A few days ago, prompted by a question from beloved reader Kermit, I got to investigating when Safeco will pass the Kingdome in total attendance. I realize now that my answer was incorrect, since I neglected to take playoff games into account, but that isn't the point of this post anyway. During my research, I got to paying special attention to the 1999 regular season. You'll recall that 1999 was the season during which the Mariners switched home stadiums.
On April 5th, the Mariners opened in the Kingdome - a stadium about which David Segui said "not a hitter in baseball doesn't like to play here." Indeed, everybody considered the Kingdome to be among the league's more hitter-friendly ballparks. But the M's played their last game in the dome on June 27th in front of 56,530 people. A 12-game road trip took them into the All-Star break, and they opened the second half on July 15th in brand new Safeco Field, a stadium which would later literally kill Jeff Cirillo dead. Allegedly built with Ken Griffey Jr.'s swing in mind, Safeco was nevertheless a very, very different environment.
How different? In 1999, the Mariners played 87 games in the first half of the season, and 75 games in the second. In the first half, the Mariners scored 527 runs and allowed 550, for an average of 12.4 total runs per game. Compared to the full-season numbers, that would've stood as the highest average in baseball.
In the second half, the Mariners scored 332 runs and allowed 355, for an average of 9.2 total runs per game. Compared to the full-season numbers, that would've stood as the lowest average in baseball.
Looking at it another way, in the first half, there were 13.8 average runs scored per game in the Kingdome. In the second half, there were 8.8 average runs scored per game in Safeco. The sample sizes are obviously quite limited, and a team's roster in the first half of a season is never identical to the team's roster in the second half of a season, but we're talking about a 36% reduction in run-scoring. That is absolutely, groin-grabbingly huge.
Between halves, the Mariners' team OPS dropped by 105 points, and the Mariners' team OPS against dropped by 101 points. The team changed, but it didn't change that much. The major difference was simply the place it called home.
Park factors. They're nuts. Never underestimate the significance of environment.