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January-February 2014

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Beyond Rain Delays: Weather's Role in America's Favorite Pastime

One of the classic lines from the motion picture Bull Durham (1988) is, "A good friend of mine used to say, 'This is a very simple game. You throw the ball, you catch the ball, you hit the ball. Sometimes you win, sometimes you lose, sometimes it rains.' Think about that for a while." But from a meteorological and physics point of view, is it really that simple of a game, even if it doesn't rain?

When most people think about weather and America's pastime, it's probably about whether to bring sunblock or if today's game might be rained out. And even that's less of a factor when seven out of the 30 major league ballparks have domes. But meteorological factors play a large role in the sport, whether it rains or shines, ranging from the microscale physics that affect the flight of a thrown or batted ball to the macroscale impacts of the shape and location of a ballpark.

The Physics of a Baseball in Motion

On the face of it, it should be pretty simple, as the motion of a smooth ball or other sphere moving through the air obeys Newton's Laws of Motion. Once a ball is moving, under Newton's first law, it will tend to move in a straight line unless it is acted on by external forces. Those forces are weight, drag, and lift. The weight of a baseball is distributed throughout the ball, but it can be thought of as single point, or its center of gravity-that point about which it rotates. Also, as a ball moves through the air, there is drag, which is the resistance of the air to the motion of the ball. This is a function of the properties of the air such, as its density and viscosity. And the final basic force on a ball in motion is the aerodynamic lift that is perpendicular to the ball's direction of flight. The complex interaction of these forces and a baseball's motion are further influenced by a number of other factors. First, a thrown or batted ball is spinning, sometimes a lot and other times, like with a knuckleball, very little. Second, the stitches on a baseball, all 108 tiny raised red stitches, further disturb the flow of the air around the ball. And third, the air through which it is traveling may itself be moving and have a wide range of temperature, humidity, and density.

When a baseball is thrown or batted its trajectory is affected by gravity (weight), the density of the air (drag) and the aerodynamics forces due to its rotation (lift).
Caption: When a baseball is thrown or batted its trajectory is affected by gravity (weight), the density of the air (drag) and the aerodynamics forces due to its rotation (lift).

The aerodynamics of air flowing around a spinning baseball is the same as that of air flowing over the wing of an airplane. Originally postulated by Sir Isaac Newton in 1672 after watching tennis being played at Cambridge, England, this effect was further described by German physicist Gustav Magnus in 1852 and is known as the "Magnus Effect." It says that the flow of air on the side of a sphere will be faster due to the combined effect of its motion through the air and any rotation of the ball. This side with the faster flow creates an envelope of lower pressure, and the ball will move in that direction. This is further enhanced by the raised seams on a baseball, which help the ball's ability to develop a boundary layer and a greater pressure difference between the upper and lower zones.

The backspin on a pitched baseball can be in excess of 2,000 rpm and significantly affects its aerodynamic properties.
Caption: The backspin on a pitched baseball can be in excess of 2,000 rpm and significantly affects its aerodynamic properties.

A typical baseball is thrown with varying degrees of spin, be they backspin (the motion of the stitches on the top of the ball is in the opposite direction of travel) or off-axis spin (as would be the case of a curveball). The amount of spin can be in excess of 2,000 revolutions per minute. It may seem intuitive, but the amount of movement due to the spin of a ball is positively correlated to the speed of the ball. It is the magnitude of all of these lift forces combined with the drag and weight that determine the ultimate path of the ball.

Air Density and Humidity and the Baseball in Flight

Importantly there is also the density of the air through which a ball may be moving. The density affects the amount of drag on the ball, and so both the distance that the ball may travel and how much it might curve. The density is a function of the interactions between temperature, atmospheric pressure, and the amount of moisture in the air.

There is considerable confusion about the effects of humidity on a baseball and its flight. "Conventional wisdom" is that moist air is heavy and therefore dense, causing more drag on the ball. Actually the opposite is true, because moist air is lighter than the same volume of dry air. The molecular weights of diatomic nitrogen (N2) and diatomic oxygen, which make up 99% of the atmosphere, are 28 and 32 atomic units, respectively. Compared to these, a water molecule (H2O) only has an atomic weight of 18 atomic units. Consequently, at a constant temperature, the more air molecules that are replaced by water molecules, the less dense the air. But the differences are slight.

