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One of the first axioms I learned when I wandered into the world of sports betting was to heed Wrigley Field’s winds. Wrigley’s proximity to Lake Michigan gave it a reputation for dramatically affecting fly balls, which would inflate or deflate the game over/under on runs. If the wind was blowing out, fly balls were expected to sail out as home runs, and the total would be unusually high. A low total typically meant that winds were blowing toward home plate, suppressing fly balls.

Vegas already knew this, which unfortunately added an additional dimension to handicapping Cubs home games. Amazingly though, this advice was extremely exploitable in fantasy baseball. An “@ChC” note next to my pitcher meant a trip to Baseball Weather Analyzer or Daily Baseball Data (two sweet resources) to examine Wrigley Field’s conditions that day. Flyball pitchers sat on blow-out days and started on blow-in days.

Chris Constancio of The Hardball Times investigated the effect of winds on HR/FB rates six years ago, and he observed statistically significant results in Chicago parks. I replicated his method on data from 2007 to the present and found that the Wrigley wind effect is stronger than ever. Over 508 games, here’s how pitchers performed in HR/FB rate, ERA, and slugging percentage allowed, split by Retrosheet’s wind field:

 Direction Definition Freq. HR/FB% ERA oppSLG In Blowing In, 10+ mph 18.9% 8.9 3.47 .364 Still Any direction 0-3 mph. 7.3% 16.7 4.17 .411 Out Blowing Out, 10+ mph 10.0% 19.4 5.84 .508 Other Any 4-9 mph, and cross winds 63.9% 12.7 4.24 .412

Harry Pavlidis, one of BP’s many Cubs experts, notes that wind direction and speed can change within a game at Wrigley Field.

Unfolding “Other” into its respective “In” and “Out” bins yields an intuitive symmetry, indicative of Wrigley’s varying wind spells:

 Direction Definition Freq. HR/FB% ERA oppSLG In In, 10+ mph 18.9% 8.9 3.47 .364 In, weak In, 4-9 mph 28.9% 11 4.00 .397 Still Any direction 0-3 mph. 7.3% 16.7 4.17 .411 Out, weak Out, 4-9 mph 12.0% 16.3 4.96 .460 Out Out, 10+ mph. 10.0% 19.4 5.84 .508 Cross LF to RF, or RF to LF 22.8% 12.8 4.16 .405

The distribution of wind events is not perfectly uniform from season to season, but even when divided into yearly buckets (so that talent is better controlled for), the outcomes are very apparent.

A considerable portion of the performance of pitchers in Wrigley was affected by the day’s wind condition. I retrieved the splits for starters with at least 45 innings pitched each in the “In” and “Out” states at Wrigley, since 1998:

 HR/FB% ERA oppSLG Name In Out In Out In Out Carlos Zambrano 7.4 16.2 3.19 3.42 .320 .370 Kerry Wood 9.0 23.0 2.63 4.67 .288 .441 Ryan Dempster 11.7 24.4 3.06 7.39 .349 .502 Jon Lieber 11.4 26.7 2.85 4.57 .323 .440 Kevin Tapani 19.4 18.8 3.47 5.38 .364 .490 Ted Lilly 7.9 19.7 2.12 5.80 .297 .563 Greg Maddux 14.9 24.1 3.39 4.20 .396 .462 Matt Clement 15.0 30.0 2.32 4.14 .285 .396 Mark Prior 5.8 22.1 2.32 5.14 .289 .502

No one was immune—not even The Professor. The effect is quite evident for starters with flyball tendencies like Wood, Lieber, and Lilly. Ultimately, the opposing wind extremes end up balancing each other out; taking the sum of all Wrigley Field numbers, the given rates are a sliver off league averages. However, the extremes might cause Wrigley pitchers to have more variation in their starts than a non-Wrigley pitcher.

The biggest beneficiary of the wind effect this season is Travis Wood. His 2.24 ERA ranks seventh in the NL, and every one of his nine starts has been deemed “quality.” Unsurprisingly, none of his six home starts featured a game with wind blowing in from the ivy:

 Date Wind Speed PA AB IP H R ER BB SO TB HR FB 20130409 In From LF 23 29 26 6.3 7 3 2 3 6 9 0 5 20130416 In From RF 5 28 24 7.3 5 2 2 3 3 7 0 6 20130502 In From CF 19 27 26 7.7 4 3 3 1 3 4 0 6 20130507 Still 2 28 26 6.7 6 1 1 2 8 10 1 3 20130513 RF to LF 5 24 21 7 2 0 0 3 2 2 0 11 20130519 In From RF 9 27 25 7 5 3 3 2 3 9 1 6

The “wind schedule” has skewed favorably for Wood in these early months. Combined with some BABIP luck, he’s evaded the regression that is looming—so far. We know he can’t sustain a .193 BABIP forever. But if he continues to serendipitously find the wind behind him, there’s a chance that his HR/FB rate will keep floating around his current 6.5 percent mark, enabling him to outperform his peripherals. For his career pitching at Wrigley, Wood’s numbers reveal a massive dichotomy:

 Direction IP HR FB ERA SLG In 46 1 39 2.54 .247 Other 63.7 5 64 3.67 .346 Out 16.3 10 25 9.39 .818

Travis Wood is a flyball pitcher like Kerry Wood before him, and the wind effect on his numbers is amplified. Hitters won’t continue slugging .818 against him with southwest winds, but if the trend is real, the .508 opponent slugging mark served by Wrigley pitchers under those circumstances in the past seven years can be expected.

Obviously, the Cubs can’t control Mother Nature, nor schedule its starters around her. Pitchers at Wrigley Field will always be, to a degree, at the mercy of its winds. The fiercest months—April and May—are nearly over, but the summer games will still see the occasional gusty afternoon. It’s a convincing reason to observe the weather, whether you’re debating a start/sit decision for a fantasy starter, or you’re the Cubs manager, pondering your offensive strategy.

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