Every broadcast in April and October mentions that the ball does not carry in the cold. It is one of those claims that sounds like folklore but happens to be straightforward physics — and unusually, it has been quantified in a peer-reviewed study large enough to take seriously.
The useful question for a bettor is not whether the effect exists. It is how big it is, and whether it is large enough to matter next to everything else that prices a total.
Warm air is less dense than cold air. Fewer molecules per unit volume means less drag on a batted ball, so the same contact travels further. Cold air is denser, drag increases, and fly balls that would clear a fence in July die on the warning track in October.
This is the ideal gas law applied to a baseball, not a hunch. Alan Nathan, the University of Illinois physicist who has consulted with MLB on home run rates, calculated the effect from ballistics and air density alone and arrived at essentially the same figure as the statistical work described below.
A 2023 study published in the Bulletin of the American Meteorological Society by Christopher Callahan and Justin Mankin at Dartmouth analysed more than 100,000 major league games and around 220,000 individual batted balls. Their finding: a 1°C increase in the daily high temperature raises home runs in an open-air stadium by 1.96%. For early-afternoon games the effect rose to 2.4%.
In Fahrenheit terms that is roughly 1% per degree.
Applied to a real seasonal swing, the arithmetic is not trivial. A game played at 14°C in late September rather than 28°C in July represents a 14-degree Celsius difference — implying something in the region of a 25% reduction in home run likelihood on identical contact, holding everything else constant.
It applies to open-air parks only. Domed and retractable-roof stadiums with the roof closed are climate-controlled, and the effect vanishes. Any weather adjustment applied to an indoor game is noise.
Home runs are not the same as runs. The study measured home runs specifically. Total scoring depends on much more: baserunning, sequencing, bullpen quality, defensive positioning. A 25% reduction in home run likelihood does not translate into a 25% reduction in the total.
Temperature is a small factor overall. The Dartmouth researchers were explicit that warming accounts for only about 1% of home runs hit annually. Ball construction, launch-angle optimisation, hitter strength and pitch velocity all contribute more to the overall home run environment than temperature does. Anyone claiming weather is the dominant input to a total is overselling it.
Sportsbooks are not unaware of temperature. Weather is a standard input, and a total posted for a cold October night already reflects it. The edge, where one exists, is narrower than “bet unders when it’s cold.”
Forecast movement between posting and first pitch. Totals go up early. Weather changes. A total priced against a forecast of 18°C that materialises as 9°C with wind off the water has a stale input, and the correction is not always complete.
Wind direction, which is larger than temperature. Wind blowing out at a park with a favourable orientation can add more expected scoring than a fifteen-degree temperature swing removes. Wind is also more volatile and harder to forecast, which is precisely why it is priced less precisely.
Interaction with park. Cold suppresses scoring most where carry matters most. A drop in temperature does more to a total in a park where fly balls are the primary source of runs than in a pitcher-friendly park where they were rarely leaving anyway.
Humidity and elevation as companions. Both feed the same air-density calculation. Dry, dense, cold air is the least favourable combination for carry.
Treat temperature as one small input among several, applied only to open-air games, and weighted below wind and below the starting pitching matchup. Check whether the forecast has moved since the total was posted, because that gap is where a stale line lives. And resist the temptation to build a strategy on it — a factor responsible for roughly 1% of home runs is not going to carry a betting operation by itself.
What it is good for is breaking ties. When your analysis has a total close to the posted number, the weather picture is a legitimate reason to lean, and in late September and October it leans in one direction more often than not.
Research published in the Bulletin of the American Meteorological Society found that each 1°C rise in daily high temperature increases home runs in open-air stadiums by about 1.96%, or roughly 1% per degree Fahrenheit.
No. Sportsbooks price weather into totals already. The opportunity lies in forecasts that change after the line is posted, and in wind conditions, which are less precisely priced than temperature.
No. Indoor and closed-roof games are climate-controlled, so outside temperature has no effect on carry. Applying a weather adjustment to those games introduces error rather than removing it.
Generally yes, in open parks with a favourable orientation. Wind can add or remove more expected scoring than a substantial temperature swing, and because it is harder to forecast it tends to be priced less precisely.
Park, weather, bullpen state and the starting matchup all feed the number before we ever look at the sportsbook’s. Members see the full reasoning behind each call.
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