Consulting the moon…
Pulling the river gauge, the cloud forecast, and the position of the moon over the gorge.
Hour by hour
Score through the night. The thin line is the moon alone; the filled curve adds the weather and the river.Why, for tonight
The moon is a gate: if it fails, the score is zero. The rest is weighted.Coming nights
Every night in the next six weeks with a usable moon. Past the 16-day forecast, the score is the moon alone. Tap one to load it above.Full moons, next 12 months
The park's viewing windows run about two nights either side of full. Times are Eastern.Kentucky State Parks publishes its own official moonbow dates and sets hours at the overlook. Check the park before you drive. This site is not affiliated with the park, the state, or the moon.
How it works
A moonbow is a rainbow made by moonlight. Same physics: light enters a water droplet, bends, bounces off the back, and leaves at about 42 degrees from the point directly opposite the light source. So a moonbow appears opposite the moon, and you only see it standing with the moon at your back, looking into mist.
Cumberland Falls gets one on a schedule because it has all the ingredients at once: a 68-foot drop with enough water to throw a steady mist, a wide gorge that lets the moonlight in, steep walls that keep the mist from blowing off, and almost no light pollution. It's the only place in the Western Hemisphere where this happens reliably. Niagara used to have one until the city lights drowned it out.
This instrument scores each night hour by hour. First the gates: the sun has to be well down, the moon has to be at least about 85 percent lit, and the moon has to be up. If any of those fail, the hour is a zero and nothing else matters. Then the weather: cloud cover and rain chance from Open-Meteo (45 percent of the score), river flow from the USGS gauge at Williamsburg, just upstream of the falls, as a stand-in for mist (30 percent), fog and haze (15 percent), and wind (10 percent). The National Weather Service forecast is pulled alongside as a second opinion, and the two are compared to tell you how much to trust the number.
One honest caveat. Rainbow textbooks say the bow drops below the horizon once the moon climbs past 42 degrees, which would make the early evening the only window. The park says midnight to 2 AM is best. Both are right: at the overlook you're standing above the mist, so a high moon just pushes the bow lower into the spray instead of out of sight. The instrument treats moon height as a soft factor, not a cutoff, and the weights are starting guesses. If you go, tell us what you saw. That's how it gets smarter.
Weather data by Open-Meteo.com (CC BY 4.0). River data from the U.S. Geological Survey. Forecast comparison from NOAA / National Weather Service. Moon and sun positions computed in your browser with SunCalc. Nothing here is collected about you; there is nothing to collect.