A swarm of associated meteoroids or meteors caused by the passage of the Earth through the dust trail left by comets, observable in November. Since the radiant is located in the constellation Leo, these meteors are known as Leonids.
Earth moves through the meteoroid stream of particles left from the passages of a comet. The stream comprises solid particles, known as meteoroids, ejected by the comet as its frozen gases evaporate under the heat of the Sun, when it is close enough. The Leonids are a fast moving stream which encounter the path of Earth and impact at 72 km/s. Larger Leonids which are about 10 mm across have a mass of half a gram and are known for generating bright meteors. An annual Leonid shower may deposit 12 or 13 tons of particles across the entire planet.
Moreover, every 33 years, when the Tempel-Tuttle comet passes by, the Leonids produce prolific meteor storms. Very large outbursts occur, during which activity exceeds 1,000 meteors per hour, with some events exceeding 100,000 meteors per hour. These spectacular showers can be observed from Earth, as in 1966 and 1999. Because of the time it takes for the swarm to disperse, it was still possible to observe some meteors in 2000. The next very large shower is predicted to occur around 2032; the comet is predicted to pass very close to the Earth.