In 2022, fourteen scientists published an answer to a question that sounds like a child's and reads like an atlas: will a warmer planet make more rainbows, or fewer? On average, more. The average is the least interesting part.
The paper, published in the journal Global Environmental Change, was led by Kimberly Carlson, then at the University of Hawaii at Manoa and now at New York University. Among its co-authors is Steven Businger, the atmospheric scientist whose research explains why Hawaii calls itself the rainbow capital of the world (a title his own work did much to justify).
Counting rainbows is harder than counting rain, because no instrument records them. Only people do. So the team taught a model to use our photographs: tens of thousands of Flickr images tagged "rainbow", screened by hand to keep actual bows and discard rainbow flags, rainbow cakes, and rainbow everything else. Each verified photo carries a time and a place. Match those against precipitation, cloud cover, and sun angle, and you get a model of the conditions under which people demonstrably see rainbows. Point that model at climate projections, and you get something new: a rainbow forecast for the year 2100.
The headline: by 2100, the average land location on Earth sees about 5 percent more rainbow days than it did at the start of this century, with the exact figure depending on how much the planet warms. The geography underneath the average matters more:
A year before the climate paper, Businger set out in the Bulletin of the American Meteorological Society why Hawaii outbows everywhere else. Trade-wind showers arrive spaced 30 to 50 kilometres apart, far enough that sunlight reaches the rain between them. Showers deepen over the warm ocean at night and bloom into morning bows. Mountains sharpen the boundary between rain and sun on every island. And the air, a hemisphere away from continental dust and pollution, is clean enough to leave the colors undiluted.
Under the 2022 projections, the rainbow capital gets richer still: the Hawaiian islands are predicted to gain a few more rainbow days per year. You can watch the benchmark live on Bowcast's Honolulu and Hilo pages.
The study's ingredients are Bowcast's ingredients: falling rain, open sun, and the geometry that puts a bow 42 degrees from the point opposite the sun. The difference is the clock. Carlson and colleagues forecast rainbows across decades to ask what a warming climate does to them. Bowcast runs the same physics hour by hour to answer a smaller question: should you step outside before dinner? The live map estimates the chance anywhere on Earth, and the science page shows exactly how.