It’s no secret that the Mariners home park is one of the more pitcher friendly ballparks in the Majors. Because of it’s asymmetrical dimensions, its especially beneficial to left-handed pitchers who can take advantage of the large area in left-center field, allowing right-handed batters to pull the ball without getting penalized by as many home runs as they’d allow in a neutral ballpark. Jason Vargas is the best current example of this phenomenon, as he’s allowed just five home runs at home and 16 on the road this season.

However, what is happening in Safeco this season looks like it goes beyond just the park effects that we already know about. For some reason, Safeco Field is just destroying offense this year in a way that it never has before.

Any time we talk about overall run scoring in a specific ballpark, we have to account for the home team that makes up a vast majority of the at-bats in that park. And, of course, the Mariners are not a particularly good offensive team, as their .293 wOBA ranks 28th in baseball, ahead of only Pittsburgh and San Diego. Even by wRC+, which accounts for Safeco’s historical impact on run prevention, they’re still 28th in the Majors in offense. So, naturally, the home of a poor hitting line-up will have lower scoring games than average.

However, the Mariners aren’t just equally below average offensively at home and on the road. At home, they’re the worst offensive club in baseball, and on the road, they’re actually okay. In Seattle, the team is hitting .201/.280/.297, good for a .260 wOBA this year. The next worst hitting team at home is the A’s, but they come in at a .295 wOBA. The gap between the Mariners and the next worst hitting team at home is as large as the gap between the A’s and the White Sox.

On the road, the Mariners are hitting .259/.310/.420, good for a .317 wOBA that actually ranks 6th in the American League. When they’ve played away from Safeco, their offense has been slightly above average. When they’ve played at home, they’ve been the worst offense in the history of the sport. These results have led to speculation about the park getting into the player’s heads and questions about whether the mental toll of watching balls die on the warning track has begun to take an effect on the team’s psyche.

Maybe it has, I don’t know. But what I do know is that this isn’t an effect that is unique to the Mariners this year. For all the coverage that their offensive home/road splits have gotten, the fact that opponents are demonstrating nearly the same effect has gone a bit under the radar. Here are how other teams have done against the Mariners pitching at Safeco and when the team is on the road.

Home: .218/.284/.328, .272 wOBA

Road: .265/.333/.466, .333 wOBA

The pitching staff’s 61 point difference between home and road wOBA is actually larger than the batter’s split. Away from Safeco, the Mariners are the third easiest pitching staff in the American League to hit against — at home, they are the hardest. If opposing batters are seeing a similar decrease in offensive performance against Mariner pitching in Seattle, this suggests that perhaps the effect isn’t so much a mental issue with the Seattle hitters and more of a blanket effect that is suppressing hitters from all teams, no matter what uniform they’re wearing.

So, why is Safeco holding down runs to such an extreme degree this year, for both the Mariners and their opponents? Let’s break down the components for both the Mariners offense and their opponents offense by home/road differential.

Mariners batters at home: 9.7% BB%, 22.4% K%, .096 ISO, .249 BABIP

Mariners batters on road: 6.9% BB%, 19.7% K%, .160 ISO, .299 BABIP

Mariners opponents at home: 7.5% BB%, 21.0% K%, .120 ISO, .261 BABIP

Mariners opponents on road: 8.0% BB%, 19.1% K%, .201 ISO, .297 BABIP

There’s a gap in strikeout rate for both the Mariners and their opponents, so it’s possible that there’s something going on with the batter’s eye or the shadows that has made it harder to pick the ball up coming out of the pitcher’s hand, but if that was the case, you’d probably expect to see a corresponding shift in walk rate as well. That’s not the case, as the Mariners are drawing many more walks at home than on the road, and their opponents are showing a minimal change in the other direction.

While it’s worth noting that strikeout rates are up for everyone at Safeco, the driving factor in the lack of offense at Safeco is the way the park has crushed hits and especially extra base hits this season. Again, breaking down the components by home/road, but only this time looking at extra base hits per contacted ball.

Mariners batters at home: 5.3% doubles, 0.4% triples, 2.2% home runs

Mariners batters on road: 6.5% doubles, 0.6% triples, 4.1% home runs

Mariners opponents at home: 4.0% doubles, 0.4% triples, 2.9% home runs

Mariners opponents on road: 7.0% doubles, 0.6% triples, 5.5% home runs

Doubles and triples are down, but it’s home run rate that’s really getting destroyed in Seattle in the season’s first three months. The Mariners hit home runs at an 86% higher clip on the road than they do in Safeco, while opponents are at 89%. In other words, Safeco is cutting home run rates for all players nearly in half compared to what they’re posting away from Safeco.

This isn’t a park effect that’s turning home runs into doubles either, as the huge BABIP gap shows that a lot of these balls that would be home runs are actually getting run down for outs. From an observational standpoint, it appears that balls are hanging up long enough for outfielders to get under them, so while the dimensions serve to keep the ball in play, it’s been the marine air that has turned those balls into outs.

And, some cursory glances at the weather patterns in the northwest seem to support that idea. The National Climatic Data Center produces maps that show relative temperature index numbers by state, and while June won’t be available for a few more days, we can already look at how the Northwest’s weather has been different than the rest of the countries in April and May.

For the country, the NCDC reports April average was 3.6 degrees warmer than normal, with only California, Washington, and the upper midwest reporting declines from the hottest March on record. May brought significantly warmer temperatures to the rust belt and to California, however, while it actually got relatively colder in the Northwest — Washington and Oregon were the only two states to post below average temperatures in May.

Looking at the AccuWeather recorded temperatures for June show that Seattle was still relatively cold, especially compared to the rest of the country. While the east coast is being bathed in 100 degree temperatures this weekend, the high in Seattle yesterday was 72. For the first few weeks of the month, the high sat in the mid-50s to low-60s. The Marine air serves to keep Seattle fairly temperate even in a normal year, but 2012 has proven to be abnormally cool in Seattle, especially relative to the rest of the U.S.

We know that there is some relationship between temperature and offensive environment in baseball, though it’s not a one-for-one tradeoff between heat and runs scored. Humidity is also a factor, and we can’t simply say that Safeco is playing as an extreme pitcher’s park simply because of the weather patterns in the northwest this year. However, as this article notes, Seattle’s climate offers two essentially two types of days in the summer – cool and humid or hot and dry. Humidity is highest in Seattle when its cooler, but as it warms up, the temperature gets further from the dew point. The recipe for a baseball to fly a long ways is hot and humid air, but that just doesn’t exist in Seattle. It’s either (kinda) hot or humid, but almost never both.

We have pretty strong observational data that shows that Safeco is suppressing offense far more than it normally does this year, and it’s probably not a coincidence that Seattle is one of the few cities in the U.S. that hasn’t really gotten around to having summer yet. When Felix Hernandez was wiping out the Red Sox last night, that was probably more good pitching than favorable conditions.

However, it can’t be ignored that both Erasmo Ramirez and Jason Vargas racked up 10 strikeout games earlier in the week, and that excellent pitching performances in Seattle have been the norm rather than the exception even though Seattle’s pitching staff after Felix is a rag-tag bunch of pitchers. Their starting rotation, in particular, is not very good from #2-#5, and yet opposing batters just can’t put runs on the board in Seattle either.

The Mariners don’t have a good offense, but they don’t have a good pitching staff, and yet, no one is scoring runs in Seattle this year. Whether it’s the weather, the batting eye, or something else that we haven’t considered, there’s something going on in Seattle that is making Safeco Field look like a more extreme version of Petco Park.