t at right angles, so that there is less square acreage in Citi’s outfield. Hey, it’s a theory!
I know you say that in jest, but it should be noted that the Wilpons were only thrifty after the Madoff scandal. At the time they built Citi Field, the Wilpons were heavy spenders.
Citi Field opened in 2009. The Mets had the 2nd highest payroll in baseball that year, behind the Yankees. They also had the 2nd highest payroll in baseball in 2008, behind the Yankees. 2007, they were behind only the Yankees and Red Sox. The Wilpons consistently had a top 2-5 payroll leading up to Madoff, and were only ‘cheap’ if you compared them to the Yankees. Back then, the Yankees were way ahead of everybody else.
fancy meeting you here Dave.
Please explain to me how a ballpark affects infield flies. Also a little cloudy on how it affects BB and K. Thanks!
There are a lot of elements in the physical environment that can affect all of them, but the most obvious as far as BB and K park factor is the size of foul territory.
Infield fly rate is affected by the size of foul territory (if it goes into the stands, a popup is a strike and isn’t recorded as a batted ball at all). Groundball BABIP is affected by the quality/consistency of the infield dirt (predictable bounces) and the length of the grass (shorter grass = faster rolling). Flyball BABIP is affected by the outfield dimensions (generally flyballs that don’t clear the fence are caught, so HR/FB% and BABIP on FB’s usually have an inverse relationship).
Then there is the broader stuff that affects every batted ball. Environmental factors (temperature/humidity/elevation/wind). The batter’s ability to see the ball (the batter’s eye). Wrigley is likely affected by the fact it hosts so many day games.
Like Jeff said, it’s complicated.
The infield fly rates being the highest at Citi than any other park remains the most intriguing and puzzling data point for me. While Yirmi’s point is correct on its face, regarding how foul territory can impact infield fly rate, it doesn’t make sense here. One would expect that parks with large foul territory would have higher rates than those with small foul territory. As I understand it, Citi field is a park with relatively small foul territory areas, so the infield fly rate is that much more mysterious.
No, you have that backwards. Larger foul territory means a higher rate of foul outs, and therefore lower rates of outs on fair balls.
maybe it’s the batters eye where the homerun apple is. I would imagine there would be a line around where a righty delivers the ball.
My Local knowledge (Born and Bred in Central Brooklyn) tells me that (1) They are close to a windy bay and (2) Because large amounts of planes fly overhead and/or nearby (LGA); Ballpark “base rate” of a google search is around 20th busiest airport in the US.
But that is just my hypothesis. Interesting data.
Excellent article presenting a real, well-documented baseball mystery. Well done. Thank you.
On the day this article gets posted, the Mets are winning 13-0 at home through 7 innings. The ole reverse jinx is on.
Clearly if Rockies hitters get penalized for hitting awards, then Mets pitchers should get penalized for pitching awards.
Based on your username, I’m assuming you’re making a dig at deGrom’s Cy young candidacy. If you look at his home and away splits this season, while he’s *slightly* better at home, the difference is negligible. But realistically they’re both having world class seasons. I honestly think Jake’s narrative of being the ‘unluckiest’ pitcher in history might put him over the top, but I wouldn’t be salty if Scherzer won it. Nola seems to be dropping out of the race.
Just because he has near identical home/road splits doesn’t mean he wasn’t significantly helped by Citi field.
Not sure why this is getting downvoted. It’s true. Single season splits are too small a sample to be meaningful. And, unless there’s some reason a particular player would *not* be helped by particular park factors, I would assume they’re still making a difference.
I would assume deGrom’s 1.66 home ERA would be more like 2.00 if he pitched those same games against the same opponents in a more neutral park. His away ERA doesn’t tell us anything about this.
For his career, DeGrom has a 2.18 ERA / .248 wOBA at home, and a 3.22 ERA / .290 wOBA on the road. Big splits.
The fact that there’s less of a gap in 2018 doesn’t mean deGrom suddenly stopped benefiting from Citi Field. It means small samples are small samples.
The idea of some kind of unintentional humidor effect is an interesting one. However, one data point that doesn’t intuitively fit that explanation, to me, is the high level of infield flies. When Chase Field went to a humidor, did the rate of infield flies jump up? This might shed some light on this potential explanation.
Do people forget about Houston? We won a World Series last year, and nobody seems to know/care about why their home ballpark doesn’t play like the hitters’ park everyone makes it out to be.
Can someone people help?
Presumably Houston makes up for BABIP suppression by yielding a lot of cheapo homers to right-handers. I’m sure the deep straightaway center field plays into it somehow too.
