THE BOOK--Playing The Percentages In Baseball

Some of this analysis may be moot since an MLB pitcher, I’m guessing, would never pitch with a ball that had any appreciable wetness or extra weight, but a ball that has slightly more weight would most definitely be an advantage to the hitter. Less action and movement on the ball and slightly less velocity, yet the ball will travel essentially the same distance off the bat since the slight loss in velocity (due to extra weight) is gained back by the ability to cut through the air better. (Deduced as I flip through “The physics of baseball” by Adair).

From a player’s standpoint, as long as you’re not going Jorge barehands and you didn’t leave your bat and/or gloves out in the rain, you’re not going to lose any grip at all. And I can’t imagine it having any mechanical impact on your swing. Visibility could be an issue during a greyed out day, but at night, shouldn’t be a factor.

In the end, pitching is so much more finesse, and gripping breaking balls with slightly moist fingers might be the biggest factor of them all.  I would imagine hitters have basically all the advantage when it starts to rain.

When I played (college), I never noticed an effect except when it got really bad to the point where it would get called.

I have played golf in torrential downpours and there is a noticeable effect on how far the ball flies in addition to the difficulty holding onto things.

This is why I brought up the Yankees game.  It had a 4 (!) hour rain delay, and it looked to me like it was raining hard.  I live in NJ, and it looked as bad there as it was here.

I’m just thinking everything about it was harder, so, how does the balance shift.  I know the fielders get hit big time.  So I wanted to know about the head-to-head batter/pitcher balance.

I mostly agree with Lee (#1) regarding the pitched baseball, but not for the same reason.  I assume (as he suggested) that the ball is never in the rain long enough to change the weight of the ball (certainly not in a MLB game).  However, humid air is less dense than dry air, so less movement (as Lee says) and less drag.  The latter means that the effective pitch speed (average speed) will be a bit higher (opposite to what Lee says).  I am guessing that neither of these effects is very large. 

The reduced air density probably has a significantly larger effect on the batted ball (and Lee has this right).  For a fly ball on a typical home run trajectory (25-35 deg launch angle), the reduced air density will lead to longer distance.  I don’t think the effect is very large, but I have not specifically looked at it.  However, it might be that the effect of reduced air density is small compared to the effect of any accompanying wind.  The wind effect itself depends on both the magnitude and direction of the wind.  An interesting effect is the downward momentum transferred to the ball by the falling water.  It would be interesting to try to estimate the latter effect, but I have not done so.

FWIW, on rainy days, scoring goes up a little, everything else being equal. I assume that is because it is more difficult for the fielders.

As far as humid air being less dense, which it is of course, causing the ball to go farther (and harder), all the research I have ever done with humidity and length of fly balls, HR rate, run scoring, etc., shows no correlation.  I have always assumed the reason is that the ball gets a little softer (or whatever else happens to it when it is exposed to humid air) which counteracts the less dense air.

And the ball certainly does get wet even though the umpire doesn’t keep it in the game very long on rainy days…

Responding to mgl:  The experiments that Lloyd Smith and I did when we studied the effect of the humidor show that it takes quite a long time (days!) for the weight of a ball to come to equilibrium when stored in a humidor.  So, having a ball out in the rain for a few minutes is not very likely to affect the weight, the COR, or the hardness of the ball.

"So, having a ball out in the rain for a few minutes is not very likely to affect the weight, the COR, or the hardness of the ball.”

Right, but if it is raining, I assume that the humidity level has been high for a very long time, at least for many hours.  No?

MGL:  Of course, these are quantitative questions.  So, let’s deal with it with some quantitative answers.  Here are some numbers that we measured.  If you take a baseball stored at a nominal RH of 50%, then put it in a humidor at a RH of 97%, the weight of the ball increases at first linearly with time.  After 100 hours, the ball weight increased by 3%, or about 0.15 oz.  These numbers to indicate the time scale over which one might expect some change in ball properties.  So, if it has been at high humidity for, say, 10 hours, I would expect 1/10 of that increase in weight, or 0.3%.  That’s pretty tiny.  So, I am skeptical of any claim that even daily changes in atmospheric conditions has enough of an effect on the ball to affect the ball-bat collision in any meaningful way.  I am agnostic about it.  I am willing to be convinced by a good argument.  But it has to be a quantitative argument. 

If indeed you are right and scoring goes up on rainy days, I would look for an explanation elsewhere.

Correction to my last post:  The ball weight increases linearly with the square root of time.  So if a 3% increase occurs in 100 hours, we expect about a 1% increase in 10 hours, or 0.05 oz.  That is still a small number, especially considering that the specs on the ball allow a total range of 0.25 oz (5 to 5.25 oz).  But let me repeat that I am willing to be convinced by a quantitative argument.

I certainly wouldn’t look at runs scored.  That could be fielders, errors, baserunning, etc.  Nothing to do with batter v pitcher.

I’d focus on K rates, BB rates, and HR rates (and HB rates too).  That would be pretty interesting.