Here is something Bill Deane asked on the SABR list:
"Hank Aaron was heard saying that the cold springs in Milwaukee cost him
a lot of home runs, as it was hard to hit the ball out of County Stadium in
the first couple of months of the season. Any numbers on Aaron's HR
frequency in his home park, 1954-65, by month?"
Here are his HR percentages, home and away, for each month over this time period:
April: 4.76%, 7.44%
May: 4.60%, 5.96%
June: 5.97%, 5.91%
July: 6.61%, 6.80%
Aug: 4.41%, 5.29%
Sep: 6.07%, 3.70%
So it sure looks like "County Stadium in
the first couple of months of the season" cost him some HRs.
Using the Baseball Reference Play Index and game logs, I looked at HR percentages in April and non-April for 1954-65 for the Braves, home and away. The percentages below include both what the Braves hitters did and what their pitchers allowed
April home: 2.62%
April road: 3.35%
Non-April Home: 2.64%
Non-April Road: 3.08%
So it looks like HR% was lower at County stadium than elsewhere during those years. The gap was slightly larger in April. Haven't had a chance to look at May by itself. Probably won't be much different from April or non-April.
In April and May during those years, Aaron had 252 ABs at home in April and ABs 607 in May. How many HRs would he have hit if he had the same % as on the road?
April: 252*.0268 = 6.75
May: 607*.0146 = 8.86
That totals up to about 15. But the road % in April of 7.44% is a bit fluky. And maybe I should be using the % differentials for all players. Maybe it should be less than 15.