Zenith Hourly Rate
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Zenith Hourly Rate is a concept used by meteor shower observers. It represents the maximum theoretical rate of meteors that would be visible to an observer, if the radiant of the shower was at the zenith and the sky was free of any light pollution. Because the ZHR is defined for optimal conditions, the actual rate of meteor seen is usually lower. ZHRs for known meteor showers range from 1 or 2 for the weakest showers that are detectable, to over 100 in the case of the Perseids, Quadrantids and Geminids.
Most meteor showers have a similar ZHR from year to year, but some show a variable ZHR, depending on how close the Earth passes to the stream of dust left by the parent comet of the meteor shower. For example, the Leonids in most years have a ZHR of less than 10, but can become spectacular every 33 years, when the Earth passes close to the parent body, Comet Tempel-Tuttle. In 1966, a ZHR of 150,000 was estimated during the 20 minute peak of the Leonid shower. Elevated ZHRs of up to several thousand were seen in 1999 and 2001.
