Five other teams have a retractable roof: Arizona's Chase Field, Houston's Minute Maid Park, Milwaukee's Miller Park, Toronto's Rogers Centre (a.k.a SkyDome) and, just christened this year, Miami's Marlins Park.
But unlike Safeco Field, whose roof is essentially just an umbrella and does not enclose the stadium, all of those other parks can become completely enclosed domes if needed, although many are equipped to essentially "open some windows" with the roof closed if desired.
Thus, two parks in particular -- Arizona and Houston -- also use the roof to protect fans from hot summer-time temperatures in addition to any rain and are closed for much of the heart of summer. For 2011, I was able to get roof data for 71 of Houston's 81 games and it was closed for 57 games, apparently including all home games in June, July and August. So no good comparisons there.
Miami closes their roof for heat and rain, according to Matthew Roebuck with the Miami Marlins. In their inaugural year at the park, they've had their roof closed on 34 of the team's first 41 games. Small sample size, but pretty obvious the roof is likely going to be closed more than open.
However, the stadiums further north do have their roofs open more often.
John Steinmiller, Manager of Media Relations with the Milwaukee Brewers, says Miller Park's roof is also closed primarily for fan comfort, which includes rain and temperature -- but not for dodging the heat. In fact, it can be for dodging the cold.
"Wisconsin springs can be quite cool, so those April and May games often have the roof closed," he said. "It really just depends on the weather pattern, we have been roof open on opening day, but we have also had snow on opening day." He added the stadium does not have air conditioning, so the roof isn't much use for heat.
For 2011, Milwaukee had the roof closed 38 times out of 81 (about 47%) and this year so far, they had the roof closed for 16 of their 36 home games (44%).
OK, how about Toronto?
They too consider both temperature and rain a factor in deciding on whether to close the roof at Rogers Centre, says Jay Stenhouse, Vice President of Communications for the Toronto Blue Jays. But neither was too much of an issue last year.
For 2011, The Rogers Centre had the roof closed just 20 times from the start, and there was one instance it had to be closed during the game. Close enough to call it 21 as I think Seattle counts a mid-game closure as a "closed roof" as well.
A tie, then, at least for 2011? Not quite. We can still say Safeco was the most open stadium if we use percentages, at least for last year. Remember, last season had a bit of a fluke in that Seattle played three "away" games against the (then) Florida Marlins at Safeco Field due to a scheduling conflict in Miami. So there were 84 games played at Safeco, of which 21 were closed.
Seattle's roof benefits two-fold
Hale says there are two benefits of Safeco's roof. One, obviously, is to prevent a rain-out or, more frequently, any rain delays. And having it rain on nearly a quarter of your games a year can really cause some headaches -- especially since while most Midwest and East Coast storms that cause rain-outs pass in a day, an individual Seattle storm can have lingering showers for two to three days, only to be followed quickly by another storm.
Just this past April, there was a decent amount of rain on 11 of the 15 days between April 16 and 30th. (Although looking to their neighbors to the south, Tacoma's outdoor Cheney Stadium seems it only had one home game rained out since the start of 2011. But I'll bet a lot of those games were pretty drippy at times.)
But second, and perhaps even more importantly, having the roof wipes out the worry of a possible rain out.
"Over half our fans come from outside King County and they travel an hour or more to get to the ballpark," Hale said. "They need to know that if they make the trip, they'll see a ballgame."
Per their media guide, if the roof is closed at the start of the game, the Mariners do have the option of opening the roof between innings if conditions warrant, provided they notify the umpire crew chief. The visiting team also has the right to challenge the opening. The Mariners can only have one opening per game (so, no off and on manipulations to mess with the hitters.)
But Hale says Mariners have rarely, if ever, used this option. Once the game starts with the roof closed, it usually stays that way no matter if the sun comes out later.
However, if the roof is open and the Mariners determine it needs to close, they have sole discretion. Play can continue as the roof closes (and often does) although the umpires can stop play if they feel they need to. It takes about 10 minutes to close the roof.
Hale says as a general rule, they try to keep the roof open as much as possible:
"We try to begin the game with the roof closed if there is rain in the forecast within an hour of the start of the game," she said. "If there is no rain, and it's determined we don't need to close it for the 'comfort' of fans, then we want to err on the side of having it open."
And as we've seen, odds are fairly decent that will include the game you're going to. But if you do manage to pick a game when the roof is closed, I'll bet you're certainly glad the roof was there!
(And perhaps maybe other stadiums are regretting they didn't add a roof:)
A lightning bolt comes down from the clouds near Nationals Park before a baseball game between the Washington Nationals and the New York Mets on Wednesday, July 18, 2012, in Washington. (AP Photo/Alex Brandon)