Touching BasesMarch 24, 2009
Fun With Hit Tracker: Home Runs Over Time
By Jeremy Greenhouse

All home runs are not created equal. Over the course of a six-month season, things are bound to change. Players wear down or maybe some heat up. In the past, we've been able to find player trends by analyzing first-half and second-half splits or maybe even game logs. But now with new data sources, we can try to find out how or why players produce different outcomes over a season. Are they lucky? Do their skills improve? Do they fatigue?

Josh Kalk used pitch f/x data to show how pitchers fatigue during starts and he unveiled wear pattern charts for specific pitchers to show how some fatigue over the course of a season.

Another great new data source that has not received the same attention as pitch f/x is Hit Tracker. Developed by Greg Rybarczyk, Hit Tracker tracks every physical aspect of the home run. So how did the distance of home runs vary over the course of the 2008 season?

"True Distance" measures how far the ball actually traveled, or how far the ball would have traveled had it landed uninterrupted. I know, if only we could project how far Mickey Mantle’s and Ted Williams' legendary shots traveled. Well, Hit Tracker can. Here and here you go.

The chart seems to show that home run distances trend upward until early August and then fall slightly. It also appears that we can say with confidence that over the course of a week, the mean home run distance will be right around 390-400 feet. The first data points on the chart are a bit whacky, since the March average was 399 feet per home run, but then the first three days of April averaged 390-foot homers per day. Hence, the five-day rolling average is somehow much lower than the same month's average. But the main observation is that from April until July, there is a rather distinct increase in home run distance—around five feet per dinger. So what causes the change? Perhaps players need some time to get into their groove, or perhaps the environment becomes progressively more conducive to home runs. But how do we measure that? Did I mention that Hit Tracker also records the two most important components a batter can control? It captures where and exactly how hard the ball is hit. With the upcoming advent of hit f/x, we might get this data for all types of batted balls. The launch angle is measured in horizontal and vertical degrees from the point of contact three feet above home plate, while the speed off bat is measured in miles per hour. I chose to use the speed off bat as a measure for the player’s skill over time. I believe that a hitter's objective when he is at bat is to hit the ball as hard as possible. Here are the results:

Well, that appears to directly contradict what we saw in the first chart. Players seem to start off hot in the opening weeks of the season, but then by late May the average speed off bat flattens out at around 104 MPH until playoff time when there is a pretty decent rise. It would make sense that the select few who are able to hit homers in October do so with more power than the average hitter.

If it’s not the hitter who controls the change home runs, then it must be the hitter’s environment. Fortunately, Hit Tracker also records atmospheric effects such as temperature, wind, and altitude. Altitude should theoretically remain constant over time, as stadiums don't traditionally switch locations. But wind and temperature flow with the seasons. Since both factors can negatively impact the distance a ball travels, I plotted the absolute average impact as well as the actual average.

The impact due to temperature is defined as “the distance gained or lost due to the impact of the ambient temperature, in feet, as compared to a 'standard' temperature of 70 degrees." Temperature, along with the Speed Off Bat appear to largely explain the opening chart which showed the average true distance of home runs over the course of 2008. I’m not a physicist, but I figure a change in one mile per hour is equivalent to about 1.5 feet per second, and the average home run stays in the air around 3-5 seconds. So if the speed off bat decreases half a mile per hour in the early months, then the batter is responsible for about a three-foot decrease in distance. Yet the average true distance of home runs increases from about 395 in April to 398 feet in July. While the batter might cause at most a five-foot dip as the season progresses, the temperature appears to rise from a minimum average of -5 feet to a high of 5, which would explain the rise in distance. I’m also not a meteorologist, but the symmetry makes sense to me as the temperature rises in the spring, then peaks around the June 21 solstice, maintaining that point through the dog days of August, until the temperature declines going into the Fall classic. Not exactly shocking results.

Putting it all together with the standard distance, which controls for atmospheric effects and simply measures how far the ball would have been hit in neutral conditions:

Looks pretty even throughout the season, with the exception that distance possibly curls up at the start and end points. This could all be contributed to small sample size, but the fact that better players make the playoffs may have something to do with it, but do better players also start out hot? I'll be sure to keep note of it over the next few weeks.

