Why Hawaii has the best rainbows on the planet

Honolulu is known as the rainbow capital of the world, while Hawaii is often called the “Rainbow State.” In a new report from UH Mānoa, Professor Steven Businger describes why Hawaii is the best place on Earth to experience rainbows. 

“Rainbows are some of the most spectacular optical phenomena in the natural world, and Hawaii is blessed with an amazing abundance of them,” writes Professor Businger.

“Rainbows in Hawaii are at once so common and yet so stunning that they appear in Hawaiian chants and legends, on license plates, and in the names of Hawaiian sports teams and local businesses.” 

“Visitors and locals alike frequently leave their cars by the side of the road in order to photograph these brilliant bands of light.”

“The cultural importance of rainbows is reflected in the Hawaiian language, which has many words and phrases to describe the variety of manifestations in Hawaii.”

“There are words for Earth-clinging rainbows, standing rainbow shafts, barely visible rainbows, and moonbows, among others. In Hawaiian mythology the rainbow is a symbol of transformation and a pathway between Earth and Heaven, as it is in many cultures around the world.”

Rainbows are optical illusions that appear when the sun shines onto water droplets. In order to get a glimpse of one on flat ground, the sun must be within about 40 degrees of the horizon. 

Professor Businger explains more about the science of rainbows, as well as the conditions that make Hawaii a hotspot for viewing them. He notes that the state’s location in the subtropical Pacific means that the overall weather pattern is dominated by trade winds, with frequent rain showers and clear skies between the showers.

“At night a warm sea surface heats the atmosphere from below, while radiation to space cools cloud tops, resulting in deeper rain showers in the morning that produce rainbows in time for breakfast,” said Professor Businger.

Hawaii’s mountains also play an important role in forming the stunning light displays. These mountains cause trade wind flow to be pushed up, forming clouds and producing rainfall.

In addition, daytime heating drives island-scale circulations. During periods of lighter winds, showers form over the ridge crests on Oahu and Kauai in the afternoon, which lead to rainbows at sunset.

The fourth contributing factor to frequent rainbows in Hawaii is the clean air that is free of pollution, continental dust, and pollen.

As Professor Businger pursued his passion for photographing rainbows, he envisioned a smartphone app that would be equipped with access to Doppler radar and satellite data so that it could alert users when the conditions are right for rainbow sightings nearby.

“Just as you can chase tornados in the U.S. Midwest, you can chase rainbows in Hawaii,” said Professor Businger.

“After a few years of false starts, Paul Cynn and I finally connected with Ikayso, a Hawaiian smartphone app developer in April of 2020. I am very excited to say that our app, called RainbowChase, is now available to the public for free.”

The study is published in the journal Bulletin of the American Meteorological Society.

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By Chrissy Sexton, Earth.com Staff Writer