Examining Home Run Rates by Ballpark

At the beginning of May, I wrote two articles about the slightly-deadened baseball’s effect on league-wide home run rates. The conclusion was pretty much exactly what you’d expect: A bouncier ball with more drag did reduce home runs, particularly among softer-hit balls at lower launch angles. In 2019, these events were the wall scrapers that barely went out of the yard. In 2021, these events are now doubles and outs, with the increase in fly outs likely contributing (at least somewhat) to baseball’s diminished run environment overall.

There were a handful of outstanding questions that I still had, one of which was the impact of the new baseball on a ballpark-by-ballpark basis. Though league-wide trends are certainly an interesting and informative way to see the effects of a new baseball on run scoring, it is also important to examine in which parks hitters are having a more difficult time getting the ball into the seats. That allows us to understand better how park effects may have been altered to different degrees as a result of MLB’s switch to the new baseball.

But it’s not just the baseball that is contributing here. MLB reportedly added humidors to five stadiums for the 2021 season, bringing the total league-wide to 10. The Rockies, Diamondbacks, Mariners, Mets, and Red Sox already had humidors in their stadiums pre-2021, but which five teams are new to that list has yet to be disclosed. We can only guess which parks now have them, but it is important to keep in mind that the ball is not the only difference.

Also important to remember when looking at ballpark-level data: The players on the home team make a huge difference in determining home run rates. It’s entirely possible that, between 2019 and ’21, a team added home run hitters to its lineup or acquired home run-adverse pitchers for its staff, or the opposite could also be true. To mitigate these effects, I only analyzed a specific slice of fly balls: those hit at an exit velocity at or above 95.0 mph, at an exit velocity below 110.0 mph, and at a launch angle below 30 degrees — the very fly balls most impacted by the new baseball in my prior analysis. I also only included fly balls hit in games on or before May 31 to control for weather effects. (That is why I am comparing 2019 to ’21.)

This is a fairly large range that is impacted by quality of hitters and pitchers. Better hitters might be hitting more 29-degree, 109.9-mph fly balls, and worse hitters could be hitting more 23-degree, 95.1-mph fly balls. I compared the samples of 95–109 mph, less-than-30 degree data from 2019 to ’21 to evaluate just how pronounced these effects were and found that average exit velocity did not shift by more than 1 mph and that average launch angles did not shift by more than 1 degree. For transparency’s sake, here are all 28 parks’ average exit velocities and average launch angles on fly balls within this range in both 2019 and ’21, excluding the Blue Jays and Rangers, who played in different stadiums in 2019 than they do currently.

Fly Ball Data by Park, 2019 vs. 2021
Park Team 2019 EV 2021 EV EV Diff 2019 LA 2021 LA LA Diff
Chase Field ARI 101.4 100.8 -0.5 26.6 26.7 0.1
Truist Park ATL 102.5 102.6 0.1 25.8 25.6 -0.2
Camden Yards BAL 102.1 102.4 0.3 25.4 26.4 1.0
Fenway Park BOS 102.1 101.3 -0.8 25.3 25.5 0.3
Wrigley Field CHC 102.9 101.9 -1.1 25.6 25.7 0.1
Great American Ballpark CIN 100.8 102.0 1.1 26.4 26.1 -0.3
Progressive Field CLE 102.9 103.1 0.2 25.8 25.5 -0.3
Coors Field COL 102.5 101.6 -0.9 25.1 25.1 -0.0
Guaranteed Rate Field CWS 102.9 101.7 -1.2 24.9 25.7 0.8
Comerica Park DET 102.6 102.0 -0.6 25.8 25.8 0.0
Minute Maid Park HOU 101.7 101.2 -0.5 25.7 26.1 0.4
Kauffman Stadium KCR 102.9 102.1 -0.8 26.1 25.7 -0.3
Angels Stadium LAA 101.7 103.2 1.5 26.1 26.1 0.0
Dodger Stadium LAD 101.6 101.5 -0.1 25.7 25.9 0.1
loanDepot Park MIA 101.3 101.8 0.4 25.6 25.3 -0.4
American Family Field MIL 101.9 101.6 -0.4 25.6 25.7 0.1
Target Field MIN 102.5 102.9 0.5 26.4 26.0 -0.4
Citi Field NYM 101.3 101.3 -0.0 25.9 25.7 -0.2
Yankee Stadium NYY 102.4 102.1 -0.3 25.5 25.8 0.2
Oakland Coliseum OAK 102.2 101.4 -0.8 26.0 25.8 -0.2
Citizens Bank Park PHI 102.6 102.4 -0.3 25.7 25.8 0.1
PNC Park PIT 101.7 101.8 0.1 25.8 26.0 0.2
Petco Park SDP 101.5 102.1 0.6 25.9 26.1 0.2
T-Mobile Park SEA 102.2 101.5 -0.7 25.7 25.8 0.1
Oracle Park SFG 101.5 102.8 1.3 26.0 26.7 0.7
Busch Stadium STL 101.7 101.1 -0.6 25.7 25.9 0.3
Tropicana Field TBR 101.5 101.9 0.4 26.5 26.2 -0.3
Nationals Park WSH 101.2 102.5 1.3 25.4 26.0 0.6
This includes all fly balls hit between 95–109 mph and at a launch angle of less than 30 degrees in these parks by batters on both teams that played here during games played on or before May 31 in each year.