Here's a chart of the three year's worth of data. Out of about 15,400 homers, Hit Tracker was missing data on less than 300 of them. The table should be read as the mean of each category, followed by standard deviation in parentheses.

Month     Amount True Distance Speed Off Bat Wind Effect Temp Effect Standard Distance                  
March     26     399.8 (25.3)  105.6 (5.7)   5.4 (17.5)  -4.0 (2.4)  396.7 (33.5)      
April     2214   395.6 (24.7)  106.1 (5.2)   1.7 (13.2)  -2.5 (4.3)  393.8 (27.1)   
May       2522   396.0 (25.3)  105.6 (5.2)   1.8 (11.7)  -0.2 (3.7)  392.1 (26.2)
June      2545   396.6 (25.5)  105.1 (4.9)   2.0 (10.6)   2.0 (3.4)  390.0 (25.8)
July      2446   397.9 (26.1)  105.3 (5.0)   2.3 (11.0)   3.3 (3.2)  390.4 (25.3)
August    2641   397.0 (24.8)  105.9 (5.0)   1.4 (9.7)    2.7 (3.6)  392.7 (26.6)
September 2508   398.0 (26.1)  105.9 (5.2)   1.5 (10.0)   0.9 (3.1)  392.7 (26.6) 
October   242    393.8 (24.8)  105.8 (5.1)   3.0 (10.6)  -2.0 (3.4)  391.2 (26.7)

I wanted to do a mini-case study applying changes in home runs over time, and the clear choice for any such study is Ryan Howard. He gives us a nice sample to work with and such a large part of his value is built on home runs. He’s been on a clear decline since his age 26 season, so we can see whether there have been changes in his home runs year by year. Plus, if you look at his day-by-day graph on fangraphs, he’s been a rather remarkable second-half hitter.


Over his career, he's held a 168 point difference in OPS between the first and second halves of the season. I'm not predicting that he'll continue the trend this year—I'm just pointing out that the trend has existed.

Howard also intrigues me since I believe he might be the best opposite field power hitter of all-time. But that’s a subject I’ll tackle another time hopefully. Again I decided to forego the launch angles and stick to the effects of speed off bat, temperature, wind, and distance. Presented without much commentary:

It's evident that he's been hitting the ball with less force in recent years. I also like that you can easily see trend lines with positive slopes each year, confirming him as a late bloomer.

Wind impact appears to be random, but you can almost make out those parabolic curves in temperature impact. A power hitter might be prone to mid-summer surges thanks to those extra five to ten feet in fly ball distance from less dense air.

Not much notable. He averaged 403 feet in 2006, 406 feet in 2007, and 398 feet in 2008. Here's his table. It includes his home runs from 2006-2008 but is missing three from 2007. It should be read category mean followed by standard deviation.

Month     Amount True Distance Speed Off Bat Wind Effect Temp Effect Standard Distance                  
April        13   414.2 (27.3)  109.1 (5.1)   5.3 (12.5)  -3.0 (4.3)   408.3 (28.9)   
May          29   394.9 (29.1)  105.7 (5.5)   3.4 (9.7)    0.5 (4.4)   390.1 (33.0)
June         24   410.7 (32.9)  105.1 (5.2)   3.2 (11.4)   3.3 (2.8)   402.5 (30.9)
July         28   398.1 (27.7)  107.7 (6.0)   2.8 (7.9)    3.0 (2.9)   391.2 (29.0)
August       26   404.8 (30.8)  108.0 (6.9)  -3.6 (13.2)   4.3 (3.0)   403.2 (35.0)
September    30   400.2 (22.9)  106.4 (4.6)   1.0 (6.9)    2.1 (2.8)   396.8 (24.8) 
October      4    390.5 (25.0)  104.5 (6.5)   3.7 (4.5)   -2.5 (6.3)   389.3 (32.2)

All data was obtained from Interested parties may contact