To recap how I got the data last year: Alan Nathan, Professor Emeritus of Physics at the University of Illinois at Urbana-Champaign wrote an in-depth article for BaseabllProspectus.com that detailed just how temperature can affect the flight of a ball when all other factors are consistent. (The point of his article was to disprove that the rise in home runs in baseball lately was due to global warming, but the math works for what I'm looking for.)
Nathan used data from Sportvision’s HITf/x and Greg Rybarczyk's HitTracker that analyzes every home run hit in baseball these days to find the correlation between a temperature change and the flight of the baseball.
He was able to conclude that if you keep all other environmental factors constant, that for a 400-foot home run on a traditional home-run trajectory, every degree you add in temperature, a ball will travel 0.25 feet farther (3 inches).
So if it's 71 degrees at Boston's Fenway Park (same 15-foot elevation as Safeco Field) and David Ortiz hits a ball the exact same way that Montero hits a ball in Seattle's 55-degree April chill, Ortiz' ball would travel 4 feet farther.
That might not seem like a lot, but consider this: Nathan also concluded that a 1 percent change in distance (so, moving a 400-foot fence in 4 feet) corresponded to a 10 percent better chance of getting a home run.
The average depth in Safeco Field's left field power alley was about 400 feet before the changes this off-season. Now it's down to 392 feet. In left field and right center, the fences were brought in 4 feet.
That means that where the fences were brought in 4 feet, it's going to have a similar effect of taking a 2012 game at Safeco and adding 16 degrees to the ambient temperature -- a 55 degree day in 2013 will now "play" like a 71-degree day would have in 2012.
In the power alley, it'll play like a relative 87 degree day (some specific spots even warmer since they're in further than 8 feet).
So this will be the equivalent of taking Safeco Field in it's 2012 configuration and moving it to a much warmer climate. Sadly for the bundled up fans, it does not actually translate into warmer air around the field!