Sports betting is all about edges, using predictive information the public tends to ignore. Sure, everyone knows the Yankees are good and lots of runs are scored at Coors Field, but far fewer people understand that weather is perhaps the biggest edge in MLB betting right now. The impact on the game is massive, yet lines don’t always reflect weather conditions properly.
Basic science tells us that hotter air is thinner air, and thinner air creates less friction on a ball heading toward the outfield fence, causing it to fly farther and often over the fence. The average runs scored per game by temperature bears this out:
Under 40 8.64
Since 2017, the difference between games under 40 degrees and over 90 degrees has been nearly two runs per game. At one run per team, that kind of swing in weather alone would turn the 2019 Seattle Mariners (4.68 runs per game, 19th in MLB) into the 2019 Houston Astros (5.68 runs per game, first in MLB). That’s massive.
The trend isn’t quite linear. Run scoring spikes more in extreme heat, which is great because Las Vegas tends to handicap these games too conservatively. Check out how opening lines compared with actual run scoring on the high end of the temperature spectrum:
Temp. Open RPG
70-79 8.82 9.01
80-89 9.05 9.45
90-94 9.46 10.26
95+ 9.97 11.96
At a league-average 70 degrees, Vegas’ opening lines tend to mirror actual runs scored almost perfectly. At 80 degrees, they diverge a bit. At 90, a bit more. Once you get over 95 degrees, an extra two runs per game more than the opening line have been scored since 2017. The 95-plus bucket is a somewhat small sample size, so the real impact is likely less than two runs, but the trend is pretty clear: Vegas doesn’t properly account for heat.
This trend is also in the closing lines, though a bit less pronounced, meaning you can still find value in games like this throughout the day. The best news of all: In this fanless 2020 season, we’ve seen games played with temperatures as high as 113 degrees. Teams that would normally close their roofs because fans in the upper decks would swelter have decided that, at field level, games are playable enough with the roofs open, even when it’s extremely hot.
The Rangers opened their roof Aug. 10 despite 98-degree heat. The line opened at 8.5 and closed at 9, and 12 runs were scored. The Diamondbacks shockingly opened the roof midgame Aug. 5 in the face of 105-degree heat. In the first three innings, with the roof closed, three runs were scored. Over the rest of the game, 18 more crossed the plate. Twice last weekend, Arizona once again decided it was fine to open the roof in 100-plus conditions, so this seems like something we could be primed to take advantage of over the rest of the season.