There might be something to be said for warm weather adding velocity if the pitcher finds it easier to get loose and having better arm action. A pitcher properly warmed up shouldn't see much difference between a warm day and a cold day, however.
I have observed our warm weather pitchers in AZ on 40 degree days, with a stiff breeze and a light rain acting likes its 110 out (shirt sleeves, quick warm up, etc). I chalk this up to being stupid more than the elements. I have posted on 'the stupidity factor effecting velocity' in other forums, but fundamentally the pitcher has to prepare himself to pitch in the elements presented.
You don't see significant velocity differences in a MLB pitcher in Coors Field on a dry, hot summer day versus the same guy in San Diego with a thick fog late at night despite changing elevation by a mile and humidity by maybe 75%.
I think the other factors posted probably make a bigger difference than the weather.