The secrets of the best rainbows on Earth

"The cultural importance of rainbows is reflected in the Hawaiian language, which has many words and phrases to describe the variety of manifestations in Hawai'i," said author Steven Businger, professor in the UH Mānoa School of Ocean and Earth Science and Technology. "There are words for Earth-clinging rainbows (uakoko), standing rainbow shafts (kāhili), barely visible rainbows (punakea), and moonbows (ānuenue kau pō), 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."

Why is Hawai'i the rainbow capital of the world?

The essential ingredients for rainbows are, of course, rain and sunlight. To see a rainbow on flat ground the sun must be within about 40 degrees of the horizon. As the sun rises to higher angles in the sky during the morning, the height of the rainbow diminishes until no rainbow is visible above the horizon. The pattern is reversed as the sun lowers in the afternoon, with rainbows rising in the east and the tallest rainbows just prior to sunset.

Hawai'i's location in the subtropical Pacific means the overall weather pattern is dominated by trade winds, with frequent rain showers and clear skies between the showers.

Businger outlines four additional factors affecting the prevalence of rainbows throughout the islands.

"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 Businger.

Another critical factor in producing frequent rainbows is Hawai'i's mountains, which cause trade wind flow to be pushed up, forming clouds and producing rainfall. Without mountains, Hawai'i would be a desert with a scant 17 inches annual rainfall.

A third factor conducive to rainbow sightings is daytime heating, which drives island-scale circulations. During periods of lighter winds, showers form over the ridge crests over Oahu and Kauai in the afternoon, resulting in prolific rainbows as the sun sets.

Due to the remoteness of the Hawaiian Islands, the air is exceptionally clean and free of pollution, continental dust, and pollen. This is the fourth factor that contributes to the numerous bright rainbows with the full spectrum of colors.

Chasing rainbows

As Businger pursued his passion for finding and photographing these beautiful light displays, he began to imagine a smartphone app with access to Doppler radar data and high-resolution satellite data that could alert users when nearby conditions become conducive for rainbow sightings.

"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," said Businger.

RainbowChase is available at: https://rainbowchase.com/ . Users can view radar and satellite images of rain and clouds, along with current and future weather, and collect rainbow photos.