Some Physics of Humidors

According to CBS Sports, MLB will require all 30 teams to keep yet-to-be-used baseballs in air-conditioned rooms this season. The point of this new regulation is to collect data to decide whether to require all the teams to use humidors to store balls in 2019. There are other side effects.

My prediction? The air conditioning will result in more home runs, not fewer. Summer humidity in places like the coasts, the South, and the northern Midwest is higher outside than in air-conditioned indoor spaces. The reverse is true in the desert such as Phoenix.

Humidity is a measure of the fraction of the air composed of water molecules. The higher the humidity, generally the more water is available to stick to surfaces; with lower humidity less water is available. The humidity in outside air is affected by temperature and atmospheric pressure. However, in controlled environments the humidity can be adjusted.

For example, you have probably noticed that water drips from the bottom of your car on hot humid days when you run the air conditioner. You’ll also find water dripping from your home AC unit. One of the first things an air conditioner does is remove water from the air. That allows the temperature of the air to drop using less energy.

The point is, air in controlled environments tends to be less humid than nearby fresh air. So, keeping baseballs in air-conditioned environments will generally tend to reduce the humidity surrounding the balls. This will create drier baseballs and more home runs.

The history of humidors in baseball goes back to 2002 when the Rockies began to store their baseballs at 50 percent humidity and 70 degrees as a method of reducing the outrageous number of homers in Coors Field. Prior to 2002, Coors gave up 3.20 home runs per game. From 2002-2010 this ratio dropped to 2.39, a 25 percent reduction.

The physics can be understood by looking at that cool beverage sitting on the table next to you. You’ll see that a few drops of water have collected on the outside of the glass since it was first poured. This process is called “condensation.” This shows us there are water molecules in the air and they can stick to surfaces under the right conditions.

Believe it or not, baseballs have a small amount of air inside them. The water molecules in this air can condense inside, particularly on the wool that fills most of the space in the ball. In 2004, a student and I published a study to test the effect of storing baseballs in environments that had humidity ranging from zero to 100 percent using NCAA baseballs. Since then Alan Nathan and others have completed a more thorough set of experiments with major league balls.

The mass of the balls in 100 percent humidity is about 0.90 ounces higher compared to balls stored at zero percent humidity. This mass change is all due to the absorption of water, which takes about one month to complete.

The increased mass of the ball will (surprisingly) result in the flight of the ball being a bit longer because larger masses feel less air drag (a bowling ball doesn’t feel the effect of the air as much as a ping pong ball). The increased mass will also slightly increase the size of the ball increasing the air drag. These two factors tend to cancel.

So, the main effect of a mass increase will be a lower launch velocity by about 0.66 mph for each one-tenth of an ounce. Below is a table to summarize the distance traveled by a 400-foot fly ball just due to the mass changes induced by storage at differing humidity.

In addition to the change in mass, the COR (bounciness) of the balls also changes. The COR goes from 0.574 down to 0.452 as the humidity grows from 0% to 100%. So, the balls coming off the bat will have a higher launch speed if the balls are stored in lower humidity, leading to more homers. A variation in COR will cause the launch velocity to change by about 1.6 mph for a 10 percent change in humidity. Here is a table.

If you need to be convinced that balls with more water will be harder to hit, here’s an experiment for you. Roll one of your baseball socks into a ball. You can hit it with a bat to get a sense of how far it will go. Now get it wet and hit it. It simply won’t go as far and you may even get wet!

This year the Diamondbacks are starting to use a humidor. One might think it is because Chase Field  trails only the Rock Yard in park factor. As Alan Nathan has pointed out, this humidor is very likely to result in a dramatic decrease in home runs. That will take care of the park factor alright!

Many of the public statements suggest the humidor is in response to the complaints of pitchers. Some claim the low humidity of Phoenix makes the balls too slippery to grip properly. Here is a small piece of evidence supporting the pitchers. In 2017 Diamondbacks hurlers threw 401 balls in the dirt at home and only 349 on the road.

The claim is the humidor will make the surface of the balls slightly more moist and therefore easier to grip. This is likely true from balls fresh out of the humidor. However, once out in the low humidity, the ball will immediately begin to change. While it takes weeks for the inside of the ball to dry out, it may only take an hour or so for the surface to dry. To be fair, there is no available data in this regard.

Why is MLB moving toward humidors for all stadiums? I suspect that unscrupulous teams may have considered manipulating the humidity of the baseballs to their advantage. If they want to protect their pitchers, they might want to store the balls above the hot dog boiler to elevate the humidity. If they want to give their power hitters an advantage, drop the humidity. Uniform storage will protect the integrity of the game in this sense.

However, as I said earlier, the great majority of teams reside in parts of the country that have high humidity in the summer. So my guess is, storage in air conditioned rooms or humidors will on average lower the humidity of the baseballs major league-wide. Thus, home runs will likely increase due to the lower mass and higher COR of the humidified baseballs.

As I hope you realize, it is easy to predict more bombs across the majors. After all, we have been on an upward trend for several years now. In addition, since we aren’t totally sure of the cause of the current home run explosion, the humidor effect will be unlikely to detectable.

Even if I am wrong, I can just claim that the balls could have been stored in lower humidity environments all long. My work here is done.

References and Resources

• Alan M. Nathan, Baseball Prospectus, “Home Runs and Humidors: Is There a Connection?”
• Alan M. Nathan, The Hardball Times, “A Humidor at Chase Field: What’s Up With That?”
• Mike Axisa, CBSSports.com, “MLB Reportedly Standardizes How All 30 Teams Will Store Baseballs in 2018”
• David Kagan and David Atkinson, The Physics Teacher, “The Coefficient of Restitution of Baseballs as a Function of Relative Humidity”
• Alan M. Nathan et al., American Journal of Physics, “Corked bats, juiced balls, and humidors: The physics of cheating in baseball”