The short answer
ZHR stands for zenithal hourly rate. It is the number of meteors a single observer would see in one hour if three things were perfect at once: the shower’s radiant directly overhead, a sky dark enough to show stars down to magnitude 6.5, and nothing blocking any part of the sky. It is a yardstick for comparing showers with each other, not a forecast of what you’ll count from your backyard.
That’s why a shower listed at a ZHR of 100 almost never delivers 100 meteors an hour to an ordinary observer. The number describes the shower; your count depends on your sky, your hour and your eyes.
The three ideal conditions
Each part of the definition corrects for one thing that changes from observer to observer.
- Radiant at the zenith. Meteors seem to stream from one point, the radiant. When it is low, many meteors hit the atmosphere at a grazing angle over someone else’s horizon, and you see fewer. ZHR imagines it straight overhead.
- Limiting magnitude 6.5. Most meteors are faint. ZHR assumes a sky where the faintest stars you can see are magnitude 6.5, which means a genuinely dark rural site with no Moon. Few people observe under skies that good.
- A clear, unobstructed view. No clouds, no trees, no buildings, and an observer who is looking the whole time.
Experienced observers who report counts to organizations like the International Meteor Organization record their actual conditions, and the counts are corrected back to these ideal conditions. That’s how a single comparable number comes out of thousands of real nights.
Why you see fewer
The corrections work in reverse for you. Start from the ZHR and every way your night falls short of ideal takes a share away.
| Factor | Effect |
|---|---|
| Radiant height | Roughly in proportion to how high the radiant is: halfway up costs about 30 percent; 30 degrees up costs about half |
| Sky brightness | The largest factor for most people. Each magnitude of brighter sky removes a large share of the faint majority |
| Moonlight | A bright Moon acts like light pollution for the whole night or part of it |
| Obstructions and clouds | Trees, buildings and patchy cloud remove meteors in proportion to the sky they hide |
| You | Breaks, phone screens and not-yet-dark-adapted eyes all lower the count |
How much sky brightness matters depends on the shower. Some streams have a higher share of faint meteors than others, so a bright sky hurts some showers more. As a broad rule, going from a dark rural sky to a typical suburban one can cut what you see to a third or less.
What it means in practice
Take a shower with a ZHR of 100, the kind of figure quoted for the strongest showers of the year. From a good rural site after midnight, with the radiant well up, a few dozen meteors an hour is a realistic hope. From a suburban backyard, 10 to 20 an hour is more typical. From a city center, you may see only the brightest handful.
Showers with a ZHR of 10 to 20 are worth watching from a dark site but are hard to tell apart from the background of random, or sporadic, meteors from anywhere bright. Sporadic meteors add a few an hour on any dark night, more in the hours before dawn.
The published ZHR is also an average for the peak. Rates climb toward the maximum and fall away afterward, steeply for some showers and gently for others. A night or two from the peak can give a fraction of the listed figure.
How to use ZHR
- Compare showers. ZHR is good at telling you which showers are worth losing sleep over and which are only for enthusiasts.
- Then check the conditions. Look up the peak date, the Moon and the radiant’s height. The meteor shower calendar and each shower’s page on Meteor showers have current dates.
- Improve what you can. A darker site, later hours and fully adapted eyes each bring your count closer to the ZHR. See how to watch a meteor shower and dark adaptation and light pollution.
Treat any forecast of a meteor “storm”, with ZHRs in the hundreds or thousands, as a prediction with real uncertainty. Such outbursts depend on Earth crossing narrow, young trails of debris, and modelers sometimes disagree on timing and strength.