What creates a rainbow?

For this discussion I wanted to shed light on the rainbow (pun intended). This has got to be a weather favorite for many. The science of the rainbow may seem straightforward at first. The two main ingredients for the creation of a rainbow are mixing water and sunlight, but the magic really comes from the alignment of you (the observer) versus the sun’s position in the sky relative to any cascading curtains of rainfall. The right positioning determines whether this spectacular display of nature manifests in front of you or not.

A rainbow in the sky is a result of both refraction and reflection when sunlight strikes, enters, and then exits tiny rain drops. Vitally, spherical rain drops act like prisms by separating sunlight into the visible color spectrum (refraction). Refracted light is then reflected back inside the raindrop only to be refracted once again upon exiting. The discrete color banding hallmark to rainbows (red, orange, yellow, green, blue, indigo, and violet) are the end result of sunlight exiting raindrops back into the sky at slightly different angles due to wavelength differences existing between individual ROYGBIV colors.

Sky conditions have to be ideal for spotting a rainbow. In fact, the sun has to be no more than 42 degrees above the horizon for a rainbow to be seen based on geometry considerations. This is why rainbows are more likely to be observed when the sun is lower on the horizon, including mornings, evenings, winter months, and higher latitudes in general. 

When you do see a rainbow, you are likely seeing what is called a “primary rainbow”. Should the sun be very low on the horizon, a higher “secondary rainbow” becomes possible that overarches the primary one (i.e., the bodacious “double rainbow”). Secondary rainbows result in the same manner to primary rainbows, but with an additional reflection inside the raindrop before exiting. Consequently, you'll notice a secondary rainbow’s colors are in reverse order compared to the primary rainbow below!

Finally, given all the alignment possibilities discussed between a perpetually moving sun and where rain may be falling, every rainbow is truly unique to an observer’s position and perspective. Perhaps you’ll be lucky enough to glimpse a rainbow today!

-JWM

Photo: Jonny William Malloy