Last week, the Astros’ Dallas Keuchel and the Yankees’ Masahiro Tanaka faced off on Opening Day in New York. The first Opening Day was pushed back a day because of wintry weather. This day was no different, with a 36-degree first-pitch temperature and 18 mph wind. The next day, I ran my daily velocity report and both of the above names were on the list of starters who’d exhibited huge velocity drops. Keuchel was down 2.5 mph and Tanaka was down 2.3 mph. By running a quick study, I found that colder weather does have a fairly dramatic effect on pitcher velocity.
To find the data, I ran a query using the PITCHf/x database. For the years 2008 to 2015, I compared average velocity from a game which started at 40 degrees or less and the average from the rest of their games. The average change in velocity was -0.95 mph, with a median value of -0.92 mph. A pitcher throwing in a cold game should expect some velocity decline.
Note: Reader yaboynate quickly pointed out that the drop because it is early in the season. I change the query around a bit, and found the average and median change to -0.58 mph. Now the rest of April would be colder and the whole month is lower to start with.
Here are how the velocity changes were distributed.
Well, the cold weather definitely limits any upside and almost half the pitchers in the sample experienced a 1 mph loss — with one in every eight experiencing a 2 mph loss relative to the rest of the season. The differences shrink as the games warm up. From 40 to 50 degrees, the gap is around -0.6 mph; from 50 to 60 degrees, around -0.4 mph.
So, it’s simple: when looking at early season velocity declines, look at game temperatures. Part of the reason for the decline could be attributed to the cold weather.
Jeff, one of the authors of the fantasy baseball guide,The Process, writes for RotoGraphs, The Hardball Times, Rotowire, Baseball America, and BaseballHQ. He has been nominated for two SABR Analytics Research Award for Contemporary Analysis and won it in 2013 in tandem with Bill Petti. He has won three FSWA Awards including on for his MASH series. In his first two seasons in Tout Wars, he's won the H2H league and mixed auction league. Follow him on Twitter @jeffwzimmerman.
Hey Jeff, just to play Devil’s Advocate, do you think the causality could be reversed? That perhaps velocities happen to be down during colder weather months because it’s just earlier in the season?
I ran the number for just games in April and the average and median drop is -.58 mph. Still a drop, just not as much
Still doesn’t control for the possible time effect, since it would be a reasonable possibility that pitchers throw harder in starts 3-4 vs. starts 1-2.