Atlanta Braves fact checker: how does humidity impact pitching?

Paul Byrd was talking with Atlanta Braves starter Mike Soroka and one of the topics that came up involved the South Florida humidity.  Let’s see what he was talking about.

Humidity vs. the breaking stuff.  Is there a correlation to more movement?  That’s what Mike Soroka is hoping as he starts tonight (game in progress) for the Atlanta Braves against the Miami Marlins.

Former big-league pitcher and FOX sideline reporter Paul Byrd sat down for a couple of minutes with Soroka and spoke about this and a couple of other topics in this interview linked below.

Mike’s hope is that the air will be a little denser and thus induce more movement on his breaking stuff.  Let’s take a quick look at the science behind that expectation.

Physics and Weather

The basic premise is correct:  air that is more dense will have molecules that are closer together.  They will strike a ball in flight more often, and that will lead to more drag.

That increased drag isn’t enough to radically alter the velocity of a pitch, but it can impact the bend of a pitch, particularly a  ball that’s already inclined to move thanks to spin and the influence of raised seams on the air around it.

There are actually multiple factors involved in air density calculations:  in simple form, it’s a combination of temperature, humidity, and altitude.  Local barometric pressure and dew point are also involved, but some of these are essentially dependent on one another.

I found a quick calculator that will assist in getting most of the math out of this post… I’ll plug in the current conditions in Miami:

• Air pressure:  29.92″
• Air temperature:  84°
• Air type:  Moist
• Humidity:  69.63% (this was filled in by the calculator)
• Dewpoint:  73°

Calculated density:  0.07243 lbs per cubic foot at/near gametime.  The altitude wasn’t part of this computation, though the barometric pressure is at least somewhat dependent on that

This isn’t going to be exact.  For one thing, the roof on Marlins Park is closed and their considerable air conditioners are running.  One of the primary tasks for air conditioners is to remove moisture from the air, which also impacts the air temperature – and in turn, the air density.

So for the sake of argument, let’s say that the game were being held in Atlanta.  Right now…

• Air pressure:  29.79″
• Air temperature:  80°
• Air type:  Moist
• Humidity:  56.21% (this was filled in by the calculator)
• Dewpoint:  63°

Calculated density:  0.07287 lbs per cubic foot at/near gametime.

Note that this is slightly more dense than at Miami.  Now let’s take the Miami numbers and vary the dewpoint at increments to see what happens.

• 78° – 0.07222, density (humidity 82.3%)
• 73° – 0.07243, density (humidity 69.6%) – actual values
• 68° – 0.07258 density (humidity  58.7%)
• 63° – 0.07269 density (humidity  49.4%)
• 58° – 0.07277 density (humidity  41.3%)
• 53° – 0.07283 density (humidity  34.5%)

Guess what?  Mike’s not correct!  It seems counter-intuitive, but with higher humidity – and all else held the same – air density actually decreases.

How Come?

The answer comes down to the composition of moist air vs. dry air.  Water molecules (2 parts Hydrogen, 1 Oxygen) are lighter than dry air – which is largely nitrogen.  The Hydrogen involved is much lighter than Nitrogen, and that’s the main reason involved.

But here’s the breakdown from somebody smarter than me:

"Water vapor (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 1 + 1 + 16 = 18 atomic units.At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become."

Now there is another factor involved:  altitude.  All other things being equal, Miami will have more dense air because of altitude above sea level (less than 5 feet) than Atlanta (1,001 at SunTrust Park or especially Denver (5,206 feet).

But that’s about altitude and pressure… not humidity/dewpoint/temperature.  In fact, the altitude change will matter more.  Miami will have 10% higher pressure than Atlanta.

That will matter… and that’s the factor that helps Soroka’s breaking pitches more than anything else.  But If he believes they are working better… we ultimately don’t really care about the reason.