Quick Study: Cold-Weather Effects on Velocity
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.
| Velocity Change | # of Pitchers | % Change |
| > 2 mph | 3 | 0.7% |
| 1 to 2 mph | 17 | 3.7% |
| -1 to 1 mph | 221 | 48.3% |
| -2 to -1 mph | 136 | 30.0% |
| < -2 mph | 81 | 17.7% |
| Total | 458 | 100% |
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 four FSWA Awards including on for his Mining the News series. He's won Tout Wars three times, LABR twice, and got his first NFBC Main Event win in 2021. 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?
Good idea.
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.
Because cold-weather games aren’t randomly distributed throughout the season, couldn’t this be due to some sort of early-season effect (e.g. conditioning or ‘seasoning’) or even some late-season effects (fatigue)? Could you test warm early-season games vs cold early-season games? That would be more convincing to show the weather was at play and not something else.
There are also cold-weather games in October, though fewer of them (and they have confounding factors). Though honestly I’ve sat through night games at Safeco in May and September that felt cold enough to have an effect on pitching. And actually, a few in July and August for that matter.
Could you do the same for humidity? People always think intuitively that humidity causes baseballs to not go as far, but in an atmospheric science class in college the professor said that humid air is actually lighter (assuming the same temp and pressure) because water vapor (H2O) has lower molecular weight than the main components of dry air (N2 and O2). Is there enough data to compare, say, HR counts or distances for similar games with differing humidity?
Humidity isn’t going to matter for HR distance. I’d have to dig around to find the spreadsheet, but I once calculated the difference in flyball distance for a typical HR hit in 0% relative humidity vs 100% and the difference simply due to the air density from the water vapor was negligible. If that surprises you, remember that it is relative humidity: air is 78% N2 and 21% O2, which adds up to 99%; the remaining 1% is Argon and a bunch of other stuff, including water vapor. So perfectly dry air has 0% H2O and 100% maximal humid air still has something less than 1%. Thus the most that water vapor could affect air density for drag purposes is something less than 1%; on a home run hit 400 feet, in the normal range of humidity found in summer ballparks, that’s going to be a variation range of a couple of feet or less.
Humidity does have an effect on the balls themselves (and to a much lesser extent, on the bats) because it softens the leather and makes them less elastic in collisions. Which is why at Coors Field they keep the balls in a humidor.
But we’re talking about pitching, not hitting. And for pitching purposes, a ball in humid conditions might be slightly easier to grip for the purposes of imparting spin. So it might actually be an advantage. But I doubt it matters much, especially considering the range of humidity is going to be much less than dry Coors vs humidor Coors. And even away from Coors, the balls are kept indoors until game time so they’re unlikely to have adjusted to outside gametime humidity (I don’t know how long it takes a baseball’s leather to come to equilibrium with changes in humidity, but I’m going to guess it takes hours if not days). The mudding-up process balls go through before the game probably vastly outweighs any effect from atmospheric conditions anyway.
Are baseballs really kept indoors right up until game time? That seems unlikely to me, unless there is some league rule about it.
In any case, humidity can be a big deal in terms of inhibiting/promoting offense. Denver is the strongest effect by far, but it’s not negligible elsewhere, esp. west coast parks.