Subject: differences in gravity at sea level & 1 and 2 miles alt.?

Debating if Denver's altitude provides any significant change via 
gravity to the rotation of a baseball thereby negating te spin a 
pitcher places on a pitch, which would allow hitters to increase home 
run production.  A friend states that it is significant, while I 
believe air temperature and humidity (which are by products of the 
level of altitude) are much more influential.

Re: differences in gravity at sea level & 1 and 2 miles alt.?