Figure 1. Test football.
Figure 2. Results of football radiation test.
On January 18, 2015, the New England Patriots and Indianapolis Colts played in the AFC Championship Football Game at Gillette Stadium in Foxborough, Massachusetts to determine which team would advance to Super Bowl 49. During the first half of the game the Colts complained that one of the footballs the Patriots had been using on their offense felt low on pressure – the ball in question had been intercepted by linebacker D’Qwell Jackson. It is thought by some that under-inflated footballs may have given the quarterback, Tom Brady an unfair advantage allowing him to otherwise grip them better.
As a result of the Colts’ complaint, at half-time the officials took the unprecedented step of gathering the other 11 Patriots balls (each team provides 12 balls for their own use on offense), taking them to the locker room, and re-checking their pressures. Normally the officials check the balls only once, prior to the game in the locker room to ensure that they are inflated within the range of 12.5 to 13.5 pounds per square inch (psi).
A total of 25 pressure measurements were taken and recorded using a combination of three separate pressure gauges; two measurements each by two alternate officials at halftime on the 11 balls retrieved from the Patriots, plus three measurements by other NFL staff on the one intercepted ball. The measurements averaged 11.3 psi with a standard deviation of 0.4 psi. Because the pressures measured at halftime were so low, the NFL suspected that the balls must have been under-inflated prior to the start of the game – they were wrong!
The NFL was aware that the pressure measurements were sensitive to temperature, and that the ambient outdoor field temperature during the first half of the game averaged 49 degrees Fahrenheit (°F) (20°F cooler than the locker room where the pressures were originally checked). So they attempted to correct for this, and in the process made a significant error. The NFL wrongly assumed that the temperature of the air inside the balls was the same as the outdoor air temperature, incorrectly predicting that the balls were under-inflated prior to the game. The NFL neglected to account for the significant effect that radiation plays in the transfer of heat.
To demonstrate this effect, we conducted a test of an official NFL football, outdoors over the period of one full day. The test ball and the temperature recording instruments are shown in Figure 1; we measured and logged the outdoor air temperature plus the ball surface and it’s inside air temperature. The results of the test are shown in Figure 2. At its coldest point during the night, the temperature of the air inside the ball measured 11.2°F cooler than the outdoor air temperature. When this correction for radiation is included, we predict that the average inflation pressure of the Patriots footballs before the start of the game would have been 12.9 psi. This is 0.6 psi higher than the prediction without the correction for radiation, and well within the prescribed range of 12.5 to 13.5 psi.