Rainbow Capital of the World | weatherology°
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By: Meteorologist Michael Karow
Updated: Mar 29th 2021

Rainbow Capital of the World

It’s a natural phenomenon so synonymous with the state of Hawaii that you can find it featured prominently on the state’s license plates, and the Hawaiian language has more than 20 words and phrases to describe its different varieties. A new paper by Dr. Steven Businger in the Bulletin of the American Meteorological Society makes the case for why Hawaii should be considered the rainbow capital of the world.

Rainbows form when sunlight (from behind the viewer) strikes raindrops. As the sun rays enter the raindrops, they get bent (refracted) and then reflected back to the viewer at a 42° angle to the incoming sunlight. Each color or wavelength of light gets refracted at a slightly different angle, so that the white light gets split into all the colors of the visible spectrum.

Thus, in order for a particular place or region to produce a lot of rainbows, it would need both rain, but also sunshine. As for rainfall, parts of Hawaii are some of the wettest places on Earth, with a rainstorm back in 2018 dumping nearly 50 inches of rain on portions of the island of Kauai in just a 24 hour period.

Hawaii is situated in the subtropics in a region that is usually dominated by sinking air as well as northeasterly trade winds. These northeasterly trade winds transport low-level moisture, often characterized by stratus clouds, from off the West Coast of the U.S. to Hawaii. As it reaches Hawaii this more uniform low-level moisture and cloud deck becomes more convective in nature, creating passing showers followed by sunny intervals, which are the perfect ingredients for producing rainbows.

Another major factor in creating Hawaii’s rainbows is its topography. The lush green mountains of the Hawaiian Islands work to lift the incoming moist air and produce orographic rain. In fact, without its mountains, Hawaii would only average 17 inches of rain per year. During periods of light winds, the topography also works to produce showers via daytime heating. The moist sea breezes converge at the higher elevation ridge lines to produce showers, which are at the perfect elevation to produce rainbows as the sun sets.

model raindrop rainbow NOAA
Conceptual model of how light both refracts and reflects within a raindrop to produce a rainbow - [NOAA]
surface map trade winds satellite Businger
Visible satellite data with overlaid surface pressure map and surface winds from June 14, 2013, showing the typical North Pacific high pressure, marine layer off the U.S. West Coast, and Hawaii's northeast trade winds - [Businger, 2021]
satellite Hawaii showers diurnal ridges
Visible satellite image from May 7, 2019 showing daytime-heating-type showers forming over the high elevation ridges on the Hawaiian Islands