How Rainbows Form

St. Patrick’s Day is a holiday often associated with images of rainbows promising a path to a leprechaun’s pot of gold. For most people, however, just spotting a rainbow is enough to brighten a day.

These amazing displays of nature form when raindrops, which act like prisms, scatter sunlight. To see one, an observer must be facing a moisture source like rain or mist with the sun at their back. The sun also needs to be at a low angle in the sky, less than 42° above the horizon. The lower the sun angle, the more of a rainbow’s arc will be visible.

Refraction and reflection inside a raindrop. Credit: Met Office

Passing from the air into a denser raindrop, the light slows and refracts. Since the different wavelengths of light bend by different amounts, the white light is dispersed into the colors of the visible spectrum: red, orange, yellow, green, blue, indigo, and violet. Red, which has a long wavelength, is refracted the least and is always on the top of a single rainbow. Violet, with a shorter wavelength, is refracted the most and is always on the bottom.

The light also needs to reflect off the back wall of the raindrop towards the viewer at the critical angle of 48° before it refracts again when it re-enters the air. A lesser angle will let the light pass through the raindrop and a larger angle will allow the light to reflect straight back out of the drop.

A double rainbow is seen when the light reflects twice inside the raindrop. Since each reflection weakens the intensity of the light, the second bow appears dimmer. The order of the colors is also reversed, with blue on top and red on the bottom.

Rainbow and faint second rainbow form after a rainstorm in Bermuda. Credit: Melissa Fleming

A Look at Rainbows and their Legendary Pots of Gold on this St Patrick’s Day

According to Irish folklore, a pot of gold can be found at the end of a rainbow. In reality, however, it is impossible to locate the terminus of this optical phenomenon.

For a rainbow to form, rain has to be falling in one part of the sky while the sun is out in another. The water droplets in the air act like prisms that refract and reflect the sunlight, revealing the colors of the visible spectrum. Red is refracted the least and is always on the top of the bow while blue is on the bottom. Since we only see one color from each drop, it takes a countless number to produce a rainbow.

That said, these colorful arcs are not physical entities that can be approached. No matter how close they appear to be, they are always tantalizingly out of reach. Nevertheless, most people consider seeing one to be a treasure with no gold required.

With a little luck, you can spot a rainbow if you face a moisture source – rain or mist from a waterfall – while the sun is at your back.

Happy Saint Patrick’s Day!

Rainbow after a rainstorm in Bermuda. Credit: Melissa Fleming

How a Circumhorizontal Arc Forms

The sky puts on an amazing light show everyday. But sometimes, it produces something special like a circumhorizontal arc.

Often mistaken for a rainbow, a circumhorizontal arc is an entirely different optical phenomenon. It is formed by the refraction, or bending, of sunlight through plate-like hexagonal ice crystals that are situated horizontally in cirrus or cirrostratus clouds. More specifically, light enters through the vertical side of the crystals and exits through their horizontal bottoms.This angled pathway produces the well-separated colors of the spectrum that we see in the sky.  They are brightest where the cirrus clouds are thickest. Oriented parallel to the horizon, a circumhorizontal arc always sits below the sun.

Rainbows, by contrast, are produced by the combination of refraction and reflection of sunlight in liquid water droplets. These arching bands of color always appear in the part of the sky that is opposite the sun.

Circumhorizontal arcs are somewhat rare. In addition to the appropriate cloud conditions, they require the sun to be very high in the sky – at least 58° above the horizon.  They are usually only seen during the summer in the mid-latitudes.

A circumhorzontal arc seen by the author last summer in Colorado. Image Credit: Melissa Fleming

A circumhorizontal arc seen by the author last summer in Colorado.  Credit: Melissa Fleming

Searching for the End of a Rainbow

At the end of a rainbow, according to Irish folklore, lies a leprechaun’s pot of gold. In reality, however, the true end of a rainbow is impossible to locate.

A rainbow is an optical phenomenon that forms when water droplets in the air both refract and reflect sunlight to reveal the colors of the visible spectrum in an arch formation. It is not a physical entity that can be touched or approached. To see them, the National Center for Atmospheric Research says you need to be both facing the source of moisture and be standing at a 42° angle to the sun’s rays.

This specific line of sight means that no two people will ever see the exact same rainbow. It also means that as you attempt to move closer to the rainbow, the further away it will appear. So, try as you might, you will never get close enough to see a rainbow’s true terminus.

In the end, rainbows are all about perception.  For many people, even without the promise of a pot of gold, the joy of sighting a beautiful rainbow is reward enough.  Happy Saint Patrick’s Day!

Rainbow appears to end in the Atlantic Ocean off Bermuda's coastline.  Image Credit: The Weather Gamut

A rainbow appears to end in the Atlantic Ocean off Bermuda’s coastline.                      Image Credit: The Weather Gamut.