The chart is sortable, so you are able to examine any changes in fly ball quality within this broader 95–109 mph exit velocity, less-than-30 degree launch angle group as you wish. I did attempt to examine the impacts, if any, of these exit velocity changes on home run rates, and there was a moderate correlation (r-squared of 0.28) that is important to keep in mind. (The r-squared between launch angle difference and home run rates was just 0.01.) Even still, after using a model to fit the change in home run rates of each ballpark to account for any exit velocity changes, there was quite a significant difference for certain parks that is more likely to be the result of non-player effects, like the baseball or the addition of a humidor.

First, here are the raw home run rates for these types of fly balls for each ballpark from 2019 to ’21. I ran a difference in proportions test to compare the rates from both years; parks highlighted in red experienced a significant change at the alpha = .01 level, and parks highlighted in yellow experienced a significant change at the alpha = .05 level. The table is sorted by raw percentage difference, but the statistical tests incorporate sample size, which is why certain rows may have smaller differences but are still significant at a more extreme alpha level:

Home Run Rates by Park, 2019 vs. 2021
Park Team 2019 HR% 2021 HR% Difference P-Value
Oakland Coliseum OAK 67.5% 35.8% -0.317 0.000
Busch Stadium STL 50.0% 22.9% -0.271 0.000
American Family Field MIL 66.7% 39.6% -0.270 0.000
Kauffman Stadium KCR 55.6% 30.0% -0.256 0.000
Camden Yards BAL 75.3% 50.0% -0.253 0.000
Dodger Stadium LAD 72.3% 47.6% -0.247 0.000
Citi Field NYM 50.0% 28.9% -0.211 0.015
Guaranteed Rate Field CWS 63.5% 43.5% -0.199 0.002
Coors Field COL 55.7% 37.1% -0.186 0.003
Comerica Park DET 49.1% 31.7% -0.175 0.010
T-Mobile Park SEA 59.4% 43.1% -0.163 0.011
Minute Maid Park HOU 65.5% 50.0% -0.155 0.006
Progressive Field CLE 67.8% 53.3% -0.145 0.055
Citizens Bank Park PHI 62.3% 47.9% -0.144 0.057
Great American Ballpark CIN 70.7% 57.8% -0.129 0.033
Angels Stadium LAA 71.7% 60.0% -0.117 0.093
Wrigley Field CHC 57.4% 47.1% -0.104 0.107
Yankee Stadium NYY 51.1% 40.7% -0.104 0.165
Truist Park ATL 63.8% 55.6% -0.082 0.219
Target Field MIN 53.7% 45.8% -0.079 0.181
Petco Park SDP 58.8% 55.1% -0.037 0.701
Tropicana Field TBR 50.9% 48.9% -0.020 0.897
Fenway Park BOS 39.5% 38.1% -0.014 0.924
loanDepot Park MIA 44.0% 43.2% -0.008 1.000
PNC Park PIT 54.3% 54.2% -0.002 1.000
Chase Field ARI 42.3% 44.9% 0.026 0.824
Nationals Park WSH 44.8% 53.8% 0.090 0.243
Oracle Park SFG 37.0% 52.5% 0.155 0.061
Among fly balls hit at or above 95 mph, below 110 mph, and at a launch angle of less than 30 degrees during games played on or before May 31 in each year.

What’s interesting to note here is that 25 of the 28 parks listed experienced some level of decrease in home run rate, which aligns with the broader trend that I broke down about last month. It is also interesting that Oracle Park in San Francisco has seen a 15.5-point bump in home run rate, likely due to the Giants moving their fences in for the 2020 season. If we’re comparing 2019 to ’21 data, that change would explain the large positive difference in home run rate there.

On the other end of the spectrum, look at Oakland, which saw a drop of almost 32 points in home run rate on these “wall-scrapers.” If you plot these types of fly balls in a spray chart, the difference is noticeable to the naked eye. In 2021, there are more fly balls in play compared to 2019, many of which were ultimately caught for an out (indicated in magenta):

Similar trends can be observed for other ballparks as well. Here’s Busch Stadium, for example:

To mitigate the effects of player-specific changes driving these shifts in home run rates, I built a simple model that incorporated a 2019–21 change in average exit velocity to the associated change in home run rate. Without getting too deep into the mathematical details, some parks — like Oakland Coliseum and Busch Stadium — still had significantly large residuals, meaning that even after accounting for any exit velocity changes, the difference in home run rate was notable. Exit velocity is not the only way to mitigate player-specific effects, but it is an important factor to keep in mind when thinking about park-specific home run rates.