For example, on a 70-degree day, the density of air that has a relatively low dewpoint of 20 degrees (15% relative humidity, or RH) is 1.198 kilograms per cubic meter (kg/m3). If the dewpoint is raised to 40°F (34% RH) the density drops by only .002 kg/m3, and by raising the dewpoint to 60°F, it only drops by another 00.4 kg/m3. With a 90-mph Major League curve ball, these differences equate to less than one-tenth of an inch displacement between the pitcher and homeplate, and would changes the distance of a 375-foot home run by less than a foot.

While the humidity of the air has a relatively minor impact on the flight of the ball, it may actually have an influence on the ball itself. In higher humidity areas, it is theorized that the ball gains weight by absorbing moisture from the air, and thus its resiliency and the distance it might travel are decreased. At the high altitudes of Denver, Colorado (elevation 5,280 feet), the low air density and the usually very low humidity contributed to one of the highest home run rates in the Major Leagues after the Colorado Rockies moved to town in 1993. To mitigate this, since 2002, baseballs for the Rockies games are kept in a humidity-controlled room at 50% RH in an effort to "deaden" them.

The temperature has an even greater impact on the density of the atmosphere, and thus the flight of the ball, than the humidity. The density, with constant pressure and humidity, at 50°F is 1.24 kg/m3, which is approximately 7% higher than on a 90-degree day, when it's 1.15 kg/m3. In the flight of a baseball, this translates to about two inches of extra "break" on a curveball, or a decrease of nearly 16 feet on a 356-foot fly ball.

But the biggest differentiator in regard to the density and thus the flight of the ball is atmospheric pressure due to the elevation above sea level. On a 70-degree day, the air density at sea level is 1.194 kg/m3, which drops by about 3% to 1.15 kg/m3 at 1,000 feet, then by another 4% to 1.11 kg/m3 at 2,000 feet, and all the way down to 0.99 kg/m3 at 5,000 feet. For each 1,000 feet of elevation change, a curveball will break three-quarters of an inch more in denser air and three-quarters of an inch less in air that's not as dense. This means that a pitch that might break as much as four inches at sea level would only break one-quarter inch at Coors Field in Denver, Colorado-a change that would mean significant adjustments for both batters and pitchers.

Thinner air has an even more dramatic effect on batted balls. A well-hit long fly ball at sea level in San Francisco, California, or New York, New York, might go 376 feet on an average day with 70-degree temperatures and moderate 34% humidity. A similarly hit ball with the same weather conditions in Atlanta, Georgia, or Phoenix, Arizona, at about 1,000 feet above sea level would go about 381 feet. But in mile-high Denver, Colorado, a ball hit the same would travel 405 feet! Again, this explains the previously mentioned placement of baseballs for Denver Rockies in a humidifier to mitigate the less dense air.

If baseball were played at the summit of Mount Everest (elevation 29,029 feet), a similarly batted ball would be a 505-foot homer. But this would pale to the same well-hit ball on the atmosphere-less and low-gravity moon, where it would travel 4,014 feet!

Wind in Play

Thus far, only the motion of pitched and batted balls through still air has been considered. However, the air is seldom calm, and the prevailing winds can significantly impact the flight of the ball, whether it is thrown or batted. The winds in a particular city are even taken into account with the orientation of a ballpark or the type of hitters that a team might focus on acquiring. And the velocity of wind does not have to be very strong to make a big difference.

A well-hit baseball on a 60-degree day at sea level will travel approximately 376 feet. However if there is a tailwind of just 5 mph, the ball will travel 415 feet, which is a home run in most ballparks. And if that wind is increased to 10 mph, the ball will travel another 40 feet and land about 455 feet from home plate. Conversely, a 5-mph headwind would decrease that 376-foot hit to just a 336-foot fly ball out, and a 10-mph headwind would keep the travel distance down to just 296 feet.

A head or tail wind also impacts a thrown ball. For example, a pitcher's almost "unhittable" 95-mph fastball may become a somewhat more hittable 90-mph pitch when thrown into a 5-mph breeze. Or with a tailwind of 5 mph, that pitch is a 100-mph blur! The bottom line is that pitchers love winds blowing in toward home plate because their pitches are faster, and if they are hit, they do not travel as far. On the other hand, batters want to see the winds blowing away from the plate, which will take something off the speed of the pitch and will also make the ball go farther.

