Ball lightning is a rare and unexplained phenomenon described as luminescent, spherical objects that vary from pea-sized to several meters in diameter. Though usually associated with thunderstorms, the observed phenomenon is reported to last considerably longer than the split-second flash of a lightning bolt, and is a phenomenon distinct from St. Elmo's fire.
Some 19th-century reports describe balls that eventually explode and leave behind an odor of sulfur. Descriptions of ball lightning appear in a variety of accounts over the centuries and have received attention from scientists. An optical spectrum of what appears to have been a ball lightning event was published in January 2014 and included a video at high frame rate. Laboratory experiments have produced effects that are visually similar to reports of ball lightning, but how these relate to the supposed phenomenon remains unclear.
Scientists have proposed a number of hypotheses to explain reports of ball lightning over the centuries, but scientific data on ball lightning remain scarce. The presumption of its existence has depended on reported public sightings, which have produced inconsistent findings. Owing to the lack of reproducible data, the existence of ball lightning as a distinct physical phenomenon remains unproven.