Q. I often see what look like basketballs on overhead electrical wires. What are they?

A. Called marker balls, they are mostly used to warn aircraft so they do not hit the wires.

Wires used for power transmission or communications and guy wires, which hold up towers, present special visibility problems for pilots of low-flying aircraft, according to the Federal Aviation Administration.

In a review of helicopter accidents involving wire strikes, published in 2003 in the administration’s safety magazine, FAAviation News, experts said that “pilots’ ability to see wires is affected by dirty windscreens, light conditions, the obscuring effects of terrain and changes in visual perspective that occur during climb and descent.” In addition, the article said, “accurately judging the helicopter’s distance from unmarked wires is nearly impossible.”