Rainbows are, without doubt, one of the most photographed and well-known meteorological phenomena on Earth. But what exactly are these events, how do they develop, and what causes their unique characteristics?

Almost every human on the planet is familiar with the multicolored arc of a rainbow spanning across the sky. Even the few readers who have never observed one have probably seen several images and illustrations of this meteorological phenomenon.

Although it usually occurs on a rainy day when the sun breaks through the clouds, it can also form under similar conditions whenever water droplets in the air interact with sunlight.

This post examines what a rainbow is, how it is formed, and looks at the different types of rainbows. It also highlights the key facts that define this meteorological phenomenon.

Before looking at the formation of a rainbow and the key facts that define it, it is important to establish a clear definition of the phenomenon.

Rainbow Definition

A rainbow is an optical meteorological phenomenon that occurs when water droplets in the atmosphere refract, reflect, and disperse sunlight into its spectral colors, which appear as a multicolored semicircular arc to an observer close to the horizon. 

It typically develops when direct sunlight and raindrops are simultaneously present in the atmosphere on opposite sides of the sky. Its appearance results from sunlight being reflected by water droplets at an angle of between 40° and 42° to the observer.

The word “rainbow” originated from the Latin word “arcus pluvius”, which literally translates to “rainy arch.”

A rainbow is a meteorological phenomenon that is, in fact, nothing more than an optical illusion. It is not an actual physical object and also has no physical location. It is simply light being reflected and dispersed that, under the right conditions, reaches an observer’s location.

It typically appears when raindrops are present in the air and are exposed to direct sunlight, low on the horizon on the opposite side of the sky. This explains why rainbows normally appear in the west in the morning and the east in the late afternoon.

The multicolored arc one observes is a result of sunlight being refracted and reflected off the back of a water droplet, which also disperses the white sunlight and breaks it up into its seven spectral colors (red, orange, yellow, green, blue, indigo, and violet).

Red (the color with the longest wavelength) bends at an angle of 42 degrees and appears on the outer edge of the bow, while violet (the color with the shortest wavelength) bends at an angle of 40 degrees and appears on the inner edge of the arc.

All rainbows are technically in the shape of a full circle, but since they occur so close to the ground, one only observes the arc visible above the horizon. The antisolar point, the theoretical center of a rainbow, is almost always positioned at a point below the horizon.

A rainbow and the sun always appear on opposite sides of the sky. As a result, when viewing a rainbow, an observer will always be standing with his/her back to the sun. (Sunlight travels past the onlooker, which gets reflected and dispersed by water droplets back to the viewer.)

With a better understanding of what a rainbow is and the characteristics that define it, one can now look at how it is formed to better understand why it occurs. The following steps detail the progression in the development of a rainbow:

1. 1. For a rainbow to develop, both water droplets and (direct) sunlight need to be present in the air.
   2. With the sun and water droplets situated on opposite sides of the sky, sunlight travels through the atmosphere until encountering and hitting the surface of individual water drops, typically in the form of a rain shower.
   3. Water has a different density than air. As a result, sunlight is bent (refracted) as it enters a water droplet. 
   4. Inside the droplet, the light also gets dispersed (broken up) into its spectral colors (red, orange, yellow, green, blue, indigo, and violet), which is responsible for the multicolored band of a rainbow.
   5. Dispersed light continues to travel through the droplet until it reaches the back of the water droplet, where it gets refracted and reflected at an angle between 40 and 42 degrees from the direction the sunlight entered the water droplet.
   6. The reflected, dispersed light continues to travel back to the surface, where an observer, positioned at the right location, sees it as the familiar multicolored arc that constitutes a rainbow.
   7. The size of the water droplet determines the size or radius of a rainbow. This is a result of the water droplet’s refractive index (the measurement of how much light is bent as it passes through a medium).
   8. Observers view rainbows as an arc because the droplets opposite the sun reflect the light back to the viewer at approximately 42 degrees all around. (Rainbows form an imaginary circle, but one only sees the arc visible above the horizon.)

There are other processes involved in the formation of a rainbow and additional factors influencing its appearance, but these are the key steps in the development of this meteorological phenomenon.

