The effect of temperature and humidity on the performance of baseballs and softballs

Sports balls such as softballs and baseballs are made from materials that are natural (cork, wool, leather) and synthetic (rubber, polyurethane) and are sensitive to moisture and temperature. It is common practice, for instance, for major league baseball parks in dry climates to condition their baseballs at 50% relative humidity. Baseball and softball are played at temperatures ranging from less than 4 C (40 F) to more than 38 C (100 F). Standardized ball tests require controlled temperature and humidity test environments. Little information is available, however, to quantity the effect of temperature and humidity on a ball's response. The following describes test procedures used to condition softballs and baseballs to controlled levels of humidity and temperature and their effect on the ball's stiffness and elasticity. While the construction of the baseballs and softballs are quite different, their elasticity and stiffness show a similar trend with temperature and humidity. Their combined effect was shown to lower bat performance as both temperature and humidity were increased.