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The headline figure is meteors per hour you could expect to see, not the shower's quoted ZHR. One relation is evaluated every 15 minutes through the night, separately for every active shower and the sporadic background:

rate = ZHR(t) × sin(h) × r^(lm − 6.5) × clear × 0.75

ZHR(t) is the shower's strength at this point in its cycle, anchored to solar longitude rather than calendar date, with a narrow peak and a broad component for streams with long wings. sin(h) is the radiant altitude, tapered below 8° for extinction and terrain. r is the population index — an exponent, not a multiplier, which is why one lost magnitude of sky darkness roughly halves the rate.

Limiting magnitude is built in linear brightness units: airglow floor at 22.0, artificial ground light from the atlas, twilight, then moonlight from the Krisciunas & Schaefer (1991) model with Kasten & Young airmass so a setting moon fades rather than snapping off. Thin cloud counts twice — it blocks part of the sky and scatters town glow and moonlight back down, so the same 60% high cloud costs nothing at a pristine site and about a magnitude at Bortle 5. Humidity above 65% adds haze. Everything is then scaled by 0.75, because ZHR assumes a perfect observer watching the whole sky at once.

Real-world Bortle is that same limiting-magnitude model turned back into a Bortle class. The atlas tells you what a site reads on a perfect moonless night; this tells you what it reads tonight, with the moon, the haze and the town glow bouncing off thin cloud all counted. It is evaluated at the zenith every 15 minutes and quoted as the median across the dark hours, so a single bright half-hour does not define the night. A Bortle 3 site under a gibbous moon genuinely performs like a Bortle 5 one, and that is the number worth planning against.

Shower parameters are hand-encoded from the International Meteor Organization's calendar and working list, with no outburst or dust-trail predictions. Cloud, humidity, temperature and dew point come from Open-Meteo, interpolated smoothly between hourly samples. Sky darkness comes from David Lorenz's 2025 light pollution atlas, baked into the app and used as a continuous measurement — Bortle class is shown for familiarity only. Beyond 16 days the cloud figure is what this place typically does at that time of year, averaged over three years and a ±3 day window: useful for choosing a week, and it tells you nothing about the night itself.