A "cloud" of dust and ice particles, most no larger than a grain of sand, trails behind the path of an orbiting comet. If Earth passes through this region, a meteor shower occurs. The small particles enter Earth's atmosphere at speeds of up to 160,000 kilometers per hour. Friction with gas molecules vaporizes the grains, producing bright meteor trails.

In a meteor shower, the meteors appear to originate from the same area of the sky, a region called the radiant. Most meteor showers are named for the location of their radiant: the radiant for the Perseids is in the constellation of Perseus.

Simulation of the Perseid meteor shower. The Perseids peak around August 12 each year, when Earth passes through the orbit of comet Swift-Tuttle. In the animation, time has been compressed. Typical meteor showers have maximum rates of tens of meteors per hour.