Compute the chance of a primary rainbow forming right now from temperature, humidity, and solar altitude.
The Sun is below the horizon.
A primary rainbow needs sunlit raindrops and a solar altitude below 42°. The tool combines current humidity, recent precipitation, and Sun altitude (computed from your latitude, longitude, and the current time) to score formation probability.
Standard forecasts do not surface optical phenomena. This tool surfaces them so amateurs can chase them.
When the Sun is above 42° altitude, no primary rainbow can appear above the horizon for an upright observer.
Only the primary bow is scored. Secondary bows require very bright sunlight and dense rain.
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