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The humidor at Chase Field: 50 games in

Friday’s slugfest muted the impact of the humidor, but how have the last 10 games gone?

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Game 50 was Friday night’s contest against the Rockies, where the sides combined for 21 runs, and all stats below are through that game i.e. excluding last night’s. Recent events at Chase Field have certainly had quite the impact. The last three games there have seen a remarkable total of fifty-three runs being scored, including both Friday, and the record setting 20-5 victory over the Padres on July 7.

Runs scored and allowed

While those were obviously extreme contests, over the last ten in total, 115 men have crossed home-plate. That’s significantly more than the same pre-humidor period in 2017, when the same ten games saw only 82 runs scored, or thirty-three fewer than with the humidor in operation. That continues the trend noted last time with the early season decimation of offense at Chase no longer quite being the case. Based on overall run-scoring numbers, if I’d to pinpoint a specific date when the humidor was “turned down,” it would be after the home-stand ending June 3. At that point, the gap between runs scored last year and this peaked at +98.

For Chase Field has been considerably more hitter-friendly since that point. It began immediately, with the first two games on the next homestand which followed, seeing Arizona win 9-5 and 13-8, and has continued through Friday night. Over the 18 match-ups between then and now, 43 more runs have been scored this year than the corresponding games last season, an average of 2.4 per game. If that new difference keeps up for the remaining 31 games in Phoenix, then we’ll end up seeing a higher number of runs scored at Chase than we did in 2017. #HumidorWhatHumidor? On the chart below, you can see the declining difference over the last few homestands.

Overall offense

The overall ERA at Chase Field is continuing to trend up as well, thanks to events like last night - the third of the year where more than twenty runs were scored in a single game. It’s now up to 3.96, an increase of almost a quarter run (0.23) since the forty-game checkpoint. ERAs in general have gone up, but for baseball as a whole, the change over the same period is only about a third of the Chase increase (MLB ERA went from 4.06 to 4.14, an eight-point increase). We’re still well short of the 4.70 figure posted here in 2017, but many more games like Friday will start to gnaw into that difference.

The home-runs in particular are beginning to fly out of the park a bit more. We’ve now reached 110 for the year, twenty-four in the last ten games. We averaged 2.65 per game last season, so based on the recent trend, this would mean about one home-run every four games NOT leaving Chase, compared to last season. Batting average on balls in play remains lower than it was - whether through our improved defense, humidorification, or most likely a bit of both. But at .288, it is also steadily regressing toward the mean. That’s now just seven points below MLB average, and 26 points higher than it was after the first 10 games. Though as Chase was +9 over MLB average in 2017, still some way to go.

Home/road splits

A quick refresher. Over the first twenty years of the franchise, the team has always hit better at Chase Field than elsewhere. The amount has varied, from a mere three OPS points in 1999, to 136 points last year, but it has been a consistent feature for two decades. Of course, run environments have shifted dramatically in that time, as we went through the steroid era and came out the other side. But it’s the difference which we’re interested in, and from 1998 through 2017, here’s the sum total of the Diamondbacks home/road offense:

  • Home: .269/.338/.442 = .781 OPS
  • Away: .248/.315/.394 = .709 OPS

Now, let’s look at those same figures for the Diamondbacks’ performance at the plate in the 2018 season. We’ve played 50 games at home and 48 on the road:

  • Home: .239/.320/.396 = .716 OPS
  • Road: .223/.296/.396 = .692 OPS

This seems to provide further evidence for the humidor being “turned down”, with our hitters now performing significantly better at Chase Field than elsewhere. Here’s how the home/road OPS have differed each of the five times we have checked in so far:

  • 10 games: -8 points
  • 20 games: -53 points
  • 30 games: +18 points
  • 40 games: +4 points
  • 50 games: +24 points

While there have obviously been fluctuations (the team couldn’t hit anywhere in May!), the overall trend has been up since the first month or so of play. While the gap is still smaller than the average over the first two decades, it has been tending up, and if I was to filter out just the last thirty or so games, it wouldn’t surprise me if the home-road split over that time was in the neighborhood of the 72 point average margin seen across the entirety of the pre-humidor era.