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.)
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