Two New Ballparks Enter the Villa

For the Tampa Bay Rays, it was the fearsome power of nature; for the Athletics, the whims of a greedy doofus. But while the cause may vary, the outcome is the same: Both teams will play all 81 of their home games this season in minor league parks. The A’s will set up shop at Sutter Health Park, also known as the home of the Triple-A River Cats; the Rays’ address is now George M. Steinbrenner (GMS) Field, the erstwhile environs of the Single-A Tampa Tarpons. (The River Cats will share custody, while the Tarpons will move to a nearby backfield.)

This is suboptimal and sort of embarrassing for the league. But it does present a compelling research question: How will these parks play? According to the three-year rolling Statcast park factors, the Oakland Coliseum and Tropicana Field both qualified as pitcher-friendly. The Coliseum ranked as the sixth-most pitcher-friendly park, suppressing offense 3% relative to league average, while Tropicana ranked as the third most, suppressing offense around 8%. Where will Sutter Health and GMS Field settle in?

I started by looking at how each park played in their previous minor league season. Over at Baseball America, Matt Eddy calculated the run-scoring environment for each ballpark in the 11 full-season minor leagues. Eddy found that Sutter Health ranked as the most pitcher-friendly Pacific Coast League park by far in 2024, allowing 31% fewer runs than the average PCL park. GMS Field played closer to neutral compared to its Florida State League peers, but it did significantly boost home runs, particularly to left-handed hitters.

But these minor league park factors can only reveal so much. They tell us, for example, that Sutter Health played as pitcher-friendly in the PCL, but the PCL is the most extreme run-scoring league in affiliated baseball, populated by high-altitude launch pads like El Paso, Albuquerque, Las Vegas and so on. GMS Field might be neutral for the Florida State League, but — by definition — all of those teams are in Florida.

Park factors are always calculated as a function of the peer group of ballparks. As the Statcast team writes on their park factor page, 100 is set to league average, and “the park-specific number is generated by looking at each batter and pitcher, controlled by handedness, and comparing the frequency of that metric in the selected park compared to the performance of those players in other parks.” In other words, to better project the major league park factors for these two new stadiums, they need to be placed in a major league context.

That might start with analyzing the dimensions of each park. Fenway Park plays as hitter-friendly because hitters bang bushels of doubles off the Green Monster; home runs fly out of Cincinnati’s Great American Ballpark in large part due to the shallow targets in the power alleys. As Martin Gallego wrote in his story on the subject for MLB.com, the Sutter Health dimensions are roughly similar to those of the Oakland Coliseum. GMS Field’s dimensions are identical to those of Yankee Stadium, a famously homer-friendly venue.

Sadly, it isn’t as simple as calculating the surface area of the outfield and calling it a day. Coors Field’s center field wall sits 415 feet from home plate; the left and right field foul poles are 347 feet and 350 feet from home, respectively. It also allows the most runs. In the thin air of Coors, pitches are easier to hit because they move less, and they travel farther off the bat because of the reduced drag.

When we talk about the environmental conditions of ballparks, we’re really talking about the air density. As Alan Nathan writes, the drag and Magnus forces are both proportional to the density of the air. As this data shows, the run-scoring conditions of a stadium are, to some degree, a function of its air density:

Even after removing Coors from the dataset, there’s still a reasonably strong relationship. (Fenway is the outlier on the top right with relatively dense air and a hitter-friendly park factor.):

To get a sense of the park factors in West Sacramento and Tampa, then, it was necessary to estimate the average air density of Sutter Health and GMS Field during game times. To start, I downloaded hourly climate data for all of 2024 from the National Oceanic and Atmospheric Administration’s Integrated Surface Database. The closest weather data station to the A’s stadium is about 10 miles away, at the Sacramento International Airport. Conveniently, GMS Field is literally next door to Tampa International Airport, which also houses one of the ISD stations.

For each scheduled A’s and Rays home game and start time in the upcoming 2025 season, I took the data from the equivalent 2024 date and start time as well as the subsequent two hours, and then calculated the air density for each hour. The Rays, for example, will open their season today, March 28, 2025, at 4:10 PM Eastern. I first calculated the air density at Tampa International Airport at 4:00 PM, 5:00 PM, and 6:00 PM on March 28, 2024, repeated that for each of the 81 scheduled home games, and then averaged the air density across all 243 observations.

Who cares about your methodology, you might be saying. Give us the dirt! Fair enough. Here is the dirt:

Given all the chatter about Sacramento’s extreme heat, that air density ranking might surprise you. At 1.178 kilograms per meter cubed, they’d rank smack dab in the middle of the pack among parks where teams played at least half their games in outdoor conditions. There are a few reasons why the air is perhaps surprisingly dense. For one, the league scheduled the A’s for a disproportionate number of night games, lowering the average temperature. And Sacramento doesn’t get super hot until the summer months. (The park is likely to play pretty differently in the summer versus the cooler spring months.) The park sits pretty close to sea level, removing any altitude effects. It’s also one of the driest parks in the sport, ranking third lowest in average relative humidity.

GMS Field is likely to have thinner air than the Sacramento park, ranking closer to the top of the league in average air density. Even though the park is only a couple dozen feet above sea level, the average temperatures during game time would have been five degrees warmer than any other stadium in the sample; the average humidity would’ve ranked third. It’s going to be hot and humid, and the ball is going to fly.

Using the outfield square footage numbers provided by this 2015 Andrew Fox FanGraphs Community Blog piece and the air density figures provided by David Appelman, the park factors of these two stadiums can be very roughly modeled. I constructed a regression equation where I could plug in the outfield square footage values and air density figures for our two parks of interest to (again, very roughly) estimate what the park factors might be. Applying these crude methods, Sutter Health looks something like a middle-of-the-pack stadium by park factor, while GMS Field figures to play like one of the more hitter-friendly stadiums in the league. Here are the full rankings with the projections for each of these stadiums:

There are factors I declined (read: did not have time) to analyze. The wind direction influences ball flight for both pitched and struck balls. (Todd Zola took a look at the wind conditions of both ballparks for ESPN.) The amount of foul territory is a relevant concern, especially going from the spacious Oakland Coliseum to a more standard setup in West Sacramento. And it’s possible the batter’s eye, as at T-Mobile Park, could affect the performance of hitters. Also, the angle of the sun. I could go on! But given the influence of environmental conditions on run-scoring conditions, I feel reasonably confident projecting at least that neither of these parks is likely to play as particularly pitcher-friendly.

My partner and I watch the reality dating show “Love Island UK.” Whenever they tease the next set of islanders, the promo copy is the same: “Two new bombshells enter the villa.” On “Love Island UK,” these bombshells stir the pot, shuffling the stale dynamics of the gathered British singles. On “MLB: Season 125,” Sutter Health Park and George M. Steinbrenner Field will play a similar role, scrambling the known conditions of baseball as we’ve come to understand it.