What’s the Deal with the Humidor?

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Alan NathanUniversity of Illinoisa-nathan@illinois.edu

El Tiante

Humidors in the News

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First this

Then this

Some of the News is Controversial

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Tim Lincecum

Even Scientists Get in the Act

Why the Humidor in Denver?

Thin air means better “carry”

•Mile high air density 80% of sea level

•More home runs!

•Let’s take a look

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The “carry” of a fly ball

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• “carry” ≡ (actual distance)/(vacuum distance)for same initial conditions

(379,20,5.2)

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HITf/x + Hittracker Analysis:4354 HR from 2009

Denver

The Coors Effect

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~26 ft

Thin Air More Carry More Home Runs

• Coors Field in Denver:– Pre-humidor (1995-2001):

• Rockies home: 3.20 HR/game away: 1.93 HR/game

• That is a huge effect

• Since 2002, baseballs stored in humidor– What effect has that had on HR’s?

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Home Runs and Humidors

• Coors Field in Denver:– Pre-humidor (1995-2001):

• Rockies home: 3.20 HR/game away: 1.93 HR/game

– Post-humidor (2002-2010):• Rockies home: 2.39 HR/game away: 1.86

HR/game

25% Reduction!

Summary and Outline of Talk

• Thin air results in lots of HR in Denver

• Humidor has reduced HR by 25%

• Two questions:– Can science account for the reduction?– Can we predict what might happen in

Phoenix?

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Effects of Humidity on Ball Properties

• Humid air makes the ball “less bouncy”– “Coefficient of Restitution” (COR) decreases– Ball isn’t hit as hard

• Humid air make the ball heavier– Ball isn’t hit as hard– But carries better

• Humid air makes the ball larger– Doesn’t carry as well

12These effects can be measured

So Here’s The Plan:

• Measure in lab how RH affects ball properties

• Use physics of ball-bat collision to predict change in batted ball speed

• Use aerodynamics to predict change in fly ball distance

• From all this, we can estimate change in home run production

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Physics Alert!!!

• The next few slides will have some details that may bore you

• Hang in and I’ll tell you when to pay attention again

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So this is what we found

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Summary of Findingswith RH increased from 30% to 50%

• Weight increases by 1.6%

• COR decreases by 3.7%

• Diameter change negligible

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For typical long fly ball (100 mph, 30 deg)- Batted ball speed decreases by about 3 mph- Fly ball distance decreases from 415 ft to 402 ft

What about Home Runs?

• We have full record of every home run hit during 2009 and 2010 (336 total)– Total distance– Batted ball speed and launch angle– Good approximation to full trajectory

• With changed balls, can ask whether the ball would have cleared the fence– Only 265 make it

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Final Result

• Actual reduction = 25%

• Calculated reduction = 27.5±4.3 %

• The agreement is amazingly good– SCIENCE WORKS

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And what about Phoenix?

• If RH goes from 20% to 50%– HR reduced by 37.0±6.5 %– A very big effect!

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More Stuff from Nick….

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Final Summary

• Storing the baseballs in a humidor has resulted in a big reduction in home runs at Coors Field in Denver

• We understand very well the effects that lead to that reduction

• We can apply our understanding to predict an even larger reduction if a humidor is employed at Chase Field in Phoenix

• I like to think that our analysis led the Dbacks to postpone their decision

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QUESTIONS?