Most meteors seen in the course of an observing session are faint ones. Only a small fraction exceed magnitude 0 which are caused by millimeter-sized meteoroids. About one in 1,200 observed meteors becomes brighter than -5 mag, while only one in 12,000 reaches -8 mag (Rendtel and Knöfel, 1989). Bright meteors are called fireballs.
The definition of a fireball is somewhat arbitrary and in the literature the required minimum magnitude varies between about -2 mag to -6 mag. In the IMO Fireball Data Center (FIDAC), we regard meteors of at least apparent magnitude -3 mag (corrected for zenith position) as fireballs. By zenithal magnitude we mean the brightness the meteor would have if it had appeared in the zenith of the observing site. As an example, a meteor appearing like -1 mag may actually be a fireball if it moves only a few degrees above the horizon. The apparent brightness decreases with the square of the distance between object and observer, and furthermore, the absorption of the light is proportional to the optical pathlength as well. In the case of a near-horizon meteor, the distance to the observer is very large resulting in a strong reduction of the apparent brightness.
To put this way round: a fireball of -6 mag zenithal magnitude terminating about 50 km above Earth's surface will appear as a meteor of -1 mag at an elevation of 5 degrees above the horizon for an observer 600 km from the event. The zenithal magnitude for visual observations can be calculated using the formula:
M = m + 5 log (sin h)
where M is the zenithal magnitude, m the apparent magnitude and h the elevation of the event above the horizon.
So-called radio fireballs are meteors caught by a receiver using the forward-scatter method whose echo- duration is longer than 10 seconds. Due to the very complicated physical relationships, a straightforward conversion of the echo duration into a visual magnitude is not possible.