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The
Mile High Effect & The Humidor Ball
(Aug-26-2007)
When I mentioned the players got it wrong in a recent poll about the
best hitting park, that it was still Coors Field and not the players’ choice
of Citizens Bank Park in Philadelphia, I got a thoughtful email response asking four things:
1) Was
I using data on Coors Field that was no longer relevant because the use
of the "humidor room" had diminished the park's offensive edge?
2) Should the Rockies be allowed
to influence the game this way?
3) What is it
exactly that boosts offense at high altitude?
4) What is it
about the "humidor ball" that is so helpful to pitchers?
For those who
don't know this story, the Rockies for several seasons now have taken to storing their baseballs
in an environmental chamber that is essentially a large humidor that controls
the temperature and humidity, and yes, it has had a very discernable impact
in lessening the offensive edges experienced at high altitude. In the
seasons before the use of the "humidor room," the number of runs
scored per inning (combination of both teams) was about 59% greater in Colorado home
games than road games. In the seasons since the introduction of the
"humidor room" that figure has been cut by more than half.
To answer
question #1, the data I used referenced only seasons where the Rockies were already
using the humidor room, something they began with the start of 2002,
which is two years before the new park opened in Philadelphia. Even making minor adjustment to equalize the
number of outs used by the home team and their opponents, Coors Field by an
overwhelming margin has been a better offensive park than Citizens Bank
Ballpark.
Runs per
Inning Compared to Road Games
Coors Field Citizens Bank
2004 to
Aug-23-07
+26% +9%
The Phillies have
had the better park for homers in that period, but Coors Field is the far
better park for singles, doubles, triples, and ultimately runs scored. It is
possible that I am still overestimating the modern impact of Coors Field. The
consistency of how the Rockies have used the environmental chamber is not clear. For example,
in 2002 they set the humidity at 40% but at some point - I believe
in 2003 or part way through 2003 - they raised it to 50% which is the
storage humidity recommended by Rawlings (along with a constant
temperature of 70 degrees). In 2006, when there seemed to be a
further dip in offense at Coors Field, a reporter claimed the Rockies were storing the
baseballs for a longer period of time in the humidor room. But even if you
just use the data from 2006-07, which is the closest we can come to getting
these parks near each other, the offensive increase of Coors Field is 17.6%
or exactly double the 8.8% offensive edge of Citizens Bank Park for those two
years.
To address
question #2, it is kind of hard to fault the Rockies for simply
storing baseballs under the conditions recommended by the manufacturer. In
fact, major league baseball is currently adding guidelines to create greater
consistency in the balls being used. The 2007 season is the first where all
30 clubs will be keeping balls in a temperature controlled setting, and it is
the first season MLB has issued a directive essentially setting an expiration
date on the balls used in games. Only balls purchased in the current year can
be used in a game. Can controlled storage humidity be far behind?
But even if the Rockies were being
allowed to doing something slightly different to offset their extreme
ballpark effect, is it really any different a case than allowing them
to have the deepest fences in baseball? We allow a lot of differences
within reason and an acceptable range. We let some teams have artificial
turf. We let a lot of teams have slightly unusual fence depth or
configuration, not just the Rockies. We let teams improve their lighting system above the required
standard, and we let them fiddle with their hitting backgrounds. We let them
adjust the thickness and height of their infield grass. Understand this is
not a case of the Rockies choosing what degree of humidified balls they will play with
according to the opponent. If there were a concern that they were going to
cheat that way, it would be easy enough to safeguard. Simply require the
storage room be run and accessed only by MLB.
Right now the Rockies are really the
only team that does anything to be sure their balls are within the range
covered by the rulebook. For example, even though there is a rule that the baseball
must weigh between 5.0 to 5.25 ounces, no one other than the Rockies keeps
track of their weight after they leave the factory, even though we know for a
fact that the weight of balls can vary beyond that range due to something as
simple as the level of relative humidity they are stored in. (An independent
study by MLB found in a sample of 3000 balls taken from storage from ML teams
- 100 from each team - that 13% of balls were underweight, even though they
all had passed the weight requirement when shipped from Costa Rica.)
