The humidor’s effect on home runs at Chase Field

Home runs were down in Arizona, but they were also down everywhere else.

This was the first year games at Chase Field were played under the effects of a humidor. The humidor was supposed to counteract the dry desert air that made Arizona a home-run-happy field just as the humidor in Colorado made Coors Field a less extreme park.

This year, the Diamondbacks just so happened to field one of their worst offenses of the millennium. This year’s Diamondbacks slugged just .398 at home, their lowest mark since 2002. Visitors also had their worst season by slugging this year, posting just a .391 slugging against the Diamondbacks at Chase Field.

Hitting away from Chase Field though, Arizona fared better than previous Diamondbacks teams. The Diamondbacks slugged .397 away from home in 2018, but that’s their fifth-best road slugging season since 2002. They also had an 85 wRC+ on the road which puts them in the middle of other Diamondbacks road teams.

It would appear that the humidor worked a little too well. Mike Petriello of MLB.com wrote that offense was way down early into the season. Last April, physicist Alan Nathan of the University of Illinois predicted that the humidor would suppress home run production from anywhere between 25-50 percent assuming the balls are kept at the suggested 50 percent humidity and 70 degrees Fahrenheit.

Looking at the year-to-year change, this bears out. Last season, there were 215 home runs hit at Chase Field or 2.65 home runs per game. In 2018, that number fell to 171 or 2.11 home runs per game. That’s a reduction of 20 percent. It’s not the 25-50 percent reduction Nathan predicted, but that’s still a substantial decrease.

When Nathan gave his talk at SABR Seminar in August, home runs were down the predicted 30 percent, but something happened toward the end of the season. On Twitter, Nathan noted that when you compare the Diamondbacks’ home/road home run numbers over the last two years, the decrease is more pronounced. On the road, the Diamondbacks hit 96 home runs in 2018 and 98 in 2017. At home, it was a different story. This year, the Diamondbacks hit just 80 home runs compared to 122 last year.

However, It should be mentioned that the ball was different in 2017 and 2016. Home runs themselves were down 9 percent across MLB in 2018. If we estimate that 9 percent of the decrease was due to the ball, hitters at Chase Field would have hit 196 home runs in 2017, meaning that home runs were down in Arizona 13 percent. That’s a blunt way to adjust for the variable of the juiced ball, but it’s a close enough approximation.

The 2017 Diamondbacks also had JD Martinez, who in 30 home games with the Diamondbacks hit 16 homers. That performance alone is enough to throw off any sort of data.

When looking beyond 2017, though, the drop off in home runs becomes less severe. From 1999 to 2017, there were 3,434 dingers at Chase Field. Over 19 seasons, that averages out to 180.7 home runs hit per year, so 2018 was down but within a Nick Ahmed-sized margin of error.

That doesn’t mean that Dr. Nathan’s prediction was wrong. This was just one season, and even with a lousy offense and good pitching staff, home run totals could vary widely. In ten years, it’s possible the 25-50 percent reduction will have happened.

This is only looking at raw home run totals as well. Nathan also predicted a decrease in average exit velocity for balls hit between 20 and 35 degrees by about 2 mph. Such a shift ought to be responsible for the 25-50 percent drop in home runs. Looking at all batted balls at Chase Field hit between 20 and 35 degrees, here are the exit velocities by year going back to 2015.

Again, there’s a year-to-drop, but it’s in keeping with the two other years from which we have exit velocity data. It’s also just one year, so 2018 may have been an outlier.

We know that the humidor at Coors Field has had a profound impact on batted balls, and Arizona shouldn’t be any different. With as poor as an offense as Arizona had this year, and with home runs down around the league, one would expect the decrease in home runs to be more pronounced.

Kenny Kelly is a writer for Beyond the Box Score, McCovey Chronicles, and BP Wrigleyville.