Doug Bonderud

Sep 7th 2020

Science of the Stunning: How Are Rainbows Formed?


Rainbows are the stuff of myth and legend. They’re impossible to ignore — these sudden arcs of vivid colors streaming across the horizon naturally inspire a sense of awe and wonder. The also awaken our scientific curiosity with questions about the form and function of these fantastic phenomena.

So let’s dive into the science of the stunning: How are rainbows formed? What types exist? And what does it all mean?

Mastering Meteorological Magic

The basic rainbow recipe is simple: water and light. But creating the ideal end result isn’t a question of whether you mix the basic ingredients well enough — it’s a question of weather.

As noted by the Met Office, two environmental conditions are required to reveal a rainbow:

  • The sun must be 42 degrees (or less) above the horizon
  • Water droplets — typically in the form of rain or fog — must be present

Sunlight entering a water droplet is refracted (bent), with each color in the visible spectrum refracting at a different angle due to differing wavelengths. These multiple rays of light are then partially reflected off the inside edge of the droplet and refracted again when leaving. If the sun is more than 42 degrees above the horizon from the observer’s point of view, the refraction won’t be significant enough to produce a rainbow.

If the sun is below 42 degrees and there’s enough water in the air, refracted light will naturally spread out into a familiar spectrum based on its wavelength. Red light has the longest wavelength at 650 nanometers and appears along the rainbow’s top edge. Violet has the shortest wavelength at 400 nanometers and appears at the bottom.

The position and shape of rainbows depend on observer location. For example, observers on the ground at sunset will often see what appears to be a complete arch with clear terminations on each end. Someone looking down from a plane at the same time may see a complete circle. In both cases, the center of a rainbow is the anti-solar point, which is directly opposite the sun.

ROY G. BIV, Redux

The familiar mnemonic device makes it easy to remember the classic rainbow color spectrum — Red, Orange, Yellow, Green, Blue, Indigo and Violet — though differentiating between blue and indigo with the naked eye is no easy task. Roy G. BIV pairs up nicely with what’s know as a primary rainbow, which is created “when sunlight strikes raindrops in front of a viewer at a precise angle (42 degrees),” according to National Geographic.

But this isn’t the only rainbow reality. In fact, multiple rainbow variations exist, including:

  • Double Rainbows: Double rainbows happen when light is reflected twice inside raindrops creating a faint, reverse-spectrum image above the primary rainbow.
  • High-Order Rainbows: Rainbows produced by second, third or fourth reflections of light are called higher-order rainbows. For example, a tertiary rainbow is the third reflection of light within water droplets and is seen when facing the sun. Tertiary rainbows are much broader and fainter than their primary or secondary siblings.
  • Twinned Rainbows: Unlike a double rainbow, twinned rainbows are two distinct, standard-spectrum rainbows produced when sunlight strikes an air mass containing water droplets with multiple shapes and sizes.
  • Supernumerary Rainbows: These rainbows are pastel-colored and can be seen under the inner arch of a primary rainbow. They occur when light rays interact across small water droplets of similar size.
  • Red Rainbows: Next on the list of rainbow variations are these monochrome manifestations. They form at sunrise or sunset when shorter color wavelengths — such as blue, indigo and violet — are scattered in the atmosphere and only red remains visible.
  • Moonbows: A moonbow is a much fainter primary rainbow produced when sunlight reflected by the moon refracts through water droplets.

Of Signs and Symbols

Beyond their visual impact, rainbows also carry cultural impact.

In Judeo-Christian culture, for example, rainbows appeared to Noah after the great flood as a sign of hope for better times. There’s also the oft-repeated legend of leprechauns guarding pots of gold. As noted by the BBC, these Irish tricksters supposedly grabbed gold from Vikings and will hand it over to treasure hunters who find the rainbow’s end. The problem? Perceived location and length vary with difference, making this an impossible task.

In Australia, aboriginal groups of the Pennefather River see the rainbow as a multi-colored serpent with the power to protect them from rain, while some Hindu traditions view the rainbow as a tool used by Indra, the god of thunder and war, to shoot arrows of lightning. In Norse mythology, meanwhile, the rainbow (or bifrost) was a burning bridge connecting Midgard and Asgard, the kingdoms of men and gods. And in Japan, land was formed from the ocean of chaos after creators descended from the rainbow bridge of heaven.

Seeing Is Believing

Environmental interactions in Earth’s atmosphere produce incredible results — from terrifying lightning storms and powerful waterspouts to stunning rainbows and beautiful aurora borealis. And while there’s a perfectly rational expiation to the question “how are rainbows formed?”, empirical evidence doesn’t negate emotional response. Instead, it highlights the human condition: the sometimes powerful, sometimes paradoxical balance of stunning beauty and scientific truth.