As Temperatures Soar, So Do The Home Runs

Yesterday was the first day of summer. Temperatures were above-average throughout most of the country. In cities hosting MLB games, temperatures were 5-15 degrees above normal, save for those games played in the paradise known as the California coast.  There were 41 42 home runs hit in MLB games yesterday. These facts are likely related.

First things first. Yesterday’s home run tally did not break the record for most home runs hit by all MLB teams on a single day. That record is 62 home runs, hit on July 2, 2002. We’ll come back to that in a bit.

Here’s a chart showing the location of each MLB game yesterday, the temperature at the start of the game (as reported by Baseball-Reference), the high temperature in June in that city, and the number of home runs hit in the game.

Games on June 20, 2012 Temp. Average High Temp. Home Runs
Atlanta at New York (AL) 94 F 79 F 9
Seattle at Arizona 77 F  (Dome) 104 F (Outside) 8
Toronto at Milwaukee 89 F 75 F 5
Cincinnati at Cleveland 88 F 79 F 2
Miami at Boston 96 F 76 F 6
Tampa Bay at Washington 94 F 84 F 1
Colorado at Philadelphia 95 F 83 F 4
Baltimore at New York (NL) 94 F 79 F 1
Chicago at Chicago 86 F 80 F 1
Los Angeles at Oakland 65 F 71 F 1
Minnesota at Pittsburgh 91 F 79 F 1
San Francisco at Anaheim 68 F 78 F 3
Texas at San Diego 69 F 71 F 0
Kansas City at Houston 73 F(Dome) 91 F (Outside) 0
St. Louis at Detroit 92 F 79 F 0

As you can see, in those games with at least four home runs, the temperature at game time was at least 10 degrees above the average high temperature for that city in June, with the exception of the game in Phoenix, which was played in a temperature-controlled dome.  The converse is not true, however. There were games played in cities which also experienced temperatures at least ten degrees above normal that did not see four or more home runs hit.

We intuitively believe that hot temperatures lead to longer and more home runs. Many of us refer to hot sunny days as “home-run weather.” And it’s true. The basics are explained by this wonderfully illustrated The Science of Baseball exhibit from San Francisco’s Exploratorium.

A few weeks ago, Alan Nathan, a Professor Emeritus at the University of Illinois-Champagne Champaign who studies the physics of baseball, provided a more detailed description of the relationship between high temperatures and home run production. Nathan wrote the article in response to Tim McCarver‘s recent comment during a FOX broadcast of an MLB game that global warming explains the rise in home runs in the 1990s and early 2000s. Nathan’s main point:

Suppose the average MLB game-time temperature were 10F higher. Fly balls on a typical home run trajectory would travel about 2.5 ft farther (about 0.6%), leading to 6% more home runs. As a more dramatic example, consider games played at the two extreme temperatures of MLB, 30F and 110F. The home run probability would be about 50% greater at the high end (110F) than at the low end (30F). This result simply confirms what everyone already qualitatively knows: balls carry better at higher temperatures, leading to more home runs. The principal contribution made here is to quantify the effect of temperature on fly ball distance and home run production.

As for the global warming connection, Nathan shoots that down.

Instead, let me focus on the period between 1980 and the present, during which the global temperature increased by about 1F while home runs/team/game increased by well over 30%. The analysis presented here is that an increase of average temperature by 1F would result in an increase in home runs by about 0.6%, a factor of over 50 below the actual increase. Clearly climate change cannot account for the dramatic increase in home runs since 1980. It can’t even come close!

Using Nathan’s calculations, it appears that the hot temperatures in New York, Philadelphia, Boston and Milwaukee likely contributed to the number of home runs hit in those games. Indeed, the nine home runs hit in the game between the Yankees and Braves at Yankee Stadium tied or set several records.

