PitchFX guru Mike Fast posted a gem yesterday (no subs. req’d), looking at pitch velocity versus temperature. Surprisingly, at least to me, the relationship between fastball velocity and game-time temperature is pretty linear, with velo increasing by one mile an hour for every 37-degrees (or so) according to Fast. This graph shows that fastball velocity gradually increases early in the season before peaking in July in August, and the change is not insignificant. We’re talking about a difference of 1.5-2.0 mph from March/April to July.
Fast acknowledges that the study isn’t perfect, as adjustments for specific pitch types (two-seamer vs. four-seamer, etc.) and climate are needed, but the early data suggests that there is substantial correlation between fastball velocity and the weather. So yeah, stop sweating March/April radar gun readings.