A meteor shower, some of which are known as a "meteor storm", "meteor outburst", or "shooting star", is a celestial event where a group of meteors are observe to radiate from one point in the sky. These meteors are small fragments of cosmic debris entering Earth's atmosphere at extremely high speed. They vaporize due to friction with the air, leaving a streak of light that very quickly disappears. For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres) this visible light is due to the heat produced by the ram pressure (not friction, as is commonly assumed) of atmospheric entry . Most of the small fragments of cosmic debris are smaller than a grain of sand, so almost all fragments disintegrate and never hit the earth's surface. Fragments which do contact Earth's surface are called meteorites.