Jhonny Peralta: Cold Like the Weather

From Andy Castrovince's latest mailbag:

I'd be interested to see Jhonny Peralta's batting average in warm weather vs. cold weather. He was red hot down in Arizona this spring, but now he seems ice cold, much like the first six weeks of last season. Great Odin's raven, am I on to something here?
-- Tim R., San Diego

I'd love to help you find those numbers, but, much like the translation of the name for your hometown, scholars maintain that the ability to calculate such statistics was lost hundreds of years ago.

Actually, I asked hitting coach Derek Shelton and media relations director Bart Swain if they've ever heard of such a stat, and neither has. You'd really have to be the obsessive-compulsive type (even by baseball statistician standards) to calculate those numbers, especially when you consider the temperature at first pitch can take a drastic dip by the last pitch.

Well, when it comes to the Indians (and especially Peralta, apparently), I am that obsessive-compulsive baseball statistician. After reading the question, I immediately thought, "I bet Retrosheet tracks weather information. Sure enough, they do.

If you want me to bore you with the details, email me. Otherwise, let's skip to the pretty picture:

The short explanation is that I took Peralta's batting average (hits/at bats, obviously) for each gametime temperature reading. As you can see, there is an upward trend. Thoughts:

• There's no context. Maybe all, or at least most, hitters follow the same trend. That would make this "revelation" meaningless.
• Correlation is not causation. It's warm at midseason, when hitters are thought to be at their best, and when the ball is said to travel better. It's cold at the beginning of the season, when hitters aren't yet in "midseason form", and at the end, when fatigue starts to set in. So, again, maybe this is a trend for everyone and not just Jhonny.
• I'd be interested to see weather-related trends for other statistics, starting with BABIP and OPS, and perhaps moving onto the more advanced stuff.