Pitch Movement – There’s something in the air

If you have been watching baseball for some time you will know the game is slightly different at Coors Field. At 5,200 feet (1,580 m) above sea level, Coors Field is by far the highest park in the majors. The next-highest, Chase Field in Phoenix, stands at 1,100 feet (340 m).

When it was built, the designers knew that the stadium would give up a lot of home runs, as the lower air density at such a high elevation would result in balls traveling farther than in other parks. To compensate, the outfield fences were placed at an unusually far distance from home plate, thus creating the largest outfield in Major League Baseball.

In spite of the pushed-back fences, for many years Coors Field has not only given up the most home runs in baseball, but due to the resultant large field area, the most doubles and triples as well.

Because of this, it is well known as a hitters park but the impact on pitchers and more specifically their pitches is less well known. Today we going to look into that. But first a quick recap of baseball physics.

Baseball Physics

Regardless of whether the ball is pitched or batted, its motion is determined by the forces acting on it. After a pitch is thrown or hit these forces are the downward force of gravity, that we all know, as well as two principal aerodynamic forces: drag force and Magnus force.

Drag force is more commonly referred to as “air resistance”.  When looked at from the point of view of the ball, the on-coming stream of air molecules collides with the surface of the ball, pushing the ball backwards. A fly ball that carries 400 ft would carry about 700 ft if there were no drag force.

The Magnus force is responsible for the curve or “break” of the baseball. The direction of the force is such that the ball breaks in the direction that the leading edge of the ball is turning. For example, a baseball thrown with backspin (e.g., an overhand fastball) has an upward Magnus force, opposing gravity, so that a typical fastball does not drop as much as it would if it were solely under the influence of gravity.

The important thing to remember about both the drag and Magnus forces is that they are both proportional to the density of the air. If baseball were played in a vacuum with no air, these forces would be exactly zero. But baseball is not played in a vacuum, and there are variations in air density at each stadium due to different altitudes, temperatures and other weather conditions.

When the air pressure is lower, pitched baseballs will be a little bit quicker. But the amount is quite small and batters can easily adjust to it. Far more important is the reduction in the Magnus force, resulting is less break on the pitched ball.

Some Maths

Now we all know a little bit more physics, let’s look at the effects of Coors Field and other stadiums in 2018.

To do this I have taken the pitch movement data, from Statcast, of every pitch thrown in MLB in 2018. Then calculated the average movement (vertical and horizontal) for each pitch of every pitcher when they were home and when they were away. With that info, I work out the difference between the two to give me a distance difference in break, home v away, for each pitcher.

Finally, I combined them together for each team, with a weighting based on a pitcher’s minimum home or away pitch count – e.g. If a pitcher threw a fastball 100 times at home and 500 times when away, then their weight would be 100. If a pitcher threw a fastball 300 times at home and 300 times when away then their weight would be 300.

The data shown from here on out is for right handed pitchers only due to there being larger samples to compare.

Coors Field

The above table is the net difference in movement, of 5 different pitches, for Colorado pitchers at Coors compared to elsewhere, in 2018. It is in inches. So the average Rockies change up will move 4 inches less when thrown at Coors, and the average of all pitches is just over 3 inches. Now, that sounds like a lot but how does it compare to all the other stadiums?

The table above is net difference for all the teams, so it could mean that a pitch moves more or less. This just highlights the difference between home and away movement. The values are also inches.

You can quite clearly see the difference between Colorado and the rest of the MLB . If you use the mean and standard deviations of these, as below, you can see that Coors is more than 3 standard deviations from the mean. 99.7% of data is expected to be within 3 SDs of the mean, which shows you how much of an outlier Coors is.

All other teams fall within one SD of the mean, so aren’t any where near as significant. But some are worth commenting on.

Kansas City and Atlanta both have the same reduced pitch movement, when at home, but on a smaller scale. On the other hand San Diego and the Dodgers experience more movement, especially in the horizontal direction, when playing at home.

The Dodgers have the highest gain in horizontal movement, but it is only half an inch. And while it is that small, I cannot attribute it to helping their pitches and being part of why the Dodgers are such a strong pitching team.

We cannot draw any conclusions, beyond Coors, but I would be very interested to see if teams/pitchers change what pitches they throw based on the atmospheric conditions and their impact on ball movement.

I started this piece with a Thunderclap Newman reference in the title so it seems perfect, to me, to end with another 1960s classic, from Buffalo Springfield, that sums this and, for me, most of the baseball stats world.

“There’s something happening here, what it is ain’t exactly clear”

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