Now, back to July 2, 2002, when the teams combined to hit 62 home runs. Here’s a chart showing the location of each MLB game on that date, the temperature at the start of the game (as reported by Baseball-Reference), the high temperature in July in that city, and the number of home runs hit in the game.

Games on July 2, 2002 Temp. Average High Temp. Home Runs
Detroit at Chicago (AL) 93 F 84 F 12
San Francisco at Colorado 92 F 88 F 10
Kansas City at Seattle 67 F 76 F 6
Chicago (NL) at Florida 82 F 91 F 4
Milwaukee at Pittsburgh 84 F 83 F 4
Cleveland at New York(AL) 95 F 84 F 3
TampaBay at Texas 75 F 96 F 3
Minnesota at Oakland 66 F 72F 3
Los Angeles at Arizona 72 F (Dome) 106 F 3
Houston at Cincinnati 95 F 87 F 3
New York at Philadelphia 95 F 87 F 3
San Diego at St. Louis 95 F 89 F 3
Baltimore at Anaheim 74 F 83 F 2
Montreal at Atlanta 74 F 89 F 2
Toronto at Boston(Game 1) 91 F 81 F 1
Toronto at Boston(Game 2) 91 F 81 F 0

We see some of the same above-average temperatures on July 2, 2002 as we saw yesterday, particularly in Chicago and Denver, which featured a combined 22 home runs. A few additional notes, though.

  • There were 16 games played on July 2, 2002, due to the double header in Boston. Of course, those two games combined only saw one home run.
  • The game in Denver was played at pre-humidor Coors Field, which yielded more home runs than any other stadium.
  • The games on July 2, 2002 were played during the so-called PED Era, when there was one home run hit every 36.9 plate appearances. This season, there’s been one home run hit every 38.2 plate appearances.
  • A funny note: Barry Bonds played in that game at Coors Field, and didn’t hit any home runs that day.

Summer is here. Temperatures are rising. We may see another day this season with more than 41 home runs.




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Wendy writes about sports and the business of sports. She's been published most recently by Vice Sports, Deadspin and NewYorker.com. You can find her work at wendythurm.pressfolios.com and follow her on Twitter @hangingsliders.


17 Responses to “As Temperatures Soar, So Do The Home Runs”

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  1. RMD says:

    Not only did Bonds not homer at Coors in the homerest day in earth’s history – he didn’t even get walked.

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    • RMD says:

      Scratch that, he did get walked once. But the Giants scored 18 runs without a Bonds homer.

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  2. Keith says:

    Actually, there were 6HR hit at Fenway last night.

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  3. steex says:

    Thanks for trying to class up my state, Wendy.

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    • steex says:

      That was supposed to include a block reference to “University of Illinois-Champagne.” Forgive me, I’m bad at the internet.

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  4. JR says:

    We also intuitively believe that Tim McCarver doesn’t know what he’s talking about most of the time. And it’s true.

    +13 Vote -1 Vote +1

  5. Kyle H says:

    My fantasy team will remember yesterday. 5 HR in the first 15 AB

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  6. TKDC says:
    FanGraphs Supporting Member

    It was 74 degrees in Atlanta and 75 in Dallas in July?

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  7. But says:

    Wendy Thurm is from the future, where beanballs count as grand slams.

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    • But says:

      Hot dogs cost 2 silver, and global cooling keeps the temperature in the 70’s in summertime.

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  8. Richie says:

    How tough would it be to simply plot what % of balls go for home runs at what temperature? Has this been done somewhere?

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  9. balticfox1917 says:

    Yes, it would be interesting to see if someone has plotted all of these weather-related factors.
    We also know that very humid air causes the ball to fly further–lighter water molecules displace diatomic molecules of oxygen–and that high pressure should depress home runs slightly also.
    Combine the three factors: heat, humidity and high pressure vs. low pressure.
    Surely, someone has looked into this.

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    • Richie says:

      My understanding of humidity was that humid, heavy air hinders the ball rising in the first place, thereby holds down distance (and so home runs).

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