But the wind is seldom a true tailwind or headwind, nor is it the same at every location inside a stadium. Due to the typical construction of a baseball stadium, the winds are anything but a consistent, smooth, constant flow. Instead the winds are more likely to be swirling, with many different speeds and directions, all affecting the flight of any given ball. The wind blowing from an angle behind the pitcher not only adds some velocity, but also pushes it sideways for more of a curve. Or when a ball is hit, the calm winds at ground level may be a 5-mph tailwind as it rises to 50 feet in its arc and 10 mph near its peak. Or, any and every combination of the above could be happening.

Stadium Design

Over the years there have been numerous efforts to mitigate the effects of the prevailing wind and other weather elements like rain, snow, and even the heat through stadium design. These attempts have been included orienting the stadium itself to block the wind, changing the playing field dimensions to shorter or longer to either decrease or increase the number of home runs, and putting "roofs" over stadiums to keep the rain out or cool in.

The gusty summer sea breezes and a constant battle with the elements in San Francisco, California, have been part and parcel of the Giants' history since moving to California in 1958. Candlestick Park opened in 1960 in a location where the seabreezes swirled and drained into the edge of the San Francisco Bay. It only took a year before this was highlighted on national television during the 1961 All-Star Game. In the ninth inning, San Francisco relief pitcher Stu Miller was blown off the mound by a gust of wind with a ball being called, allowing the tying run to score from third. In 2000, the Giants moved to AT&T Park, which was purposefully located in an area with considerably lighter winds, and the grandstands were even oriented in such a way as to block those breezes.

Currently, there are seven major league baseball stadiums that have gone even further to mitigate the weather by having either domes or retractable roofs. The only fixed roof is in Tampa, while there are retractable roofs in Houston, Texas; Tampa and Miami, Florida; Milwaukee, Wisconsin; Phoenix, Arizona; Seattle, Washington; and Toronto, Canada. The purpose of the roofs varies in relation to the local climate. In Phoenix, the retractable roof (and air conditioning) is used to combat the average 99-degree maximum temperature during baseball season. The northernmost roofed stadiums (Seattle, Milwaukee, and Toronto) control for precipitation, plus early and late season cold weather, while in in the southeast (Houston, Tampa, and Miami), the intent is to mitigate summer heat, humidity, and showers.

In locations without roofs, baseball teams must deal with the possibilities of rain delays and rainouts and the resulting negative impacts on schedules and revenue. The highest number of rainouts is in the northeast, with Boston, Massachusetts; Pittsburgh, Pennsylvania; Cleveland, Ohio; both New York teams; and Philadelphia, Pennsylvania, all having at least 21 cancellations between 2000 and 2010. During that same decade, the five California teams have only seen about a half-dozen rainouts combined.

But having a roof doesn't guarantee that you won't get rained out. In September 2004, rain and flooding from Hurricanes Frances and Jeanne caused games to be canceled in Tampa, Florida, despite the roof, and the same happened with Hurricane Ike in Houston, Texas, in 2008. And even stranger was a 2003 game that was rained out at Olympic Stadium in Montreal, Canada, when the retractable roof got "stuck" open!


Baseball is not only plagued by the rain, but at times the cold has made it less than hospitable for the "boys of summer." Historically, the beginning of the season in April will see some below-freezing overnight temperatures, but it is rare that even night games are sub-freezing. However, with Denver, Colorado, joining the Major Leagues in 1993, there have been games played in weather more suited for ice hockey. The coldest of these has been documented in the night game played between the Colorado Rockies and the Atlanta Braves on April 23, 2013. The temperature at the time of the first pitch was a very unspring-like 23°F, and snowmen could be seen scattered in the stands among the over 15,000 fans. This smashed the previous first-pitch low temperature record of 28°F.

The World Series, played in October, has also has had issues with wintery weather. The third game of the 1997 World Series in Cleveland against the Florida Marlins was played with wind chills down to 15°F and occasional snow flurries. Ironically, the warm weather Marlins hung on to win 14 to 13! In 2008, the fifth and final game of the World Series took four days to play, as it was plagued with heavy rain one day, snow the next, followed by flurries and then rain showers the third. It wasn't until the fourth day that the Phillies would finally beat the weather and Tampa Bay Rays. And the seventh game of the 2001 World Series in Phoenix, Arizona, got a double whammy, with the game being delayed first by a dust storm and then rain, an even rarer event in October in Arizona!

So, despite being a simple game, the weather can indeed make playing a baseball game a battle of not only the teams on the field but also a battle with the elements.

JAN NULL, Certified Consulting Meteorologist, is founder of Golden Gate Weather Services, a part-time faculty at San Francisco State University, and a former Lead Forecaster with the National Weather Service.

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