As a former resident Denver, I can personally attest that there is a very noticeable difference in the air at a mile high as compared to the air at the relative sea level of Penn State or my home in Maryland. Whenever I return, stepping off the airplane in Denver International Airport is like learning how to breathe again. It feels like your lungs are doing the same work in order to access less oxygen, and that’s because they are. That’s why many people get altitude sickness and why three of the five longest field goals in NFL history happened to take place in Mile High Stadium. But there’s another quality to the Coloradan atmosphere; it’s extremely dry.
Baseball teams in Colorado notice the difference as well. In the first nine years after the Colorado Rockies baseball club entered the MLB, their ballpark, Coors Field, was known as a hitters park. According to MLB Reports, their team batted an average of .331 at home, an great number for an individual player and patently absurd for an entire team to sustain over the course of almost a decade.
Many thought the dry air could be the cause of the Rockies batting success. Baseballs are made of a rubber core tightly wrapped in dozens of meters of yarn and covered in leather. The yarn was thought to become heavier and softer in humid conditions while it was harder and compressed less (therefore making the ball go further when hit) in dryer conditions. There was no way, however, to isolate that from the confounding variable of the thinness of the air, until 2002 when the team installed a humidor to keep their baseballs in storage. If the yarn were soaking up atmospheric moisture in other locations and not doing so in Denver due to the lack thereof, maybe keeping the baseballs moist could help normalize the Rockies’ insane hitting statistics.
Their null hypothesis in this case was that keeping baseballs in a humidor wouldn’t affect their hitting stats while the alternative hypothesis is that it would decrease them.
Eleven seasons in (by 2013), the null hypothesis was unequivocally rejected. In that time, the Rockies have seen their team average plummet to .315, an extremely significant drop. 
The sample size on each side of the humidor installation is also large enough to control for years where the team might have had an exceptionally good or exceptionally bad season.
The team’s minor league affiliate, the Colorado Springs Sky Sox, followed suit by installing a baseball humidor in 2012. While the sample size of two seasons looked at by the Denver Post is relatively small, the results are very pronounced. While hits and batting average have both fallen by between two and three percent, the number of home runs, where the aerodynamics of the ball would be most effected by moisture or lack thereof, decreased by 14.9%.
Clearly, the use of the humidors served exactly the purpose the originators of the idea believed it would serve.
Photo Refrence:
http://www.rockieszingers.com/2014/04/14/key-pitching-altitude/
References:
http://www.profootballhof.com/news/long-field-goals/
Tyler- this was a really cool post. I am a huge baseball fan, so I found it really interesting that the Rockies were actually getting an advantage due to the altitude with which their stadium stood at. I also found it cool that three of the five longest recorded field goals have been in Colorado- who knew the air had an impact of the flight paths of balls? This is an article I think you will enjoy- http://www.beyondtheboxscore.com/2014/11/6/7159813/keeping-it-dry-what-would-happen-if-all-mlb-parks-installed-humidors. It takes a look at the impact humidors would have if they were used in all ballparks, not just ones in Colorado. Enjoy!