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Weather forecasting and analysis, space and historic events, climate information

11:30 AM | ***Baseball is back and so is the "Home Run Weather" app!

Paul Dorian



It has long been contemplated that weather conditions have an effect on the distance a baseball can travel. More home runs are seemingly hit on hot days or on days with the wind blowing out.

Who can forget the many games at Wrigley Field that have featured numerous home runs as the wind raced out towards Waveland Avenue? Mike Schmidt cranked four home runs on just such a day at Wrigley Field in April of 1976. That game featured nine home runs and 34 total runs and a wind blowing out strongly ahead of a cold front. Not surprisingly, most of the games with four home runs hit by an individual player have occurred with temperatures of at least 80 degrees or with a strong wind blowing out. Conversely, most people would agree that fewer home runs are hit on cold days or with the wind blowing in. The old Candlestick Park in San Francisco frequently offered such weather.

The distance that a baseball travels is indeed impacted by atmospheric conditions. In general, the less dense the air is, the farther a baseball can travel. Humidity plays a crucial role in air density. Air with higher humidity is actually less dense than drier air. This may be contrary to perception and many baseball fans have no doubt heard baseball announcers incorrectly use the phrase “heavy humid air” on a hot summer night. Dry air is mostly comprised of diatomic oxygen and nitrogen (i.e. O2 and N2) whereas water vapor (H2O) is composed of one oxygen atom and two hydrogen atoms and the moist air has a lower overall atomic mass than dry air. Thus, at a constant temperature, the more water vapor that displaces the other gases, the less dense the air will become.

Additionally, hot air is less dense than cold air and higher altitude air is less dense than air at sea level. It is for this reason that so many home runs were hit in Colorado before the humidor was put into place. The elevated humidity in the humidor that stores baseballs for the Rockies home games effectively reduces the distance that a ball will travel in multiple ways: 1) by adding slightly to its weight through absorption of water, 2) by causing the size of the ball to increase slightly which increases air drag and 3) by reducing its “bounciness” factor.

Utilizing sophisticated math and physics, meteorologists and software engineers at The SI Organization, Inc. have investigated this topic which mixes science and baseball. And the results of their efforts are available in a free, real-time baseball weather application called Home Run Weather. Developed for the iPhone and Android devices, the app relates live temperature, atmospheric pressure, humidity, field orientation, wind direction, wind speed and the drag coefficient of a baseball to the user to determine if local weather conditions, for any big league park, are favorable for home runs being hit. Twenty-four hour forecasts are available in addition to live weather.

Two approaches were combined to arrive at a “home run favorability” index, which the app displays on a scale of 0 to 10 (least-to-most favorable). The first approach was to analyze actual weather conditions and home run data over several seasons (Citizens Bank Park was chosen as the venue for this study). The second approach uses a theoretical, physics-based model that determines the distance a ball will travel based on the temperature, relative humidity and atmospheric pressure.

The Citizens Bank Park study yielded some interesting, but perhaps not too surprising, findings. First, temperatures and dew points had a clear trend line relationship with home runs, as generally more home runs were hit in hot and humid air than colder, less-humid air. It was also found that 13-percent more home runs were hit when the wind was blowing out than other wind conditions. Additionally, 6-percent more home runs were hit in the daytime as compared to nighttime games.

A full major league season of testing on the app index produced very encouraging results with respect to the average number of home runs hit per game. The low home run favorability index values (0-3) had an average of 1.38 home runs per game for the full season. The moderate index values (4-7) had an average of 1.95. And the high values (8-10) had an average of 2.47 home runs per game. In addition, the home run favorability index correlated very well with average total runs scored per game. The low home run favorability index values (0-3) had an average of 7.07 total runs per game for the season. The moderate index values (4-7) had an average of 8.08. And the high values (8-10) had an average of 9.45 total runs per game.

Whether you're a fan at the park who's interested in a home run forecast, a spirited fantasy owner or baseball analyst, Home Run Weather should provide some useful information and conversation material. For more details, visit the "Home Run Weather" page on The SI Weather website (just click on the baseball).