My measuring the change is velocity from time a ball leaves a pitcher’s hand to the time it crosses the plate, one can calculate the drag coefficient of the ball. That correlates to home run distance.

In total, the changes in ball drag explain about 25 percent of the variation in the ratio of home runs to fly balls over the last four years.2 Wind and weather can also influence drag, and although I controlled for those when I calculated the league-wide numbers, I also double-checked my analysis by looking only at Tampa Bay’s stadium, which is indoors and air conditioned. In the Rays’ home park, I found an even stronger correlation between home run rates and the ball’s drag coefficient.3 It doesn’t exactly come as a shock, but this is clear confirmation that air resistance influences home run rates.

I also found a significant decrease in the drag on the ball in general over the past few seasons, with the MLB-wide average drag coefficient dropping by about 0.01 from 2015 to 2017.4 That might not sound like much, but Nathan’s calculations show that even a change that small can add up to 5 feet of distance on a well-hit fly ball,5 which in turn would be enough to make 10 to 15 percent more balls leave the yard in a given season.

Of course, a ball that slows down less probably helps pitchers strike out batters. That’s what we are seeing, more home runs, more strikeouts, fewer other hits.

Hat tip, BBTF.