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	5 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.

Elias Says Yankees & Braves combined 9 HR tied most in any Yankees game since ’51, Mickey Mantle’s 1st year with team es.pn/MkT3np

— ESPN Stats & Info (@ESPNStatsInfo) June 21, 2012

1st #Yankees game in Live Ball Era in which 2 different pitchers allowed 4 HR in a game

— Mark Simon (@msimonespn) June 20, 2012

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.