Is This The Best Place In The World To See Rainbows?

Rainbow over east Oahu. Image credit: Steven Businger

As if anyone needed another reason to visit Hawaii, a newly published research paper explains why the archipelago is the best place in the world to see rainbows. Appearing in the Bulletin of the American Meteorological Society, the study breaks down the science behind everyone’s favorite weather event, while also describing the factors that make Hawaiian rainbows so much more spectacular than those seen elsewhere.

Clearly a man who loves his job, study author Steven Businger’s paper reads like something of a love letter to the rainbow, beginning with a lamentation of the fact that the phenomenon is not given more attention on the educational curriculum. From then on, it’s all sunshine and rainbows, and if ever there was a research paper to put you in a good mood, this is it.

Before dealing specifically with Hawaii, Businger gives a detailed account of the rainbow effect, explaining how the famous philosopher René Descartes first discovered that each water droplets contributes a “sparkle of color” by projecting a “cone of light” at an angle of exactly 42 degrees to the incoming sunlight. “One of Descartes’s great insights is that each observer sees their own rainbow made up of light rays from a unique set of raindrops that happen to be located in an arc at a 42° angle away from the shadow of that observer’s head,” he writes.

Rainbow over Honolulu Harbor. Image: Minghue Chen

As such, “a friend sharing the experience nearby will actually be enjoying a different rainbow.”

Moving on to discuss the unique qualities of Hawaiian rainbows, Businger explains that four distinct factors contribute to the awesomeness of the archipelago’s colourful skies.

Firstly, the position of the islands within an atmospheric circulation known as the Hadley Cell ensures that Hawaii is visited by north-westerly trade winds on the majority of days. This produces patches of convective showers, interspersed with areas of clear sky, allowing sunlight to mix with rainfall and generate rainbows.

The overnight warmth provided by the ocean, meanwhile, causes air to rise in the early morning, allowing rainbows to form “in time for breakfast”. Conversely, evening rainbows are produced as temperatures rise through the day, creating currents of air. “Under these conditions showers form over the ridge crests over Oahu and Kauai in the afternoon, resulting in prolific rainbows as the sun sets,” says Businger.

This mountainous topology is another key factor in the Hawaiian rainbow show, as the peaks force the trade winds to rise and cool, producing rainfall. Without these mountains, the archipelago would be something of a desert, with next to no rain and, therefore, no rainbows.

Finally, Hawaii’s remote location ensures that its air is largely free of pollution, continental dust, and pollen, which means there are very few aerosol particles that could scatter sunlight and dampen the glory of a rainbow.

An example of supernumerary bows beneath the primary bow. Image: Matt Champlin

All of these factors combine to allow Hawaiian rainbows to persist for hours at a time, with Businger claiming that they can last for up to 6.5 hours during the summer and 8.5 hours in the winter.

It’s hardly a surprise, therefore, that rainbows are so prominent in Hawaiian mythology and culture. Within the Hawaiian language, for example, “there are words for Earth-clinging rainbows (uakoko), standing rainbow shafts (kāhili), barely visible rainbows (punakea), and moonbows (ānuenue kau pō), among others,” explains Businger.

Of course, while Hawaii may boast the clearest, most abundant, and longest-lasting rainbows on the planet, you can still get lucky and have your mind blown by a killer rainbow elsewhere, as the video below demonstrates.

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