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“The number one reason, for me, is the calming effect of the humidor. That’s made it a more realistic game for pitchers. And not just our numbers-opposing numbers are down, too. It’s created an environment that keeps the balls from shrinking.”-Rockies manager Clint Hurdle

Two weeks ago I offered a few theories on why run scoring at Coors Field is down this year in response to a rash of articles on the “humidor effect” last month. Through games of June 15, some back of the envelope calculations show that run scoring is about 5% higher at Coors while home runs are up about 16%, whereas historically those numbers would be around the 25% and 50% range respectively. Obviously small sample-size caveats apply, but in that article we found that through the first two months of the season, run scoring and home runs have never been this low in Coors, a fact that verified Hurdle’s statements and justified all the talk around the league.

Among the theories offered in that column were that a better pitching staff and weaker hitting have the effect of dampening park effects (in other words, the effects are still there, but the team’s makeup conceals them), soggier baseballs (made so by leaving them in the humidor longer) make it harder to take full advantage of the environment, and that longer grass has some effect on batted balls. This week we’ll explore the last two of these theories in a little more detail, with the help of some additional data. So, as Will Carroll says, powered by my recent trip to Fenway Park, let’s go.

Soggier Baseballs

As mentioned in the previous column, it has been reported that the Rockies are now simply using balls that have been in the humidor longer. The effect of that practice is ostensibly to have the baseballs absorb more moisture, making them heavier with a lower coefficient of restitution (COR).

COR is a measure of the “bounciness” of the ball, or more precisely, it’s the ratio of the velocity of the ball when rebounding from a hard surface with its initial velocity. Major League Baseball specifies that the COR must be between .514 and .578, as measured by firing balls at 85 feet per second at a wall of ash.

As any kid can tell you after they leave their baseball in the yard all night in a rainstorm, combining these effects should serve to decrease the distance balls can be hit. Although humidity in the air has little if any effect on a ball’s flight, in his book The Physics of Baseball, Robert K. Adair notes that R.C. Larsen found that:

…the weight of balls stored at 100 percent humidity for four weeks increased by 11 percent and the coefficient of restitution at an impact velocity of 25 mph decreased by ten percent–when dropped on concrete from a height of 20 feet, the humidified balls will bounce only 80 percent as high as the balls stored at low humidity.

This effect would translate into a 30-foot difference for fly balls at normal Major League velocities, but storing the balls at 50% humidity would have a smaller impact, obviously.

So is this what’s happening?

Using data from STATS, Inc. here are the average fly ball distances, and the percentage of fly balls that go for home runs at Coors Field since 2000:

Year    Avg FB       %HR
2000       336      .174
2001       340      .192
2002       334      .155
2003       334      .149
2004       336      .167
2005       333      .139
2006       329      .113

There are two main points to take away from this table. First, with the humidor installed for the 2002 season, you can see that the average fly ball distance has decreased around five feet, from 338 before to 333 after (if this season’s rate holds. And secondly, although the 2006 data only includes the first two months of the season, fly balls are traveling even less distance this season. This provides some better evidence than park factors alone, that not only has the humidor had an effect, but that the effect has increased this season, making it likely that soggier baseballs are the main reason.

Ah, but what about the weather? As pointed out in the previous column, spring weather in Denver is not exactly predictable, nor is it often pleasant, which could explain the lower run scoring this season. Once again we have some data to look at for games at Coors Field going back to 2000.

Year    Avg Temp  April/May Temp  April/May Wind Speed
2000       74.1       68.0              10.1
2001       74.5       66.0               7.6
2002       73.5       64.6               9.2
2003       73.4       65.4               8.0
2004       68.9       57.2               7.6
2005       73.1       63.1               9.0
2006       66.8       66.8               8.3

This table shows the average game time temperature for each season, along with the temperature and average wind speed in only the first two months. Although temperature and wind speed can sometimes be misleading since wind patterns play a role, the table gives us no reason to suspect that the weather has been particularly harsh this year. In fact, the average temperature this season has been a bit warmer than in any of the previous five seasons.

And because I knew you’d ask, here are the average fly ball distances, percentage of home runs on fly balls, game time temperature, and temperature for the first two months of the season for all other outdoor ballparks, dating back to 2000.

Year    Avg FB      %HR  Avg Temp  April/May
2000       322     .134     72.7     67.4
2001       322     .131     73.3     67.7
2002       321     .122     73.3     65.3
2003       321     .125     72.0     64.7
2004       323     .133     72.8     68.0
2005       324     .125     73.4     66.4
2006       325     .131     67.1     67.1

You may have noticed that although the average fly ball distance in 2006 at Coors is still four feet farther than in other outdoor parks, the percentage of fly balls hit for home runs is actually less. This is the case since the dimensions at Coors are still long (347′ and 350′ down the lines, 390′ and 375′ in the alleys, and 415′ in center). As a result of the larger dimensions, as the average fly ball distance decreases at Coors, the percentage of fly balls that are home runs will decrease rapidly.

Even though home runs are up about 16% at Coors this season overall, when you break it down you’ll notice that Rockies pitchers have given up just 17 home runs on the road in 291 innings of work, while allowing 32 in 293 home innings. Their stinginess on the road is easily the best mark in baseball. On the other hand, Rockies hitters have actually hit fewer home runs at home (26 versus 36, albeit in two more road games), so the positive effect for home runs thus far may be a bit of a mirage.

When you add it all up, one might project that because there is still plenty of room for bloopers and Texas Leaguers to fall, Coors will continue to have a positive effect on run scoring but end up being a more neutral home run park.

Longer Grass

A second theory mentioned in the previous column, as hinted at by Rockies broadcaster George Frazier, is that the grounds crew at Coors Field is keeping the grass longer this season. Since that was written I’ve verified that the infield grass is indeed being kept longer, and as you might imagine, that reason is to help out the Rockies ground ball pitchers, particularly Aaron Cook and Josh Fogg. While I don’t know exactly how much longer the grass is, I also understand that some other teams, notably the Diamondbacks, are not pleased.

But has the longer grass had an effect?

The following table shows the number of non-bunt ground balls hit at Coors Field over the past few years, and the number and percentage that went for hits.

        GB  GBH    PCT
2003  2100  474   22.6%
2004  2037  468   23.0%
2005  2108  497   23.6%
2006   828  158   19.1%

So again it would appear that ground balls are not making it through the infield quite as frequently, and the difference between 2006 and the previous three seasons is statistically significant at the 10% level–that is, there is a greater than 90% chance that the difference we’re seeing reflects an underlying reality. One might then postulate that a key contributing factor is the length of the infield grass.

Making a Case

Summing up, it would appear from this data that the way in which the humidor is being used, along with the effects of longer grass, are probably the two biggest reasons that run scoring is down at Coors this season.

Throughout the first eleven years at Coors Field the Rockies record at home has been 490-392 (.556) while on the road just 339-543 (.384). If indeed leveling the playing field in terms of the way baseballs feel has contributed to their 16-16 record at home and 18-18 record on the road through June 17th, folks in Denver will learn to welcome a little more humidity in their summer fun.

Thank you for reading

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Dan Fox


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