Air Density and How it Impacts Baseball Games

With the Weather a Bit Quiet Today, I Figured I would post this article. 

 

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 Air Density and How it Impacts Baseball Games

By now, I hope you have read my daily reports where I mention air density and wind for each game. Wind is self explanatory, if it is strong and blowing out, that is great for the batter, strong and blowing in, great for the pitcher and varying degrees in-between depending on how strong the wind is and its direction in or out of the ballpark.

Air density is not quite as simple but it is not rocket science either. There are three factors that impact air density: elevation above sea level, temperature and the amount of water vapor in the air (often referred to as humidity).

The 1st factor mentioned, elevation, I already wrote about here. This is the only factor of the mentioned 3 that will never change staying at the same ballpark, thus this factor is easy to figure out. It also why Coors Field in Denver will always be a "10" because of the huge advantage it gives hitters.

As you can see, Coors has the highest elevation and it is really not even close. Elevation does impact other ballparks (think of Pac Bell park in SF sitting on the bay vs. the 724 feet of elevation that PNC in PIT has) but I do not automatically assign a 9 on my scale for Chase or Turner Field like I assign a 10 for Coors. Air density decreases dramatically as you increase in elevation. The difference Coors makes on baseball games is well documented and is, quite honestly, remarkable.

Temperature has an impact on air density and it is quite simple: the warmer the temperature, the less dense the air is and the farther a batted ball will travel. Right off the bat (nice pun huh? @RSandersDFS on twitter would be very proud) I will say that temperature will not affect the distance traveled of a batted ball nearly as much as a strong wind blowing in or playing at Coors. However, let's see if I can show you its impacts:

Density is on the column at the far right. A normal atmosphere at a temperature 20C (68F) is 1.2041 kg/cubic meter. If you just raise the temperature and nothing else, air density goes down and vice versa.

Why is that? Very simply, heating any substance (in this case air) causes molecules to speed up and spread slightly further apart, occupying a larger volume that results in a decrease in density.

So what does that mean to a high, flyball to deep centerfield? An exact measurement is tough to do. Does warm air give it 10 more feet of travel? Well, it depends on how much warmer the atmosphere is. I do not think saying that a ball hit 380 feet on a very cold day traveling 390 feet on a hot day is unreasonable. I found this article that talks about how far balls may travel due to differences in air density by a physics professor from the University of Illinois. Seems like he was very interested in the 2007 World Series between the Red Sox and the Rockies.  He believes that air density plays a bigger role than some people may think.

All in all though, I do not think it can be unquestionably said that a batted ball will travel exactly x feet at 55F and x+10 feet at 80F. But, we can say that a warmer atmosphere will give a batted ball a boost.

We now come to humidity. Simply put, the higher the dewpoint of the air is (ie. the more water vapor it contains) the farther a baseball will travel. Water molecules ( H2O, molecular weight of 18) actually weigh less than a molecule of air (air is mainly composed of diatomic Oxygen, which has a molecular weight of 32, and Nitrogen, which has a molecular weight of 28). As water molecules are added to the air, the average atomic mass decreases. Since density is equal to mass divided by volume, decreasing the mass also decreases air density.

Much like temperature, humidity does not play nearly as big a role in batted ball distances as elevation or wind does. But it does make a difference! I would say that temperature and humidity play about the same role in determining how far a batted baseball can travel.


Also, please keep in mind, that as temperatures and dewpoint rise in summer, the wind also changes direction and speed. A case in point is Fenway Park in Boston. As June comes around, warm and muggy days and evening become common. Accompanying this different airmass is a south or southwest wind. Instead of a cool wind off the ocean from the Northeast that is blowing in from right, a warm and humid southerly wind blows out towards the Green Monster. So the increase in temperatures and dewpoint help hitters but probably not nearly as much as the wind blowing out to left.

And, batters generally are much more comfortable when it is 75 degrees vs 45 degrees. Have you ever used a wooden bat when it is cold and made contact with a baseball? It stings! Plus, you are all bundled up and your muscles may be tighter. Again, this is a bit of an inexact science. You really can not put a distinct measurement on how much air density and other factors hurt or help a batter, you can simply say that it does.