The annual Perseids meteor shower can usually be counted on to deliver one of the best astronomical fireworks displays of the year.

Two of the variables that can affect the viewing of this spectacular sky show are clouds and a bright moon, both of which, unfortunately, will be a factor to some degree this year in the Lower Mainland.

However, the good news is that the cloud cover tonight (August 12 to 13, the peak of the five-week-long transit of the debris fields) is predicted to be patchy, with clear periods, and the moon, although near full, will not be dazzling enough to obscure all of the meteors (or shooting stars, as they are sometimes called).

The Perseid extravaganza is caused when the Earth passes through (or, in this case, sideswipes) streams of debris left by the 26-kilometre-wide comet Swift-Tuttle as it orbits our sun every 133 years (the next appearance is due in 2126). The tiny fragments of rock and dust—most the size of sand grains but some as big as marbles—streak through our atmosphere at about 60 kilometres per second and are vapourized, causing the classic fiery streak through the night sky.

A "normal" Perseid shower (so called because they seem to be radiating from the direction of the constellation Perseus) usually delivers about 60 meteors (they are called meteoroids while still in space and meteorites if any survive the scorching fall to the ground) per hour, with irregular "storm" years bringing as many as 150 per hour.

Meteors spotted before midnight tend to have longer bright "tails" and are relatively few in number, but the frequency will pick up in the predawn hours.

It is best to get out of town to see the Perseids, away from artificial light sources, but any dark and secluded area will offer a viewing opportunity. You should allow at least a half-hour for your eyes to adjust to the darkness.