Also that whole “let’s put a giant flag pole in the field of play“. Would have loved to be in that design meeting. “No, a flag pole is too dangerous.” “Oh, you might be right. What if we put it – hear me out – on a hill?”
yes i know that is removed now and is irrelevant, it just seems so absurd to me that i can’t help it
Well, in the sixteen seasons the hill was there, zero (0) players were hurt. So I understand a lot of people (baseball fans) are not athletic, but hills aren’t really that big of a deal for players. Especially considering said incline was 420 feet from home plate.
@nb, be as it may that LF is a short porch, it’s far from being the only stadium that provides “cheap” home runs. I am more so considered with the overall run suppression there and why it players like a pitchers’ paradise despite having such a notorious reputation to be a hitters’ haven.
But it just seems like national writers avoid Houston at all costs, so maybe I should just do my own analysis.
Interesting article Jeff. Couple of points from a Mets fan who has been on the field several times at different charity events:
1) The hitter’s backdrop is fairly awful. There is a huge scoreboard in dead center with breaks between the left field and right field stands. While there is a black backdrop about 20 feet high in dead center, the electronic scoreboard is above that and can easily affect a hitter’s view, especially from tall (Syndergard-like) pitchers coming over the top.
2) The black backdrop is off center and a lefty is releasing his pitch in front of seats that are always occupied.
3) The wind from the river behind the stadium blows around the scoreboard, which means fly balls to both right and left center are impacted, lowering BABIP as more balls are caught on the warning track.
4) The pitcher’s mound is really, really high which to me means balls come in just a little higher than other stadiums, which leads to more infield flies, which means a lower exit velocity off the bat, and lower BABIP.
It would be So Mets if we found out that the mound was not actually the correct height.
More like the RS, right up there with stealing signs.
My immediate thought was also the batter’s eye. A lot of these are inadequate and/or overly quirky.
The one I have noticed the most is vs. LHP in Fenway (esp. RHB) – the ball is basically coming right out of the white shirts in the RCF bleachers (I’m sure the Sox know this, which might have influenced the lefty-heavy staff).
LHP’s vs RHB’s at Fenway has to be one of the most offense-friendly splits in the majors. Even if you’re right about the batting eye, that wall 300′ away more than makes up for it.
Hey, that first one is testable!
I downloaded all the wonderful splits by pitcher from here. 2012-2018, I’m getting the same .025 drop in BABIP at Citi and IFFB% +.017. Baseball Prospectus gives us pitcher’s listed height (such as it is)…
This data doesn’t confirm the hypothesis, though. Whatever effect there is, pitchers at middle heights seem to have the biggest impact, with less change in BABIP for both short and tall…
Breaking it down:
Short (=77″): 14% of BIP; -.017 BABIP; +.006 IFFB%
Hmm, looks like the table didn’t come through, but essentially short pitchers (=77″) showed the smallest impact to BABIP at Citi while medium height (73-75″) showed the biggest. Also, lots of pitchers are listed at 76″ and had a slightly smaller impact than the 73-75″ group.
delete
This article is fantastic and incredibly fascinating.
I only have two questions:
1. Is there any evidence that this was or was not an issue before 2012?
2. When the Mets moved the walls in 2012, didn’t they change the colors of the walls too? I seem to remember that being the same time they changed everything to blue… I wonder if the color is visually disruptive to hitters.
Could it be LGA?
Is it possible to run an estimate of whether the exit velocity delta explains the BABIP delta? Take each ball’s speed and angle, record its hit probability with actual speed and with a what-if higher speed. Enough added hits to make up the BABIP?
Great article.
One thought, seeing Miami and Houston down by Citifield in the charts, and knowing both cities are quite humid, I wonder if it is worth computing different park factors for retractable dome stadiums, an open and a closed roof park factor, yes single-season numbers might result in a small sample size but wondering if 3-year numbers might yield something interesting. Also wondering for Citifield to try and tease something out by looking at day-night and/or monthly numbers over the past 3-years, to see if any of that could explain the latest Mets’ Mystery.
I think it’s worth noting that Dodger Stadium is almost the exact same measurements at every spot, so comparing the two stadiums directly would help. Dodger is 2 spots behind Citi in Home – Road BABIP, but in every other stat here Dodger’s middle of the pack. So if it’s not the walls or foul territory, it could be…
1. Grass/dirt: no idea, never been on the field, no one’s ever asked about the dirt. The grass is the most common variant of grass in baseball (https://www.baseballpilgrimages.com/grass-at-major-league-ballparks.html )
2. Air: Queens is different from the Bronx in the water (which determines the wind); LI sound and Flushing Bay very different than the Harlem River. Considering Moltar’s news on there never being a wind study, seems plausible. Can’t find any studies on the wind since its opening either.
