If Coors' dry air is what makes the ball fly, then why is it that Nats Park gets more "lively" when it gets humid?
Or is it the thin air at Coors, and not the dry air?
Or is it that pitchers' breaking balls just don't break as much, and end up center-cut? After all, the Rockies scored lots of runs, too.
Assuming no breeze, here are the major factors affecting the distance of a struck ball, in order of importance:
1) Altitude - higher altitude means thinner air, and significantly so.
2) Temperature - higher temp means thinner air, as it expands as gasses and fluids do when heated.
3) Humidity - higher humidity also means thinner air, drier air is more dense and thus balls tend to fly further in the summer.
Note that Coors has 1-2 working for it, which way overwhelm the relatively meager negative impact that drier air has.
As for effect on pitches, the thinner air means less break on sliders/curves/2-seamers which obviously works significantly in the hitters' favor, more than offsetting the slight pitcher advantage of greater pitch speed as drag on pitchers is reduced and the ball crosses the zone having lost less velocity due to drag than normal.