The Leonids are caused by countless separate trails of debris left by comet Tempel-Tuttle, which orbits the Sun every 33 years, each time on a slightly different path. Most of the debris is the size of sand grains or smaller. Each vaporizes when it enters the atmosphere, creating a streak of light.

Outbursts of activity can occur when Earth passes through the middle of a relatively recent debris trail. Brief but intense flurries of activity -- with many hundreds of meteors per hour streaking across the sky -- occurred last year and in 2001.

Most Leonid meteors are typically not visible until after midnight local time, when the part of Earth you are on rotates so that it faces the direction Earth is moving on its orbital path through space. Just as bugs slam into a car's front bumper, meteors are scooped up more readily by the leading edge of the planet.

A fireball dazzles. It can be colorful and bright enough to cast a shadow. Fireballs are plainly visible even from the brightest cities.

The Leonid meteor shower is known for fireballs. The reason lies in part with the source of the material.

Comet Tempel-Tuttle orbits the Sun in the opposite direction as Earth. Its debris, boiled and blown off by solar radiation, continues orbiting the Sun along the same general path, so it meets Earth head-one. Leonids hit the atmosphere at a relative speed of 160,000 mph (72 kilometers per second), twice the speed of many other shooting stars visible on a typical night. The speed adds to the intensity of each meteor's brightness.

Despite the modest expectations for this year's Leonids, many avid meteor watchers will brave several hours of chilly weather just for the chance to see a few bright shooting stars and fireballs.

Earth will pass near one trail, laid down in 1533, at 2:28 a.m. EST (7:28 GMT) Wednesday. The timing favors North and South America, for several minutes surrounding time the hourly rate of meteors could range between 30 and 100, according to two separate predictions.

However, most of these meteors will likely be faint, astronomers say, and difficult to see except from remote locations away from all light pollution.

Other Leonids will be produced throughout Wednesday morning by the "Filament," an amalgam of many trails, no longer individually recognizable, deposited over millennia by Tempel-Tuttle. Rates from the Filament could reach nearly one per minute, figures Peter Jenniskens of NASAs Ames Research Center.

"As with all predictions, these rates assume dark skies," said Joe Rao, SPACE.com 's Night Sky columnist. "City dwellers could see considerably reduced rates."

Passages near or through distinct trails are typically brief events, providing outburst that can last just a few minutes to perhaps an hour.

The passage through the Filament, though, will last for up to 24 hours, centered around 12:25 a.m. EST (5:25 GMT) Wednesday. Western Europe and Africa are well positioned to see Filament meteors, while most of the Americas should catch a good portion of this activity too.

The Filament is expected to contain a higher percentage of large bits of debris, generating hope of several fireballs.

Meteor forecasting is tricky business, however. Rates could be lower or higher and brief bursts of activity could come at any time.

Meteor watching is easy. Binocular and telescopes are of no use, because the streaks come and go too quickly.

It is important to select a site with a broad view of the sky and few local lights, seasoned meteor watchers say. Porch lights and street lights will render fainter meteors invisible.

Dress more warmly than you think necessary. Bring warm drinks, but not alcohol, which impairs the ability to spot shooting stars. A lounge chair or blanket will allow you to lie back for a full view of the sky, avoiding neck strain.

Leonids can appear in any part of the sky. But if you trace each one back it will point to a hub of origin, called the radiant, in the constellation Leo, the Lion. The radiant rises above the eastern horizon around 11:30 p.m. local time. By 3 a.m. it is high in the sky.

The radiant is above Jupiter, an obvious beacon that rises around 1 a.m. local time and shines brighter than all stars. Nearer dawn, a thin crescent Moon will rise, hanging in the sky below Jupiter. Its light will not be bright enough to seriously impair viewing of the Leonids. But blocking the Moon with a building or tree could allow a few more of the fainter meteors to be visible.

For best results, face the general direction of the radiant but scan as much of the sky as possible, experts advise. Be attentive to your peripheral vision.

A small number of other meteors, not associated with any named shower, could appear at any time. They will streak in different directions unrelated to the Leonid radiant.

The next meteor shower this year will be the Geminids on the night of Dec. 13-14. But a bright gibbous Moon will outshine many of the Geminids this year. Not until the August Perseids will shooting stars again compel an all-night vigil.

That leaves Wednesday morning, Nov. 19, as the best meteor-watching opportunity for several months.