Rain and sunlight are the key ingredients of rainbows, but many other environmental factors are required to consistently pull off this visual trick, as new research shows.
“Rainbows are some of the most spectacular optical phenomena in the natural world, and Hawaii is blessed with an amazing abundance of them,” opens a new science paper published in the Bulletin of the American Meteorological Society. The paper explains why this is the case, listing such factors as sea surface temperature, prevailing trade winds, and topography, among other variables.
In an associated press release, Steven Businger, the sole author of the paper and a meteorologist from the University of Hawaii at Mānoa, said the cultural significance of rainbows in Hawaii is evident in the indigenous language.
“There are words for Earth-clinging rainbows (uakoko), standing rainbow shafts (kāhili), barely visible rainbows (punakea), and moonbows (ānuenue kau pō), among others,” he said “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.”
At their most basic level, rainbows are easy to explain: These iconic multicoloured arcs are caused by the reflection, refraction, and dispersion of light travelling through water droplets. We see rainbows when the Sun is situated at around a 40-degree angle to the horizon.
“Billions of raindrops distributed along a cone at the correct viewing angles contribute to the phenomenon we perceive as a primary rainbow,” according to the paper.
During the morning as the Sun rises in the sky, the height of possible rainbows shrink, while the reverse happens during the afternoon, with the tallest rainbows appearing before sunset.
But there’s more to rainbows than water droplets and the correct angle of sunlight. As the new paper points out, Hawaii’s location in the subtropical Pacific is ideal; the archipelago’s weather is heavily influenced by northeast trade winds, which appear nine days out of 10 during the summer, and six of 10 days during the winter months. Rain showers are frequent in this climate, with clear skies appearing during the intervals.
Warm sea surface temperatures heat the air close to the surface, while “radiation to space cools cloud tops, resulting in deeper rain showers in the morning that produce rainbows in time for breakfast,” said Businger in the University of Hawai’i press release.
At the same time, mountains redirect trade winds up into the sky, assisting with cloud production on the lee side. Indeed, mountains are hugely important in Hawaii, as the islands would be deserts without them owing to scant rainfall, compared to the hundreds of inches of rain the archipelago receives annually, according to the research.
Daytime, or diurnal, heating helps wind to circulate around the islands. When winds are light in the afternoon, rain showers will form over the ridge crests and mountain slopes — a set of conditions that leads to a plethora of rainbows prior to sunset.
Another factor has to do with how clean the air is in Hawaii, a result of its isolation.
“Hawaii is known as one of the most remote island chains on Earth,” writes Businger in the paper. “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 colours even at low angles when the sun is close to setting.”
So not only are rainbows abundant in Hawaii — they’re also visually spectacular.
For those of you who live in Hawaii or are hoping to visit, there’s a new app called RainbowChase to help you find the colourful arcs and a pot of gold. The app, developed by Businger and his colleagues, allows users to view radar and satellite images of rain clouds, in addition to tracking the local weather conditions.