Today's New York Times article about Coors Field in Denver wasn't really clear about the meteorological factors determining how far a ball travels.

Generally, air is less dense at a higher altitude than at a lower altitude. Therefore, in Denver (elevation 5,280 feet), baseballs will generally travel further. This is the primary cause of the high home run total at Coors Field.

In addition, humid air is less dense than dry air. Therefore, dry air is not at all a contributing cause of home runs at Coors Field. The caption to the photo says that home runs and Coors Field "go together" even though balls are humidified. This isn't a scientifically accurate assessment of cause and effect. Coors Field is a mile above sea level, and that's always going to contribute the most to a high number of home runs.

Recommended reading: The Physics of Baseball by Robert Adair.