The difference in the distance a 375 foot homerun travels when the humidity is 20% and when it is 60% is less than a foot. And the difference in the amount that a 90 mph curveball breaks is only a tenth of an inch. (There may be some absorption of moisture by a baseball on a particularly muggy day, but those amounts have been found to also be negligible).
The temperature (and thus the air density) has a slightly greater impact with the difference in the distance of a home run on a 50 degree day being about 16 feet less than on a 90 degree day. The altitude above sea level and corresponding air lower density difference means a 375 foot homer at sea level would travel about 405 feet at Coors Field in Denver.
But the meteorological element with the greatest impact is the wind. Just a 5 mph tailwind will carry that 375 foot home run to 415 feet and a 10 mph wind will translate to an epic 455 foot blast.
The bottom line is that if you want to see really long homeruns go to Denver on a hot dry day with the wind blowing out!
A more detailed treatment of the topic can be found in last year’s Jan/Feb issue of Weatherwise Magazine (http://www.weatherwise.org/Archives/Back%20Issues/2014/January-February%202014/rain-delays-full.html )
Jan Null, CCM
Golden Gate Weather Service