PETCO Park is no doubt an extreme pitcher's park, but there is a rumor that it is more hitter-friendly during day games due to the "marine layer." Is this a real thing or just an anecdotal rumor? I ran some numbers to find out.
Last week, I gave you a pre-Christmas present (or lump of coal, depending on your opinion) wherein I detailed the park factors and impacts of San Diego's PETCO Park. I focused on the impacts of the park's dimensions on right handed power hitters, since the Padres just collected a nice batch of new righty sluggers. At the end of that article, I teased you all with something I heard from a former San Diego resident. He claims that the "marine layer"' descends on downtown San Diego in summer afternoons and leaves by evening, resulting in fly balls traveling further in day games than in night games. I promised you all that I would investigate this story and see if there is any truth there.
To dive into this research, I used retrosheet.org extensively. They have game logs for every game in the last 70+ years. I pulled the game logs for every game in PETCO Park since 2008, focusing primarily on runs scored by both teams combined. The following table shows the results of this first step in investigating the day/night splits. The table shows the combined runs per game for both teams in all games since 2008, games in 2013-14, and pre-2013 games. Why was 2013 chosen to split out? That was the year the fences were moved in, especially in right field, so I expected that to affect run scoring.
| Petco | All Games | 2013-2014 | pre-2013 |
|---|---|---|---|
| Day | 7.56 | 6.68 | 7.89 |
| Night | 7.02 | 6.84 | 7.09 |
From this, it looks like run scoring was in fact higher in day games prior to 2013, by a significant margin. Then, in the last two years, day games had fewer runs scored per game than night games, so it doesn't look very convincing. Runs scored isn't showing us any real day/night effect, so what else can we look at? By using the individual game statistics for all games in baseball in 2013 and 2014 and then focusing on games at PETCO, we can look at the effects of day games on doubles and home runs, specifically. The table below shows the results of this research. Both the total home runs hit in PETCO and the homers/game are shown.
| Petco | 2013-2014 Total HR | HR/game |
|---|---|---|
| Day | 78 | 1.66 |
| Night | 169 | 1.47 |
Aha! Now we see that the ball does appear to carry further in day games than in night games, at least in the last two years. There are clearly more home runs in day games. The next table looks at the same information for doubles.
| Petco | 2013-2014 Total 2B | 2B/game |
|---|---|---|
| Day | 116 | 2.47 |
| Night | 338 | 2.94 |
Doubles don't show the same effect, so it looks like it is isolated to homers. Just to make sure this is showing a real effect, like a good researcher, I need a control. I used game data from all MLB games in the last two seasons and ran the same day/night analysis, shown in the table below.
| All MLB | 2013-2014 Total HR | HR/game |
|---|---|---|
| Day | 2897 | 1.79 |
| Night | 5951 | 1.83 |
In all MLB games, there is almost no difference in home runs hit between day and night games, with night games having slightly more. What this tells us is that PETCO does appear to boost home runs in day games. Keep this in mind for daily leagues and leagues with daily lineup changes. You may want to avoid Padres pitchers that are prone to giving up fly balls in home day games and you will want to start Kemp, Myers, Upton, and Norris in those same games. Maybe San Diego will push MLB to allow them to have more day games to take advantage of all their new sluggers more? As always, Tschus!