Leonid meteors are dust particles that have come off Comet Tempel-Tuttle. Most of this dust is still following the comet fairly closely in space. The comet takes 33 years to complete an orbit around the Sun, and planet Earth ploughs through its main dust cloud when the comet returns to our vicinity every 33 years. In the years when this happens, a strong shower or storm takes place. In the years in between, a very small number of Leonid meteors are seen in mid-November.
Some meteor showers produce about the same rate of meteors around the same date every year.
Regular annual showers happen when the dust from a comet has spread around the whole of the comet's orbit, something that takes place gradually over a long period of time. An example is the Orionids, a shower in late October each year caused by dust from Halley's Comet. Looking ahead to 1999, Comet Tempel-Tuttle will still be relatively nearby and some astronomers are predicting that the Leonid meteor display could be better next year than this. If that were to happen, then Europe is expected to be the ideal location. Most of the Leonids weigh about 1 millionth of a gram - not much more than a particle of cigarette smoke.