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discover the secret of the most beautiful rainbows in the world

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A researcher from the University of Hawaii at Mānoa explains why the archipelago of the same name is considered the rainbow capital of the world. In his paper, he details both the physics of the phenomenon and its influences on the culture local. The article appeared on February 26 in the American Society Bulletin.

If rainbow lovers had to choose a dream destination, it would be the Hawaiian archipelago without hesitation. Indeed, this set of islands located in the northern subtropical Pacific is considered the capital of the rainbow world. A title that we owe both to the particularly high intensity and frequency of the phenomenon in the region.

Due to its strong presence, the rainbow occupies a considerable place in Hawaiian culture. ” [Celle-ci] is reflected in the Hawaiian language, which has many words and phrases to describe the varieties of manifestations seen in Hawaii »Notes Steven Businger, author of the paper. As such, the locals speak respectively ofdouble rainbow, of humiliate, of punakea or eventhe rainbow is your night in the presence of a double rainbow, greenish, barely visible or due to the luminosity of the moon. However, these are just a few examples, as the author has identified at least 22 different appellations.

Rainbow east of O’ahu. Credits: Steven businger.

A combination of circumstances for an explosion of colors

How to explain that the Hawaiian archipelago knows such a profusion of colors? Initially, the occurrence of a rainbow obviously depends on the simultaneous presence of rain and light rays. In addition, the solar or lunar star must not be located too high above the horizon. On flat ground, for example, the angle should be less than 42 °. Beyond that, the phenomenon merges with the surface and its observation is no longer possible from the ground.

But what makes the archipelago a special place is its geographical position and its relief. Indeed, the islands are far from any source of pollution and are exposed to the trade winds. While flying over the imposing volcanic landforms, the air is forced to rise on the windward side. Also, a semi-permanent showers regime affects the region. Without its mountains, Hawaii would experience a much more arid climate. As for the high purity of the air, it makes it possible to generate very intense and elegant colors.

Relationship between the angular elevation of the sun and the resulting rainbow. Beyond 42 °, the latter is no longer visible from the ground. The largest occur when the sun is just above the horizon. Credits: Steven Businger, 2021.

Otherwise, “ at night, the sea surface heats the atmosphere from below, while radiation to space cools cloud tops, resulting in more marked showers in the morning who bring rainbows in time for breakfast »Explains Steven Businger. We are talking about differential heating. A similar phenomenon occurs during the day when the ground heats the overlying air and causes the development of showers on the foothills. They peak at the end of the afternoon, at a very fertile time of day for rainbows. A fascinating spectacle that nature has been constantly renewing for millennia!

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