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    Is weather playing a role in MLB's record-setting home run spike this season?

    By Olivia Miltner, AccuWeather staff writer
    August 01, 2017, 12:37:44 PM EDT

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    Home Run

    Carlos Santana home run (Photo/Erik Drost/Flickr)


    The first few months of the 2017 Major League Baseball season were record-breaking.

    In June, players hit more than 1,100 home runs, more than in any other month in MLB history. This put the league on track for more than 6,000 home runs by the end of the season, which would smash the record set in 2000.

    Experts have yet to decide why the MLB has seen such a spike this season and a general increase in home runs over the past three years.

    Most potential explanations revolve around aspects like how baseballs are designed or if players are adjusting their swings to hit more home runs. Some believe batters are trying to increase the launch angle of their hits and the speed of their swings, which should result in more home runs. However, one topic is already known to impact home run rates in certain areas: weather.

    When the Colorado Rockies entered the MLB in the 1990s, people immediately noticed the number of home runs at the Rockies' stadium, Coors Field, was higher than anywhere else in the league, Alan Nathan, professor emeritus of physics at the University of Illinois, said.

    That was due to an atmospheric trait, lower air density, which can have a big influence on ball flight. Lower air density can help balls fly longer distances. The air is less dense at Coors Field since it sits at an elevation of 5,200 feet above sea level.

    “If I’m comparing Major League Baseball venues, then the most important difference is the difference between Denver and everywhere else,” Nathan said. “The Denver effect is very well known... the elevation plays a big role. It shows up pretty much immediately.”

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    Air density has a few major components. The most influential is elevation, Nathan said, while the second is temperature. The third, humidity, plays a smaller role.

    “People know, players even know, understand intuitively if nothing else, that the ball simply does not carry as well in cold weather as it does in warm weather,” Nathan said. “The temperature effect is not as huge [as elevation], but it’s a big effect.”

    Mace Michaels, a meteorologist for the Twins, said the players and grounds crew he works with, as well as long-term baseball fans, tend to acknowledge this trend based on their own experiences and observations.

    “They tend to say the baseball flies better when it’s hotter or more humid,” Michaels said.


    The chart above shows the average number of home runs per game in red and the average air density in green for when those home runs were hit.

    These climate characteristics combine to create conditions less or more favorable for home runs in certain fields. One of Nathan’s studies, for example, looked at how climate conditions affected ball fly distance in Arizona and San Francisco.

    “Arizona is about 1,000 feet higher in elevation than San Francisco and has a temperature about 17 degrees warmer, both of which contribute to lower air density and therefore longer distance,” Nathan said. “What would typically be a 400-foot fly ball would travel about 10 feet farther in Arizona than it does in San Francisco, and it’s partly elevation, partly temperature.”

    Coors Field now keeps baseballs in a humidor, which increases the relative humidity at which baseballs are stored and lowers their "bounciness" while increasing their weight. This was meant to help balance out the elevation effect, and after the humidor's implementation, the rate of home runs was cut by 25 percent.

    The Arizona Diamondbacks are also considering the install of a humidor, and Nathan predicted it could reduce home runs hit at Chase Field by 37 percent, even more than in Coors Field because Phoenix has lower relative humidity than Denver.

    Air density and its contributing influences are well understood, but wind is anther weather condition that has a large impact on how far a ball flies. The wind's effect is especially difficult to measure.

    Wind conditions, Michaels and Nathan said, vary significantly, not just at specific points in time during a ball game, but also within the field itself. This makes quantitatively accounting for wind difficult, Nathan said.

    “A direct northwest wind in Target Field doesn’t mean all the balls that fly into left field will be knocked down," Michaels said.

    "The wind kind of swirls in our park, so it has a huge amount of difference. I can walk 10 feet in the outfield and have the wind at my face and 10 feet later have the wind at my back,” Michaels said. “I’ve walked out there in all different types of wind, and you can’t even begin to figure it out.”

    Outdoor fields are particularly exposed to weather variability since grounds crews don’t have the same ability to control conditions during a game as indoor fields.

    Although weather conditions have the ability to impact how far baseballs travel, Nathan said he doubts weather-related effects are responsible for the uptick in home runs this season.

    Instead, he said most researchers are looking into other variables that affect how a ball flies, such as the speed at which the ball comes off the bat, the design of the baseball and the way players swing their bats.

    Even when looking at why some fields are considered more home run-friendly, Nathan said the explanations are usually not weather or climate related.

    Besides a few notable exceptions like Coors and Chase fields and Globe Life Park in Arlington, Texas, where summer temperatures can reach the mid-90s F, home runs in different stadiums are often affected by stadium characteristic like the size of the field and how fences are built.

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