This post has been a long time coming. I've been saying for months that I am going to post a blog dedicated to educating the masses about the real impact of the so-called "thin air" in the Mile High City, and I've finally decided to get around to posting it. Through the years, everyone from the casual baseball observer to the most ardent sabermetric statistician has postulated that many offensive statistics at Coors Field can be attributed in large part - sometimes even completely - to playing in the high altitude. You are about to find out that, while there is definitely some truth to that, the logic behind most of the conversation is flawed. I am going to tell you about the effects of the altitude, much of the reason for inflated offensive statistics, and the infamous humidor. The Coors Field Factor likely does not stem from what you think it stems from.
Believe it or not, there have been many university studies done on the effect of altitude on the flight of a baseball. I have read several of these studies, and the statistics seem to show that a baseball travels anywhere from 4-10% further in the 5,277 foot altitude at Coors Field than it would at sea-level. Most of the numbers seem to be in the 5% range. To expound on this point further, it is helpful to simulate the situation. Take a baseball game played in two different cities - say, Miami and Denver - with the exact same pitcher, the exact same hitter, the exact same pitch, the exact same weather conditions, and the ball struck exactly the same. A ball that would travel 400 feet in Miami would travel about 420-430 feet in Denver. To put it another way, a ball that would die on the warning track in Miami would land in the first row of seats in Coors Field. This is due to simple physics - there is less pressure in Denver due to the higher altitude, so the oxygen molecules in the air are more spread out, thus causing less drag on the baseball. While there is a difference in the two, the effect of altitude is actually rather miniscule compared to what one may think or hear.
While it is true that Coors Field is still a very, very good offensive ballpark, and it still relinquishes its share of homeruns, it is a fact that the numbers are nowhere near what they were in the first ten years of the park's existence. In fact, Coors Field ranked 13th among Major League stadiums in homeruns in 2009 at 2.2/game, tied with Chase Field and Comerica Park. This is still above average, but it goes to show that the altitude does not affect the flight of the ball in ways that you may think that it does. It also demonstrates the effectiveness of the much-talked-about humidor.
What caused baseballs to jump off of bats in the early days of Coors Field? It is actually quite the opposite of the effects of high altitude. Take something that is sealed in a material that doesn't breathe - a bag of potato chips, for example - into a higher altitude, and you will find that it expands due to the lack of pressure. In breathable materials, the opposite happens, since there is less moisture in the air at higher altitudes. To understand the effect that the region plays on baseballs, you have to know a little more about the geography of Colorado. In Colorado's semi-arrid climate, the air is very, very dry. When relative humidity gets above 30% along the Colorado Front Range, we say that it is "muggy" outside. In air this dry, what happens is that moisture and oxygen are removed from materials that breathe, such as leather. To demonstrate, take a new pair of leather boots that fit comfortably on your feet, and go into the mountains. You will find that they tighten as the moisture in them is removed, and they will become quite stiff and uncomfortable.
In fact, that is exactly how the humidor came to be. Tony Cowell, a Coors Field electrician, had this very thing happen to him in the early part of this decade during a hunting trip in the Rocky Mountains. As he thought about it, he realized that perhaps this could be a contributor to the homerun totals at Coors Field. He took his theory to Colorado Rockies president Kelli McGregor, who started measuring some old baseballs that were laying around. Sure enough, they were significantly smaller than the new baseballs, and even older baseballs from other parks. As it turns out, moisture and oxygen being drawn out of baseballs in storage was the single largest contributing factor to the flight of baseballs at Coors Field. As they dried, the cores became dense, the baseballs became slick and harder to throw, and the baseballs essentially turned into oversized BB's. There was no cushion to soften the blow of the bat. The difference between hitting a standard baseball and a Coors Field baseball would be akin to the difference between throwing a rock and an apple at a wall. This is how the humidor came to be. Since 2002, baseballs have been stored in the Coors Field humidor (which is similar to a walk-in meat freezer) at 70 degrees and 50% humidity. They no longer fly the same, and pitchers have an easier time gripping them. From 1993-2001, baseball games in Denver averaged 13.83 runs and 3.2 homeruns per game. From 2002-2006, those numbers changed to 12.25 and 2.58, respectively. In 2006 alone, average runs per game had dropped to 10.72, a full 3 runs per game lower than what they had been.
This brings up two questions: Why was and is there still such a stark difference in home/road batting averages for Rockies players, and why does Coors Field still relinquish high offensive outputs? I'll answer the second question first. What makes Coors Field so attractive for hitters are the tremendous hitter's eye in center field and the extremely wide gaps in the outfield. The power alleys in left- and right-center field are nearly 430 feet deep. It is a haven for line drive, doubles hitters. Still, Petco Park, Comerica Park, and Citi Field all have comparable outfields, but they don't relinquish nearly as many hits. This is because outfielders in Coors Field still play much deeper than they do in any other ballpark to guard against the ball carrying more than it does anywhere else. This opens up a lot of ground in front of the outfielders. While I haven't seen any statistical data, I would venture to guess that much of the reason that Coors Field is such a great offensive park is due to the fact that it is easier to hit a single here than anywhere else. Routine fly-balls drop in for singles, bloopers find the grass, and line drives that may hang up and be caught somewhere else fall in because the outfielders play so deep.
The answer to the first question is a combination of the altitude and the dry air, and it is also the answer to the question of why it has been so difficult for pitchers to be successful here. The fact that outfielders don't play as deep anywhere else as they do in Denver also contributes. The main factor, though, is that the ball doesn't break in Denver as well as anywhere else, even today. This is where the "thin air" comes into play. Without the help of heavy air, pitchers really have to snap the ball to effectively throw a breaking ball. Not only can this play mind games with the pitcher, it also wears them out, and when you combine that with the fact that dehydration is much harder to defend against as you go higher, it can make pitcher's lives hell. Also, prior to the humidor coming into play, the slicker, denser balls were hard to grip and hard to throw. I have to believe that this contributes in large part to the Rockies' collective offensive woes on the road through the years. Coming off a long homestand where the ball doesn't break as much can be devastating to a team when they go on a road trip, particularly in the West where they've had to face pitchers like Randy Johnson, Curt Schilling, Jason Schmidt, Brandon Webb, Tim Lincecum, and many more dominant arms through the years. When you combine breaking pitches that the players aren't used to with the changes in the way the game is played in other parks, you could imagine how difficult it is for the hitters to adjust.
I hope that this has been interesting for some of you to read. The Coors Field Factor is a very real thing, but it is nowhere near what it used to be, and it is still used to unfair ends in a lot of analyses. It has made for some very entertaining baseball games through the years, and it has given seamheads something to talk about since 1993. It will continue to be talked about, and it will continue to be an interesting topic. It is not nearly as significant as it once was, though, and hopefully its effects won't be overstated in the future.
The Rockies have run promotional commercials during games since 2007. This is one of the original Rockies commercials from that year. Pitchers Jeff Francis and Aaron Cook are sorting through baseballs in the humidor prior to a game.