The Rockies have found that
when stored at the 50% humidity, 70 degree temperatures, recommended by
Rawlings that their balls tend to weigh 5.12 ounces. When the Rockies examined balls
stored before they controlled their environment, they found some of the balls
did not come close to the required range of weight and
circumference. The Rockies had unknowingly been breaking the rules, and what they are
doing now only brings them back within the rules.
Some of you may wish
to skip over the next few paragraphs as answering questions 3 and 4 are more
science than baseball questions. And before you think I am geekier than I am
- which I suppose is considerable - for having so much of this at the tips of
my fingers if not the top of my head, let me explain that I worked with the
Rockies at one point precisely because I was fascinated by the Mile High
effect, and my interest in weather effects on baseball actually goes all the
way back to a thermal environment study that played a key role in
starting my baseball career 26 years ago.
There are many
factors that account for the dramatic impact on offense called the Mile High
effect. Perhaps the most unavoidable relates to a significant reduction
in air resistance. Even with zero wind, air resistance still exists based on
the density of molecules in the air. Technically that air resistance is a
combination of density and viscosity, but we can toss out viscosity as
this varies so slightly there are no meaningful extremes to consider. Air
density, however, can vary quite a bit.
The three
variables influencing air density are humidity, temperature, and air
pressure. Of the three, air humidity - the amount of water vapor in the air -
has the least impact on air resistance. The difference in air density
between, say, the extremes of humidity at 10% and 80% - which is well beyond
the average differences between the most extreme ballparks - is all of 1%.
While it is for the most part insignificant, it is interesting to know the 1%
gain is actually at the HIGHER humidity. All other things being equal, a ball
will carry ever so slightly further in humid air due to lower air density.
Water vapor has an atomic mass of 18 units, and in moist air it is
replacing the heavier molecules of diatomic Oxygen (32 units) and diatomic
Nitrogen (28 units), thus resulting in lower air density. The perceived
impact of "heavy" air - humid air – restricting the flight of a
baseball has nothing to do with a change in air resistance. It is the impact
of humidity on the ball itself. That's why the Rockies humidor room has
made a difference. They are putting the balls in the humidor, not the
ballpark.
Temperature is
much more significant than humidity in affecting air density. The hotter it
is, the more molecular motion that is going on, which
expands volume and decreases density. For about every 5-6 degrees
difference in temperature (Fahrenheit) there is a 1% difference in air
density. The maximum difference in average temperature during the baseball
season is between Milwaukee and Miami with an average difference of 20 degrees. All other things
being equal, air density would average about 3-4% less in Miami than Milwaukee. (All
other things aren't actually equal in this case because Milwaukee is about
700 feet higher in elevation than Miami. See the
next paragraph.)
The largest
influence on air density is air pressure. Even the presence of
a high or low pressure weather system can affect the carry of a
baseball through the air, but altitude generally has a larger impact on air
pressure, and it is variable that is always there. For about every 1000 feet
you are above sea level, air density goes down a little more than 3%. (That
percentage starts to diminish at the extreme elevations but by then you are
also talking about elevations higher than the tallest place on earth.)
By temperature we have an extreme difference in air density between
major league cities of 3-4%, and that is generally the maximum spread for the
influence of altitude if you throw out Denver. That
would leave the tallest major league cities as Phoenix (1100 ft
above sea level) and Atlanta (1010 ft above sea level). But we aren't throwing out Denver, and air
density in Denver is about 16% less due to reduced air pressure compared to the
numerous cities near sea level in the major leagues.
But reduced air
resistance is not the only intractable element to the Mile High effect.
Changing the air pressure on the sides of the ball is the basis of all
movement of a spinning baseball beyond its forward motion and its
gravitational fall. Every curveball, every slider, the different movement of
the 2-seam and 4-seam fastball, and every variance between a sinking and
sailing fastball - all of it - is tied to air pressure. When you are working
with less air pressure to begin with, the amount of movement resulting from
changing air pressure by spinning the ball would be reduced. Harvard
physicist Robert Adair estimates the reduction in a curveball's movement
is 25% less at Denver's altitude. Straighter fastballs and less movement on
breaking balls is a real part of the Mile High effect. It's a big reason why
Coors Field is the first ballpark to have a relatively huge
impact on strikeout rates, lowering them to a degree that no other ballpark
has ever come close to matching.
