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Out of Thin Air—the New Coors

Of course, as everyone knows, Rockies pitchers are at a big disadvantage: they have to pitch at one mile above sea level. The high altitude leads to increased offensive levels, which leads to those high ERA's for Rockies pitchers. So, what's going on this year?

Humidor (noun): a case or enclosure in which the air is kept properly humidified

If you've been paying attention to this sort of thing, then you've probably heard about the "humidor." The Rockies have been storing their baseballs in a humidity-controlled environment, causing the balls to absorb some moisture and hence lose some "bounce." They also presumably become slightly heavier. The result has been that Coors has been much more pitcher-friendly than in the past, not just for Rockies pitchers, but for opposing hurlers, as well.

There was a lot of talk and a fair amount of complaining about the Rockies' humidor in the press earlier in the season. One of my favorite articles was this one, where Jeff Cirillo claims that the Rockies are cheating by deadening their baseballs. Now, Jeff Cirillo went to USC, so I suppose he must be reasonably intelligent, but he sure says some, well, silly things in the article. Talking about the baseballs at Coors, he said, "It's all spongy and it's big and it's waterlogged. They're illegal baseballs. They are non-flying baseballs." And then, "You wouldn't think that they'd be cheating. They are."

Um, Jeff, isn't there one point that you are overlooking? The fact that ... wait, Cirillo's own manager Ned Yost put it nicely:

Both teams are playing with the same balls. It doesn't matter if they're mushy, if they're square, if they're triangular ... That's nuts, man. That's just a waste of time, discussing it.

Actually, back in the day, Connie Mack, owner-manager of the Philadelphia Athletics (for a mere 50 years), was occasionally accused of freezing baseballs to deaden them. Apparently, if you take a ball out of the freezer a few hours before game time, the outside of the ball returns to normal temperature, although the cold center leads to a deader ball. In the late 1960s, manager Eddie Stanky and the grounds crew of the Chicago White Sox were accused of deadening their baseballs by storing them in a humidor. Curiously, everyone referred to them as "frozen baseballs", even though they were not frozen, merely humidified.

Getting back to today's subject, as you might imagine, pitchers do not necessarily agree with Cirillo's viewpoint. Chris Capuano, who'd given up two home runs in a loss the previous day, remarked, "It didn't look like like Garrett Atkins hit a sponge ball."

It's amusing that Cirillo is the one complaining. His years in Colorado marked the beginning of the falling-off-a-cliff decline phase of his career. After posting OPS+'s of 122 and 118 in Milwaukee the previous two seasons, his two years in Denver (2000-2001) saw him slip to 96 and 98. Of course, his raw numbers looked pretty good, because at the time the Coors effect was still going strong, as we'll see below. More recently, Cirillo went 0-for-4 in his one appearance at Coors this year. Non-flying baseballs, no doubt.

Run Scoring in (and out of) Colorado

So, what's going on in Colorado? Is the ball really deader or are Rockies pitchers simply better this year, as many have stated in the press? The way to answer the question is to compare the number of runs scored in Rockies home games to the number scored in Rockies away games. This comparison ensures that you are comparing Coors Field with the "average" NL park (it's not quite average, because of the unbalanced schedule, but it's close enough). The quality of play is the same for both sides of the comparison, since, to a good approximation, the same players are playing in Rockies home games and their away games. This ratio of (total runs at home) divided by (total runs away) is the simplest form of park factor.

So, if the ball is much deader in Colorado this year, we should see a significant drop in the park factor. The graph on the right shows the Coors Field (and Mile High Stadium) one-year park factors, calculated with a single season's worth of data. There are several things to note from this graph. First, there's quite a bit of variation in the the park factors, despite the fact that they are all measuring the same thing (starting in 1995, when the Rockies moved into Coors Field). One-year factors are susceptible to large statistical fluctuations, which is why most analysts use multi-year park factors.