3: Humidity: Queens is, in fact, more humid than the rest of NYC. Not sure by how much, though. 14 people got Legionnaire’s disease in 2017, by the way, which only happens in very humid environments. And you can ask anyone who’s been to the US Open, Flushing is gonna be humid a LOT.
Final verdict: fuck if I know. As someone who’s watched almost every Mets game since mid-2015, I can say that I haven’t noticed misjudging a lot of fly balls becoming/not becoming home runs. So for that, I want to say it’s not the wind. (Plus I’d probably have felt it in the outfield nosebleeds, and people sitting around the Great Wall of Flushing would’ve said something.) So… probably the humidity?
*shrug*
(Also, we can monitor this over time. So if the humidity gets higher and the Home-Road split gets worse, well, the Mets have become the first team to suffer because of global warming.)
Also, I compiled every significant, multi-season Met’s stats from the Citi Field era. The only hitters who’ve been able to consistently hit better or equal at home have been Cespedes, Wright, Duda, Conforto, Wilmer, TdA, and Nimmo. What do they all have in common? Their bats are well above-average in power (with the exception of Wilmer). Grandy would be the only guy with power from the Citi Field era who did consistently better on the road. Some of them have smaller sample sizes and some had poor BABIPs in a split or two, but it seems like a theme.
The most penalized for hitting at Citi? Rosario, Murphy, Lagares. These are/were guys with speed and whose wRC+ would be driven on their ability to get base hits.
This is a very roundabout way of saying, yes, Citi doesn’t suppress HR but does base hits. But based on how power hitters perform there, it seems that it either doesn’t suppress XBH, or it’s a bandbox once the ball is hard enough in the air.
This doesn’t make sense to me. If it suppresses exit velocity, it probably suppresses homers and extra-base hits. Whatever causes that is probably also why the fences had to be moved in.
I believe the fence was moved in just cuz 390 ft. itself was suppressing HR (and there was a spike in HR after they moved it in). I didn’t look at the players’ HR totals, but it was pretty reliable that the “sluggers” weren’t too penalized by the stadium. I’ll definitely look at HR totals on home/away splits for the players in the sample, though.
It really seems like it has to be something wrong with batters eye making batters late on everything. Idk what else could increase popups and suppress exit velocity.
To me the most likely answers by far are humidity or batter’s eye. Chase Field obviously was a hitter’s haven in large part due to the lack of humidity making the balls drier and bouncier, but it also has one of the best batter backdrops in baseball – a black 30 foot tall fence all the way across CF. Many, many hitters have mentioned loving it there because of that.
Of course, it’s the Mets so the answer is probably that they store their baseballs in a sauna or something stupid like that.
**EDIT: the other possibility is foul ground. Have no idea where Citi Field ranks in that regard but more foul ground should suppress BABIP.
You know, you’d think more teams would deliberately mess with targeted parts of the batter’s eye to help their own pitchers over others, going on handedness and release point.
Seems it might have worked for Jered Weaver.
I know this is a bit late (great article BTW) but I think I have a reasonable explanation that you might find interesting. If you look at the amount of drag at Citi relative to other parks, it doesn’t really stick out. So it really can’t be wind or humidity or the balls. If it were any of those, it would show up in the drag factors (see article here https://www.fangraphs.com/community/the-mystery-continues-has-the-2018-ball-been-de-juiced/).
What does really stick out is the extreme IFFB rate for all hitters. It is really off the charts different. It would seem to me that it has something to do with vision/batters eye, etc. The reason balls are coming off the bat slower is because the hitters aren’t seeing the ball as well and swinging less aggressively AND the quality of the contact is also lower (really high IFFB rate). For example, if you look at EV based on strike counts on well hit balls, it is clear hitters are less aggressive as the strike count increases – 99.3 for no strikes vs. 98.2 for two strikes (only well-hit >90 mph). On all balls, EV is 89.0 for no strikes and 87 for two strikes which takes into account both the factors above (this is across all stadiums). I thought that maybe there were higher strike counts at Citi due to more foul balls but this didn’t turn out to be the case (at least due to a higher foul ball rate). The average strike count was slightly higher at Citi but it didn’t rank near the top of the list. So it is likely some “field condition” but probably not an atmospheric one.