The balls on power lines are called marker balls, visibility markers, or aerial markers, and they serve a safety purpose for low-flying aircraft and to protect the electrical infrastructure. They make the power lines more visible to pilots and help prevent collisions with the lines. The Federal Aviation Administration mandates that the balls should appear on wires that cross canyons, lakes, and rivers, as well as those that surround airports. The diameter of the balls must be at least 36 inches on wires that cross canyons, lakes, and rivers, but the FAA allows 20-inch spheres on power lines fewer than 50 feet above ground level and within 1500 feet of an airport runway end. The balls are usually orange, but they can also be yellow, red, or white. They weigh about 17 pounds each and are placed on power lines near mountain passes, in deep valley areas, near major freeway crossings, and around airports.