How Are Rainbows Formed? Overview and Ideal Conditions

Rainbows are a welcome sight after drab and dreary rainstorms. However, despite being one of weather's most beloved phenomena, they can't be forecast. These optical illusions form spontaneously when drops of water (raindrops, or mist from everything from lawn sprinklers to waterfalls) disperse light into its component colors via two processes known as refraction and reflection. 

Ideal Conditions for Spotting a Rainbow

Creating a rainbow isn't as simple as mixing together sun and water; otherwise, rainbows would follow nearly all rainfall. It's how these two ingredients interact that determines whether or not a rainbow forms.

Look Skyward Just After a Morning or Afternoon Rainstorm

One of the best times to find light and drops of water paired up is near the end of a rainstorm when the sun peeks out from behind rain clouds, and a sprinkle of raindrops are still floating in the air where they can catch the sunlight.

As sunlight shines into a raindrop, light waves travel from air to water. Because water is denser than air, the light wave slows down and bends or “refracts” as it enters the raindrop. Once inside the bead of water, light travels to the curved back surface of the droplet, bounces or “reflects” off of it, then journeys back through the droplet and emerges out its other side. The light refracts again as it exits the droplet, and upon re-entering the air, scatters in all directions—up, down, and sideways—while journeying to observers’ eyes.

It’s this refraction that gives rise to the smorgasbord of colors rainbows are famous for. Remember that “white” light is composed of all the visible colors in the electromagnetic spectrum: red, orange, yellow, green, blue, indigo, and violet.

As sunlight is refracted, each of its component wavelengths of light refracts by a slightly different amount, and also bends at a different angle, which causes the light beam to fan out and separate into its individual color wavelengths. Violet wavelengths, which have the highest frequency, are refracted the most. They travel toward an observer at the sharpest angle—40 degrees from the path sunlight initially entered the drop at—which is why it’s the innermost color in a rainbow’s arc.

Meanwhile, lowest-frequency red refracts the least. It travels toward an onlooker's eyes at an angle of 42 degrees from the path of sunlight and thereby comprises the outermost color band. The other five colors of light travel at angles between these two. 

Although each droplet disperses the full spectrum of colors, only one color is seen per raindrop. For example, if green light reaches your eye, the violet light from the same drop will pass over your head, and the red light will fall toward the ground in front of you. If you've ever heard it said that rainbows are unique to each observer, this is why; everyone sees their own rainbow created by a different set of water drops and different sun rays.

Stand With the Sun Behind You

In order to see the resultant rainbow, an observer must be positioned just so. The sun should be at your back, and the water droplets, in front of you. 

Another thing about the sun: It should be sitting low in the sky. If it's directly overhead, as is the case with a midday sun, the sun angle will be too high to form the needed 42 degree angles with our eyes, and a rainbow won't occur. 

The Higher the Ground, the Better the View

From the ground, a rainbow takes a curved "bow" shape because its rays travel toward an observer's eyes when looking 40 to 42 degrees upward, to the right, and to the left. If you're on a mountain or a plane, though, you'll also be able to look downward at an angle of 42 degrees (the ground will be too far away to cut off your view). This is why rainbows seen from lofty heights appear as 360-degree circles. (Remember: full rainbows are not the same as glorys.)

When Rain Slackens or Sunlight Dims, Rainbows Fade

If you do spot a rainbow, be sure to enjoy it while you can. It’ll only last as long as raindrops remain suspended in the air where they can easily catch sun rays, and as long as the sun is shining. In other words, rainbows are short-lived sights. Rainbows caused by rain showers last mere minutes at best, while those associated with waterfalls or similar water features can last up to a few hours. Of course, in late 2018, a rainbow shimmered over Taipei, the capital city of Taiwan, China, for nearly 9 hours.

Rainbow Variations

As if classic rainbows aren’t wondrous enough to behold, small changes in their ingredients, such as the light source and water drop size, create a number of rainbow variations.

Double Rainbows

If light reflects not once, but twice while inside a raindrop, a secondary rainbow will form. Because this re-reflected light exits the drop at a 50- rather than 42-degree angle, this secondary bow sits above the primary arc. Its colors are also reversed (red appears on the bottom and violet on top). Secondary rainbows are fainter, too, since some of the light leaks out of the water droplet during the extra reflection and is lost to the air outside.

Curious about the dark, shadowy region sandwiched between the twin arcs? In truth, this area, which is named "Alexander's band" after the Greek philosopher Alexander of Aphrodisias who first described it around 200 AD, isn't any darker than the surrounding air. Because the light inside the primary arc and outside the secondary arc undergoes enhanced scattering from the raindrops and therefore looks brighter, the in-between area simply appears unlit in comparison.

Moonbows

As their name suggests, moonbows, or lunar rainbows, are powered by moonlight rather than sunlight. Because the moon is 400,000 times more dim than the sun, expect a moonbow's colors to be more muted than those of their daytime twins.

Fogbows

If water droplets are too small for light to refract, as is the case with very fine fog or mist droplets, a “white” or “ghost” rainbow, or cloudbow, will form. According to the Washington Post's Capital Weather Gang, cloud droplets are typically 20 microns across, whereas rain drops are 2 millimeters (20,000 microns) in diameter. This means that light waves don't have enough time to fully split into their component colors. Instead, they bend and spread, or "diffract," creating a hazy white arch.