A lot has been made of the large number of HRs at the new Yankee Stadium so far this year. AccuWeather speculated that the way wind traveled travelled through the new Stadium was responsible for the additional HRs. Greg Rybarczyk determined that the dimensions of the park are not, in fact, exactly the same as the old Yankee Stadium. AccuWeather since came to the same conclusions as Greg, that it was a change in outfield dimensions and not wind that was responsible for the change.
Greg determined that the biggest difference in outfield dimensions is in a portion of right field where the wall is between 2 and 9 feet closer to home plate and 2 feet shorter than in the old Yankee Stadium. This portion of the wall was already relatively close to home plate, the famous short porch in right.
The HR numbers are way up, and I wanted to see if they were up in this location that both Greg and AccuWeather identified as having the biggest difference in outfield dimensions between the two stadiums. So I looked at the HR per ball in air rate by angle for the old Yankee Stadium from 2005 to 2008 (I am using the GameDay data and that is the extent of it) and for Yankee Stadium so far this year. The thick lines is the estimate and the thin lines indicate the standard errors. An angle of -45° corresponds to the 3B line, 0° to right up the middle (second base) and 45° to the 1B line. Here is the rate for right handed batters.
In left field there is almost no difference in HR rate, but in right field there is a slight increase in 2009 compared to pre-2009. The confidence intervals overlap so the difference is not statistically significant. Along the right-field line there is actually a drop in HR rate, but since there have been so few balls hit there is no statistical confidence in this difference.
Here is the same image for left handed batters.
Here there is a real statistical difference. Between about 5° and 35° the HR rate has been statistically higher in 2009 than pre-2009. This data is for all hitters and is not corrected for level of hitter, as park factors are. So it could be that there have just been more power lefties hitting at Yankee Stadium this year compared to 2005-2008. But since the largest increase in HR rate is in the same area of largest outfield fence change I think it is that fence change that is responsible.
The new Yankee Stadium has an even shorter porch in right field, and, it seems, LHBs will be the primary beneficiary.
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