Baseball Humidor Aids Fair Play in Denver The World Series resumes tonight, and as the Rockies play Boston in Denver, you might hear something about a humidor. It's where the baseballs are kept, because in the Mile-High City, low humidity causes the balls to dry out, which makes them better home-run candidates.
NPR logo

Baseball Humidor Aids Fair Play in Denver

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript
Baseball Humidor Aids Fair Play in Denver

Baseball Humidor Aids Fair Play in Denver

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript


Time now for Science Out of the Box, our weekly segment when we try to answer our question of basic science and learn about the basic work of scientists.

(Soundbite of music)

SEABROOK: This week, with the World Series resuming tonight in the Mile High City of Denver, our question is about baseball. If you watched tonight's game, you're going to hear a lot about a humidor, and not because they're smoking cigars up in the sky boxes. This is a climate-controlled vault where the Colorado Rockies store their baseballs to keep them as moist as they would be in a more humid climate.

Alan Nathan is a professor of physics at the University of Illinois, and he's an expert in the physics of baseball, particularly as played by the Boston Red Sox.

Professor Nathan, how much does a baseball dry out in a dry climate like Denver's?

Professor ALAN NATHAN (Physics, University of Illinois): Well, there's no question that water eventually dries out of the ball to which makes the ball physically a little bit different than balls stored in a typical 50 percent humidity kind of environment.

SEABROOK: So how does that affect the game?

Prof. NATHAN: Well, you know, I'm not sure there are any real precise data about that. We have a lot of statistical information. The two things that people have said happens are that drier balls, since it has less water in it, weighs a little bit less.


Prof. NATHAN: It has an effect on the ball-bat collision. You could hit a lighter ball further than you can a heavier ball.


Prof. NATHAN: But that's somewhat counterbalanced by the effect of the air drag on the lighter ball - is greater than on the heavy ball. You know, you throw a whistle ball and you see it's greatly affected by the air drag.


Prof. NATHAN: But those are fairly small effects, I think. I think that the major issue with the humidity is what is known as the coefficient of restitution of the ball. It's sort of the bounciness of the ball. You know, you drop a super ball, and it bounces nearly up to the height that you dropped it from. And you bounce a baseball, it hardly bounces at all.

That bounciness plays an important role in that collision between the ball and the bat. It's much easier for the bat to transfer energy to a bouncy ball than a ball that's not as bouncy.

SEABROOK: What does the coefficient of restitution, the bounciness, mean for the actual game?

Prof. NATHAN: Okay. So if I look at the measurements comparing the bounciness of the ball literally at zero percent humidity in a completely dry atmosphere compare it with, say, 50 percent humidity, which is the humidity in which the humidor in Denver is set�


Prof. NATHAN: �then that would reduce the distance of a long fly ball by about, say, 14 or 15 feet.

SEABROOK: Oh, plenty enough to stop it from going out of the park anyway.

Prof. NATHAN: It's certainly plenty enough to stop it from going out of the park. So, yeah, it's significant.

SEABROOK: What about the fact that the air through which the ball is flying is dry in Denver? How does that affect the speed or the trajectory or whatever?

Prof. NATHAN: That's very interesting question. Most people, if you ask them their gut reaction, they will tell you that a ball will travel farther in dry air than in humid air.

SEABROOK: Right. Because there'd be less drag on the ball because the air is less dense, right?

Prof. NATHAN: Well, it's - no, no�

SEABROOK: Not less dense.

Prof. NATHAN: Be careful not to mix apples and oranges here. It's true that the air in Denver is less dense, that makes the ball go farther for sure.

SEABROOK: That's the altitude, though, don't you think?

Prof. NATHAN: That's the altitude, right.

SEABROOK: Oh, I see.

Prof. NATHAN: The dryness effect is a little different. Actually, all other things being equal, a ball will travel farther in humid air than in dry air. And the reason is very easy. It's because a water molecule weighs less than an air molecule. But that's not the major effect in Denver. The major effect is really just the altitude effect itself.

SEABROOK: Fewer air molecules per cubic, you know, inch or whatever.

Prof. NATHAN: Per cubic - yeah, exactly. At a given temperature, the density of air in Denver is about 80 percent of the density at sea level. To appreciate how important the air resistance is, the - sort of the standard 400-foot home run�


Prof. NATHAN: �you know, the benchmark home run, if that ball were hit in a vacuum where there is no air�


Prof. NATHAN: �that ball would probably travel over 700 feet.

SEABROOK: It would just go and go and go.

Prof. NATHAN: It would just keep on going. So the air resistance, the drag is actually a very important effect, and there's 20 percent less drag in Denver than, say, at Fenway Park.


Prof. NATHAN: So�

SEABROOK: �the Major League Baseball can't control the fact that the Rockies play in Denver and the Marlins play, you know, at closer to sea level in Florida.

Prof. NATHAN: Right. And in fact, the reason for the humidor is to try to use a different way to try to compensate to that effect. That was the motivation for the humidor, is to compensate for the tendency of balls to go farther in Denver by making the ball a little bit less lively, so it's not hit quite as hard. Now, that's the whole point of the humidor.

SEABROOK: Alan Nathan is a physics professor at the University of Illinois.

Thanks so much for joining us.

Prof. NATHAN: Yeah. Thank you.

Copyright © 2007 NPR. All rights reserved. Visit our website terms of use and permissions pages at for further information.

NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.