Another
intractable but minor factor of the Mile High effect is a bit of extra
fatigue from not getting enough oxygen during times of exertion. The relative
percentages of the gases in the air are the same at high altitude (oxygen is 21%
of the gases in air regardless of the elevation) but with the reduced
pressure there is less of all of the gases in each cubic foot of air, which
means less oxygen is taken in with each breath. Fortunately people can fully
adapt to this even at elevations much higher than Denver's but it
isn't instantaneous, and it isn't helped by moving back and forth between
altitudes 5000 feet apart.
But there is
another element to the Mile High effect that is easily controlled, and
surprisingly the tip about its potential magnitude came not from a study or
experiment done by the Rockies but by one commissioned by MLB itself. In 2000
they had the University of Massachusetts-Lowell - which actually has a "Baseball Research Center" - study the difference in distance when hitting the
extremes of weight and circumference allowed for major league baseballs. That
is, which would go farther and how much farther, a 5.25 ounce ball with
a 9.25 inch circumference, or a 5.0 ounce, 9-inch ball. It was not that
surprising to find it was the smaller ball that went further. It had less
weight to be influenced by gravity, and it had less surface area resulting in
less air resistance. They calculated that a home run swing, one that would
drive a mid-range ball (5.125 ounces and 9.125 inches in circumference) 387
feet, would go 421 feet if it were the smallest legal ball and 372 feet if it
was the largest legal ball. That’s an incredible difference. Two perfectly
legal balls could differ almost FIFTY
feet on the same home run swing. (I suppose that impact is mitigated somewhat
in actual practice by the pitcher being able to throw the smaller ball
slightly faster as well.)
High altitude
reduces relative humidity and dew point. In short, it creates ideal
conditions for things to dry out quickly and remain in a state of dryness.
The baseballs in Denver would get so dry they would lose weight through lost
moisture and shrink a bit. In several cases the lost weight and shrinkage was
greater than what U-Mass was testing. The greatest shrinkage that the Rockies found in a ball
in their old storage room was one that weighed 4.6 ounces and was 8.5 inches
in circumference. The dried out balls were also harder and slicker, to a
point where more than a few pitchers would complain about not being able to
get a good grip on the ball. In fact, the Rockies said in 2002
that their initial goal in setting up the humidor room was to improve the
grip on the ball.
Better grip and
the balls don't go as far. That's the edge being gained by the
"humidorized" baseballs. It is far from a perfect solution to the
Mile High effect, but it was a big step in bringing it into a reasonable
range, and that is good news for the Rockies as an
organization. When I worked briefly with the Rockies in the
pre-humidor days, I saw a lot of wrong-headed decisions being made simply
because it was so difficult to comprehend a park effect being that gigantic,
and literally everyone had the experience and training to approach things
from the perspective of building a successful club at low altitude. It
wasn’t working, and I don’t think it ever would have worked. They either had
to radically change their thinking and their approach, or they needed to
somehow get the ballpark effect within a range that “low-altitude” mentality
could handle.
Before the
humidor room, Colorado had a ballpark effect like no other in the history of major
league baseball. It was so far above the norm that the last ballpark to have
even half that type of impact was Philadelphia’s Baker Bowl way back in the 1929-37 period. Among ballparks in
my lifetime none had even a quarter of the impact of Coors Field through
2001.
With the humidor
room Colorado still has an extreme hitter’s park, but at least now it is in
the range of “comprehension.” People can more easily wrap their minds around
this type of park effect, make better decisions. Just as important is that
their pitching is no longer working under the pressure of trying to contain a
juiced offense. That makes it a lot easier to develop a pitching staff with
less volatile swings in health and performance. The biggest effect of the
humidor ball will ultimately be its enhancing the ability of the Rockies to build a
ballclub.
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