Ultimately, the answer to why certain parks might have been more impacted by the new run environment than others is difficult to unpack. As mentioned, we don’t know for sure which parks added the humidor. Additionally, the interaction between outfield space and outfield wall height may also play a role, as could climate and, more specifically, weather so far this season. And, as always, there’s definitely randomness in here as well. But perhaps more than understanding why we’re seeing such significant changes at the ballpark level, it’s first important to acknowledge that some huge differences exist in how the baseball (and potential humidor) is affecting how the ball flies in each environment.

Devan Fink is a Contributor at FanGraphs. You can follow him on Twitter @DevanFink.

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2 years ago

This is a fascinating thing but I do think you would need to take into account the “getting better pitchers” / “getting worse hitters” aspect of this in a more direct way. The Orioles got way, way better pitchers; the Mets thumpers have all been injured; the Brewers both lost all their thumpers and their pitching got way better. But there are a few stadiums that are harder to explain. These are St. Louis, Seattle, and Cincinnati. What’s going on there is straight-up weird.

2 years ago
Reply to  Devan Fink

Could there also be a weather issue of comparing a full season to a partial one? Sorry if I missed it, but is the 2019 data just the first two months as well?

2 years ago
Reply to  sadtrombone

I agree that personnel changes are a tough confounder for park analysis to overcome. Though as a frequent viewer of Brewers games, I can attest that I’ve seen far more balls that appear to be hit “on the screws” (or thereabouts) go nowhere this year. Visual contact quality that chased outfielders back toward the warning track the last few years is now resulting in outs caught by stationary fielders. Even the crack of the bat oft proves misleading. As my eyes and ears continue to fool, it’s hard not to presume more external forces are at play.

All this is to say I would be pretty shocked if AmFam/Miller is not one of the stadiums using a humidor.

Smiling Politely
2 years ago
Reply to  g4

I feel this way about Dodger Stadium, too (and the data! wow!), but in my most paranoid state, I cannot imagine anyone thinking, “Let’s secretly install humidor *in Los Angeles*”

2 years ago
Reply to  sadtrombone

Not counting openers and ranked by WAR, the White Sox rotation in 2019 (starts):

Lucas Giolito (29)
Reynaldo Lopez (33)
Ivan Nova (34)
Carlos Rodon (7)
Dylan Cease (14)
Hector Santiago (2)
Odrisamer Despaigne (3)
Dylan Covey (12)
Ross Detwiler (12)
Ervin Santana (3)
Manny Banuelos (8)

And in 2021:

Carlos Rodon (9)
Lance Lynn (9)
Dylan Cease (11)
Lucas Giolito (11)
Michael Kopech (3)
Dallas Keuchel (11)
Jimmy Lambert (1)

Which group would you expect to give up more homers?

2 years ago
Reply to  sadtrombone

Better pitchers/worse hitters might be a factor, but what is the coefficient next to that variable? A statistically significant change in pitcher and hitter quality in just 2 years seems unlikely.

The ball seems to be the story here. Oakland with a 32% decline in HR% on hits with that LA/EV…6 teams with a >25% decline… These are astounding numbers.

2 years ago
Reply to  msl216

Give me a break. Pitchers aren’t any better. This is ALL on the hitters. When you swing for a home run EVERY time, because you stupidly think that “exit Velocity is a stat that matters, you are going to miss a lot of times. Batters of past generations were SMART. They would shorten up their swing with 2 strikes, just trying to make contact. A base hit is better than a strikeout, after all. Or, if they only need a base hit to win, or a sacrifice fly or slow ground ball. And the bottom of the order rarely tried to hit a home run. And if a team was stupid enough to leave one entire half of the infield wide open for you? Thank you very much for the free bunt base hit. Don Mattingly sure as hell wouldn’t se stupid enough to hit into a shift.

Nolan Ryan would legit have 7500 strikeouts, 12 no hitters and AT LEAST one 25 strikeout game if he pitched today and were allowed to stay in as long as he did. Not because he’d be better. ALL because the hitters are idiots. Just another example of how analytics is destroying the sport for many many fans. There is nothing exciting about strikeouts and home runs. Even the home runs are very fleeting excitement, followed by bases empty and more strikeouts. Even pitchers’ duals aren’t exciting anymore because they usually are the result of bad hitting and not great pitching. A team striking out 15 times in a game is not a rare occasion when they ran up against great pitching. It is something that happens to almost every team once a week. and too often in 2-1 games or 1-0 games, it is not pitchers painting the black and keeping the batters’ off balance. It is hitters swinging at balls that bounce 8 inches in front of home plate because they decided before the windup that they were going to swing.