Also, there has been a dip in the park factor this year, and its current value of 1.08 is the lowest in Rockies history. A more general downward trend seems to have started with the 2001 season. In fact, the article linked above states that the Rockies starting using the humidor in 2001. So, even if things are jumping around due to statistical variation, it appears that the humidor is having a significant effect on the offensive levels at Coors Field.

One can investigate further to see if the downward trend in the park factor is compatible with a heavier, soggier ball. You can look at component park factors, or how a park favors (or disfavors) different parts of offensive output: home runs, non-home run hits and strikeouts, for example.

We expect that both home runs and non-home run hits will decrease with the soggy ball, a point that I think needs no explanation. What about strikeouts? Coors Field reduces strikeouts because breaking balls don't break as much in the thin air. And since most strikeouts are obtained on breaking balls, there are relatively fewer strikeouts at altitude. However, we wouldn't expect the soggy ball to have any effect on the number of strikeouts at Coors Field, since a bouncy dry ball and a dead wet ball behave just the same if you miss them entirely.

The next graph shows the component park factors for home runs, non-home run hits and strikeouts. Looking at the trends in the "humidor era" (2001 to the present), we see pretty much what we expected. Home runs seem to be affected the most, although non-home run hits are also down since 2000.

This year, the home run park factor has shown only a modest decrease, while the non-HR hit factor has dipped more strongly. Earlier in the season, THT's Dave Studeman and Baseball Prospectus' Dan Fox noted that fewer ground balls were getting through for base hits at Coors Field this year. This effect would reduce the non-HR hit factor, which is what we see in the graph. I've also calculated the park factor for batting average on ground balls, and my results are in agreement with the previous findings:

Coors Park Factor - Batting Average on Ground Balls
2003: 0.937
2004: 1.016
2005: 1.117
2006: 1.011

Lastly, you can see from the graph that we observe little change in the strikeout park factor, which agrees with our reasoning, above.

Home/Road Splits

Another way to look at this issue is to consider the home/road splits of the Rockies pitchers and hitters. First, let's look at the team splits:

              Home R/G       Road R/G
Pitchers       4.74           4.87
Hitters        5.53           4.31

Let's have a look at some individual players to see who likes to play in Coors and who fares better on the road. First, the starting pitchers:

              Home ERA     Road ERA    Home - Road
Francis        4.02         3.77         +0.25
Jennings       3.28         3.97         -0.69
Cook           3.73         4.62         -0.89
Fogg           6.45         4.99         +1.46
Kim            4.03         6.78         -2.75

On the other side of the ball, we see a more familiar picture. Most Rockies hitters still perform better at Coors Field. Here are a few Rockies regulars:

              Home OPS     Road OPS    Home - Road
Atkins         1.011        .904       +.107
Holliday       1.130        .828       +.302
Helton          .983        .795       +.188
Hawpe           .800        .929       -.129
Carroll         .927        .591       +.336
Sullivan        .657        .796       -.239
Torrealba       .683        .787       -.104

Who Cares?

Well, you should, if you play fantasy baseball. Until this year, most fantasy players avoided Colorado pitchers like the plague. In my keeper league, only closer Brian Fuentes and starter Jeff Francis were owned at the start of the season. That changed as some sharp-eyed folks started noticing the success of Jennings and Cook, and even Kim at home.

On the flip side, if current trends continue, you might not see mediocre offensive players putting up huge numbers in Colorado in the future. I know that several Rockies hitters still performed much better at home this year, but remember that the humidor has a big dial on it and that gives Rockies management a knob for turning down the offense as much as they please. Who knows what their plans are?

But it's not just about going after Rockies pitchers and perhaps devaluing their hitters somewhat. All fantasy players have to make lineup decisions and you might want to re-think benching all your pitchers' starts in Coors Field next year. Likewise, you might not want to promote Joe Average to the starting lineup, simply because he'll be playing four games in Colorado.