Science Sunday: It’s not the heat, it’s the humidity (at Coors Field)

Two CU physicists say to look elsewhere for the decline in Coors Field offense. The humidor is not the culprit. But the National League’s former “official physicist” disagrees.A couple of University of Colorado physicists say the Colorado Rockies baseball team has it backwards with their famous humidor: moist baseballs should fly even farther than dry ones.

In a paper published online, CU-Boulder physicists John Bohn and Edward Meyer found that baseballs stored for two months at a humidity of 30 percent to 50 percent actually flew farther than dry baseballs.

This finding calls into question the experimental results from five seasons of baseball at Coors Field. Since 2002, in an attempt to counterbalance the effects on baseballs of playing at Denver’s mile-high elevation, the Rockies have stored all their game balls in a humidor set at 70 degrees Fahrenheit and 50 percent humidity.

The effects of elevation on aerodynamics of baseballs have been studied. A 400-foot home run hit at sea level becomes a 430-foot homer at Denver’s elevation. Coors Field became known as a great hitter’s park, and a graveyard for pitching careers. But that was before the humidor.

Home run totals in Denver have declined from 268 in 2001 to 185 in 2007, from the most among major league parks, to only the tenth most.

Most fans and writers have attributed the decline in offense, and corresponding improvement of the pitching, to the humidor. But Bohn and Meyer found that two months in the humidor increased the diameter of the balls by 0.26 percent and their mass by 1.6 percent.

Bigger, wetter, squishier balls come off the bat a little more slowly than dry ones. But this effect is more than counterbalanced because they retain their velocity longer. Because they are heavier, they take longer to slow down, and they travel farther.

Bohn and Meyer said:

“We have shown that storing baseballs in humidity controlled environments can slightly increase their size, mass, and density. This change, in turn, has small consequences for the aerodynamics of a ball in flight. Humidified balls will break slightly more, by about 0.2 inch, while batted balls may travel farther, on the order of a couple of feet. Both effects appear at first counterintuitive, but follow from the aerodynamics of spheres in air. Both effects are also common in experience: a baseball and wiffle ball are the same size, but the denser baseball curves far less and can be batted much farther. Moreover, the effect on the batted ball may be largely negated by the decreased coefficient of restitution of the humidified ball.”

So how to explain the decreased scoring at Coors?

“The search for the influence of the humidor on the game at Coors Field then shifts away from aerodynamics to other possible effects. An intriguing possibility is that the humidified balls are easier to grip, allowing pitchers to put a greater spin on a humid ball than on a dry one. In addition, a batted dry ball may be able to have more spin imparted in the collision, thereby allowing the drier ball to travel farther.”

The CU paper is not likely to be the final word on the issue, because it contradicts earlier research to a certain extent.

In his classic 1994 book The Physics of Baseball, Yale University physicist Robert Adair said:

“R.C. Larsen found that the weight of balls stored at 100 percent humidity for four weeks increased by 11 percent and the coefficient of restitution” – the promptness with which the ball pops back into its original shape after being struck with a bat – “at an impact velocity of 25 mph decreased by 10 percent when dropped on concrete from a height of 20 feet. The humidified balls will bounce only about 80 percent as high as the balls stored at low humidity.

“If that proportional decrease in elasticity holds at larger impact velocities, the swing of the bat that would drive a `dry’ ball 380 feet
will propel a ball stored at high humidity only 350 feet.”

After reviewing the Bohn and Meyer paper, Adair — who once held the impressive title of “Physicist to the National League” — said in an email:

“As I read the paper, it comes to the conclusion that the humidity does not much affect the flight of the ball — from pitcher to batter or from batter to fence. The differences the authors find are more or less within the uncertainties in their — or our — knowledge of the drag and Magnus forces.  This has always been my studies’ opinion.  I can quibble with some of their statements, e.g., that the Magnus force does not much affect the distance home runs travel — it does for line-drive home runs.  But that is not too important. 

“But I am very much disturbed that they never even mention the effects of humidity on the elasticity of the ball.  That is surely a much larger effect than  aerodynamic effects and likely to be quite important.  The elasticity certainly changes greatly as one goes from 0 percent humidity to 100 percent humidity (at room temperature) as stated in my book.  Hence, I am very comfortable with the general view that the Rockies have reduced home runs by raising the humidity of their  ball storage from 30 percent to 50 percent.”

Pitchers and catchers report for spring training in about five weeks.

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Dan Whipple

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