Zenithal hourly rate (ZHR)
The number of meteors one observer would see in an hour under perfect conditions, with the radiant directly overhead.
Meteor counts depend hugely on conditions. A city observer with a low radiant might see a handful of meteors, while someone under a perfect sky sees dozens. To compare showers fairly, astronomers use the zenithal hourly rate (ZHR). It is the number one person would see in an hour if the sky were dark enough to show stars as faint as magnitude 6.5, nothing blocked the view, and the radiant was at the zenith, the point directly overhead.
ZHR lets you compare showers and spot unusual activity. The Perseids usually reach a ZHR of around 100 at their peak, the Geminids often about 150, and smaller showers, such as the Ursids, only about 10.
The important thing to remember is that ZHR is an ideal figure. In real life you will almost always see fewer, often half or less, because the radiant is lower, the sky is brighter, or the Moon is up. So don't be disappointed if your count is lower than the ZHR quoted. Observers who carefully record their counts, sky quality and the time help scientists work out the true ZHR.
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Level: Explorer Ages 10 and up