Zenithal Hourly Rate
The Zenithal Hourly Rate ( ZHR ) of a meteor shower is the number of meteors that under a sky with the visual limiting magnitude of 6.5 m an observer sees within an hour, when the radiant of the stream is in zenith and there are no obstacles in sight.
Since this ideal situation can not happen exactly, the true, observed rate is always lower than the theoretical rate from the list of meteor streams. By means of the following formula, the observed rate in the Zenithal Hourly rate is approximately normalized:
N: number of meteors of the observed meteor stream
R: Population Index
: Limiting magnitude of the weakest observable with the naked eye star
T: observation time in hours
H: height of the radiant above the horizon to the center of the observation period