Hawaii is the best place to experience rainbows, here’s why

Which is the best place to experience a rainbow in all its glory on Earth?

It's Hawaii, according to Steven Businger,.a professor who is also the Chair of the Atmospheric Sciences Department at the University of Hawaii in Manoa.

Rainbows, some of the most spectacular optical phenomena in the natural world, are very common in Hawaii.

So common that they've become part of its cultural discourse. "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 (uakoko), standing rainbow shafts (kāhili), barely visible rainbows (punakea), and moonbows (ānuenue kau pō)," said Businger in a recent paper on rainbows and Hawaii.

He added that rainbows denote a transformation and a pathway between dimensions in Hawaiian mythology and are seen as celestial paths that Hawaiian gods use to come down to Earth from their home in the godly realms.

Businger explains the reason behind the abundance of rainbows in Hawaii:

Trade winds

Northeast trade wind conditions occur over Hawaii 9 out of 10 days during the summer and 6 out of 10 during winter and the airflow around the North Pacific high takes air from just off the West Coast and transports it over Hawaii in just over a week’s time. During this traverse, the marine boundary layer gradually deepens as a result of entrainment or mixing at the top and warming from below. Low marine stratus common along the West Coast gradually changes to closed cell convection and eventually transforms to open cell convection, as the air passes from California to Hawaii. Therefore, Hawaii’s trade wind weather is characterized by convective showers with clear skies between the showers. The spacing between the showers is sufficient (∼30–50 km) to allow sunlight to reach the rain beneath the showers, producing optimal conditions for rainbow sightings. At night, a warm sea surface heats convection from below, while radiation cools cloud tops, resulting in deeper rain showers in the morning with rainbows showing up.

The mountains

Businger says that without mountains, Hawaii would be a desert with a scant ∼430 mm annual rainfall as estimated from TRMM data over the proximate ocean. The impact of the Koolau Mountains of Oahu on the trade wind flow is apparent in the sharp gradients of cloud fraction and rainfall over the island. Although each Hawaiian island has unique topography and attendant orographic effects, in every case the mountains produce sharp gradients in clouds and rainfall, which are key to abundant rainbow sightings.

Diurnal heating

The volcanoes of Maui and the island of Hawaii are tall enough to block the prevailing trade winds. Therefore, these islands experience the most consistent sea breezes and mountain–valley circulations During periods of light winds, all the Hawaiian Islands experience island-scale circulations driven by diurnal heating. Under these conditions, showers form over the ridge crests over the islands of Oahu and Kauai in the afternoon resulting in prolific rainbows.

Precipitation physics

Hawaii is known as one of the most remote island chains on Earth, says the author, adding that Hawaii’s remoteness means that the air is exceptionally clean and free of air pollution, continental dust, and pollen. As a consequence, there is less scattering of sunlight by aerosol, and the sunlight contains the full spectrum of colors even at low angles when the sun is close to setting.