Although the principles defining a rainbow and its formation remain the same, small variations and additional factors can result in different types. The following are some of the most common examples besides primary rainbows:

• • Full-Circle Rainbow: Although only the arc of the rainbow is usually visible from the planet’s surface, it is possible to view a full circular rainbow from an elevated position, for example, from an airplane or high mountain peak.
  • Double Rainbow: A raindrop can have more than one internal reflection. This often leads to a double rainbow with the second bow visible on the outside of the primary bow. Its spectral colors are inverted, with red on the inner and violet on the outer edge.
  • Twinned Rainbow: As a result of different-sized raindrops and originating from the same base, the very rare twinned rainbow sometimes occurs. Unlike a double rainbow, the spectral colors are in exactly the same order in both rainbows.
  • Reflected Rainbow: When viewed from across a relatively calm body of water, light first encounters and reflects off the water droplets in the atmosphere, but also gets reflected by the water’s surface before reaching the observer, resulting in a reflected rainbow.
  • Monochrome Rainbow: On rare occasions, rain showers close to the horizon can result in the scattering of green, violet, and other spectral colors with shorter wavelengths. This may lead to the formation of spectacular red (or monochrome) rainbows.
  • Fogbow: A fogbow occurs when sunlight encounters tiny water droplets, usually in the shape of fog. The microdroplets scatter the different spectral colors to such an extent that they basically cancel each other out and display a predominantly white bow. 
  • Supernumerary Rainbow: A supernumerary rainbow occurs when smaller pastel-colored bands appear inside the violet (inside) boundary of the primary rainbow. They are the result of the presence of water droplets smaller than 1 millimeter in size.
  • Higher-Order Rainbows: As already described, multiple reflections of light can take place inside a water droplet, resulting in a double rainbow. But even more reflections can occur, resulting in third and fourth-order rainbows, which are seldom visible, though.

There are even more variations of the primary type of rainbow, but these are some of the most commonly observed ones.

Previous sections of this article already addressed the definition and formation of a rainbow. The following list provides the reader with some of the key facts and characteristics that define a rainbow:

1. 1. A rainbow is an optical meteorological phenomenon that occurs when water droplets refract, reflect, and disperse sunlight at an angle of 42 degrees to an observer on the ground.
   2. The multicolored arc is a result of water drops breaking (dispersing) the white sunlight into its spectral colors and reflecting it back to observers on the ground.
   3. The size of a water droplet determines the radius/size of a rainbow. This is a result of water’s refractive index.
   4. The size of a water droplet also determines the brightness of a rainbow. Water drops larger than 1 millimeter in size produce much brighter and well-defined colors than smaller droplets.
   5. A rainbow is not an actual physical object and can never be reached.
   6. All rainbows are technically in the shape of a full circle, but one only observes the arc visible above the horizon since the observer is usually situated on or close to the planet’s surface.
   7. The theoretical center of a rainbow is called the antisolar point. (The position exactly opposite the sun.)
   8. A rainbow-producing water droplet can have more than one internal reflection, resulting in a double rainbow.
   9. A rainbow and the sun always appear on opposite sides of the sky. (The sun is to the back of the observer viewing the rainbow.)
   10. Several different types of rainbows exist, including double rainbows, supernumerary rainbows, twinned rainbows, and full-circle rainbows. 
   11. In Norse Mythology, rainbows were seen as a bridge (called the Bifröst) connecting mortal Earth (called Midgard) to the gods (in a location called Asgard).
   12. The dark piece of sky that exists between a primary and secondary rainbow is called Alexander’s band.
   13. The longest-observed rainbow ever occurred in Taiwan on November 30, 2017. It lasted for almost 9 hours (8 hours 58 minutes).

Although this list does not contain all the data available about rainbows, it highlights the key facts and characteristics of this optical phenomenon.

Although rainbows may sometimes appear to be physically reachable, they are not physical objects and can never be approached or touched. They have no fixed location. They are optical phenomena created by sunlight interacting with water droplets, and their position depends entirely on the observer’s location.

This means that as the observer moves, the rainbow seems to shift as well. Each person viewing a rainbow is essentially seeing their own version of it, formed by light reaching their eyes at a specific angle.

To see a rainbow, an observer needs to stand with the sun positioned low behind them, while water droplets are present in the air in front of them. This usually occurs during or shortly after a rain shower when sunlight breaks through the clouds.

For the best viewing conditions, the sun should be low on the horizon, and the observer should look toward the opposite side of the sky where the rain is falling. Clear skies in the direction of the sun and rain on the opposite side will significantly increase the chances of observing a clear, well-defined rainbow.

As this article illustrates, although a rainbow can sometimes appear almost magical and supernatural, it is a simple optical, meteorological phenomenon that results from the refraction, reflection, and dispersion of sunlight by water droplets in the atmosphere.

This article describes what exactly a rainbow is and how it develops. It also looks at the different types of rainbows, as well as the key facts defining this optical phenomenon.

Until next time, keep your eye on the weather!