MC- Oak is a pitcher friendly park and ChiW is a hitter friendly park so don't look at just ERA's.
Oak
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The stadium was originally known as a pitchers' park because of breezes blowing in from nearby San Francisco Bay, but after centerfield skyboxes were added in 1995 for the return of the Raiders, this deflected the normal wind patterns. The power alleys were also shortened by 13 feet, which has helped batters significantly.
After the partial enclosure of this cavernous stadium, prior to the 1996 season, flyballs have carried much better. Prior to 1996, the 1987 Oakland Athletics had the team record for home runs with 199; in 1996 and 1997, the team hit 243 and 197 home runs respectively, indicating how much more run-friendly the newly refurbished Coliseum is.
Between 1979 and 1995, only once did the ballpark have a run index of over 100 (103 in 1989). In 1996 and 1997, the newly renovated ballpark had run indices of 103 and 108, respectively. But from 1998 to 2000, the run index dropped to 88, the lowest in the American League. (Only Dodger Stadium in Los Angeles and Qualcomm Stadium in San Diego are lower over this period.)
It's still a tough place to score, though it's not as extreme as it used to be. The reasons are:
1) The foul territory is immense. I mean, gigantic. The rounded playing field leaves seemingly acres between the baselines and the crowd - the backstop is 60 feet away and remains almost as distanced from the playing field from 1st to 3rd base.
2) Winds coming in off the San Francisco Bay hold up fly balls that get up too high in a wicked jet stream.
3) The thick natural grass holds up a lot of ground balls, benefiting ground ball pitchers.
4) Starting in midsummer, shadows often fall across the plate, making it harder for hitters to pick up pitches.
The wind patterns slightly favor left-handed batters, though statistically the effect seems not to have shown up in the past few years.
Defense: The right side of the infield and outfield suffers a severe sun problem, making tracking flyballs a problem. Because of the immense foul territory, speed at the corners of the outfield is more important than in any other ballpark. The thick grass favors infielders who have limited range but strong throwing arms.
Before the renovations in 1996, the smooth, round outfield wall made it relatively easy to play outfield here, and the number of doubles and triples was well below average. However, since the renovations, the number of outfield errors is much higher (thanks to the straight walls and quirky angles that have replaced the round outfield walls) and the number of doubles and triples has skyrocketed.
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