DATA DEPENDENT: Is pitching or the weather responsible for World Series home runs?

Is there any correlation between weather and home runs during the World Series? Is better pitching responsible for less HRs or is it the colder temperatures?

There’s been no shortage of articles written on the subject and even some scientific studies. Back in 2012, Alan Nathan took a mathematical model approach. Using data from the 2009 and 2010 seasons, he designed an aerodynamics model using the initial position of the home run ball and its velocity and was able to reconstruct the entire trajectory of the balls, including landing point and hang time. He calculated that the mean temperature from the data set 72.7F with a standard deviation of 10.2F. He then used the data to answer the question: What would have been the home run distance had the temperature at the time been the mean temperature, 72.7F?

Ultimately, he concluded: 

“Suppose the average MLB game-time temperature were 10F higher. Fly balls on a typical home run trajectory would travel about 2.5 ft farther (about 0.6%), leading to 6% more home runs. As a more dramatic example, consider games played at the two extreme temperatures of MLB, 30F and 110F. The home run probability would be about 50% greater at the high end (110F) than at the low end (30F). This result simply confirms what everyone already qualitatively knows: balls carry better at higher temperatures, leading to more home runs.” ( )

A few years later, P.J. Drane and J.A. Sherwood conducted a study at the Baseball Research Center at the University of Massachusetts. They physically tested what physicists like Nathan calculated mathematically – how much does temperature and humidity affect the flight of a baseball?

Drane and Sherwood conducted tests on batted balls that were cooled or heated from 40 degrees Fahrenheit up to 120 degrees F. Their data indicated that the lower the temperature, the slower the ball traveled after being hit. 40 degree balls traveled at a velocity 2 percent less than the 120 degree balls. In other words, the canonical 400 feet home run hit at 120 degrees would have fallen 8 feet shorter at 40 degrees. That is easily the difference between the more desirable side of the outfield fence and the less.

In honor of the 2020 World Series, we’ve decided to take quick and admittedly crude look at whether temperature has played a historic role in keeping HR numbers down (since the mid to late October temperatures are often some 40 degrees cooler than in July) or whether it’s down to better quality pitching.

We charted the average daily temperatures of each World Series game going back to 1945 and then plotted the average temperature for the series against the total number of HRs. The least amount of dingers was hit in 1950 when the New York Yankees took on the Philadelphia Phillies. It was a quick series with the Bombers sweeping, taking two at Shibe Park and two at Yankee Stadium. The second least was in 1945 with 3 HRs as the Detroit Tigers took on the Chicago Cubs. However, the fact that this series went the full 7 games with only 4 home runs surely counts for something.

The most home runs were hit during the 2017 World Series between the Houston Astros and the Los Angeles Dodgers. They split 25 HR between them, though some will argue that Houston’s numbers should carry a scarlet astersik for eternity.

36 World Series had double digits home runs. Of those, 21 went the full 7 games. Logically, those series have a greater chance of having more home runs but it is far from given.

In terms of temperature, the coldest average temperature series — 46.233 degrees — was held in 1979 between the Pittsburrgh Pirates and Baltimore Orioles. 7 home runs were hit. The hottest average temperature series — 71.775 degrees — was held in 1949 between the New York Giants and the Cleveland Indians. They split 5 home runs between them. It’s worth noting that the Giants swept the series.

The average temperature of all World Series between 1945-2019 was a cool 58.977 degrees. The average number of home runs was 10.446 per series.

A scatterplot chart of World Series HRs by temperature yields the following:

Clearly, the trend is toward more home runs as the temperature increases, even after eliminating outliers. On the flip side, lower temperatures tended to result in less balls leaving the park. (Pardon the shaky trend line.)

A line chart comparing average temperatures with home runs year-on-year tracks fairly closely. (NOTE: The big gap in the mid-1990s represents the 1994 strike year.)

So what to make of this crude comparison? Nothing conclusive, mainly because there are so many other factors that also play roles in HR totals. From a purely science based perspective, barometric pressure and humidity play roles. According to Robert Adair in The Physics of Baseball, the 400-foot home run will go about three feet farther for every one-inch reduction in the barometer.

Humidity has a counterintuitive effect. Again, Adair, “Humidity per se has little effect on the flight of the ball. Indeed, since water vapor is a little lighter than air, if all other factors are the same, a ball will travel farther if it is exceptionally humid, though only by a few inches.” 

Still more, elements like wind need to be factored in.

The most significant factor that probably needs to be taken into consideration is the altitude of the baseball park being played in. Since the the retarding force on a baseball is proportional to the density of the air it is travelling through, batted balls travel farther at higher altitudes. Famously, Coors Field proves this to be true.

And then there’s the obvious human element, namely great pitchers. It’s reasonable enough to postulate that the post-season features some of the best pitching rotations. Discounting their contribution would be wrong. 

But still, it’s hard to argue with the fact that low-temperature World Series tend to feature fewer home runs.

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