Regarding humidity the balls absorb the moisture and will not go as far but the air is actually thinner the higher the humidity so ball the can travel further, these two variables work against each other so the result in ball travel is negligible. But the theory is that the pitcher gets less movement on his pitches with low humidity (so you get more overs) and when the humidity is 50%-70% the pitchers seem to excel (you get more under's). There are a bunch of factors that come into play, whether the theory is right or wrong the data doesn't lie. And there is no huge edge betting with the humidity knowledge....it is just another factor. The times when it has really factored in and become a great capping tool is when the humidity at a certain stadium is more of an anomaly than the norm. For example, Arizona and Colorado almost always have low humidity so look for a day when the humidity is unusually high and visit the rest of the data to see if the under looks like a good play, and vice versa look for low humidity in stadiums that normally have high humidity and that is where you can find an edge.