A tennis ball is a ball designed for the sport of tennis, approximately 6.7 cm (2.7 in.) in diameter and is usually bright yellow-green, but in recreational play can be virtually any colour. Tennis balls are covered in a fibrous fluffy felt which modifies their aerodynamic properties.
Tennis balls must conform to certain criteria for size, weight, deformation, and bounce criteria to be approved for regulation play. The International Tennis Federation (ITF) defines the official diameter as 65.41-68.58 mm (2.575-2.700 inches). Balls must weigh between 56.0g and 59.4g (1.975-2.095 ounces). Yellow and white are the only colours approved by the United States Tennis Association (USTA) and ITF, and most balls produced are fluorescent yellow (known as "optic yellow") the colour first being introduced in 1972 following research demonstrating they were more visible on television. Tennis balls are filled with air and are surfaced by a uniform felt- covered rubber compound. The felt traps the air flow boundary layer which reduces aerodynamic drag and gives the ball better flight properties. Tennis balls begin to lose their bounce as soon as the tennis ball can is opened and can be tested to determine their bounce. A ball is tested for bounce by dropping it from a height of 100 inches (2.54 m) onto concrete; a bounce between 53 and 58 inches (1.3462 - 1.4732 m) is acceptable (if taking place at sea-level and 20°C / 68°F; high-altitude balls have different characteristics when tested at sea-level). Modern regulation tennis balls are kept under pressure (approximately two atmospheres) until initially used.
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