The Science of Rainbows in the Desert

Arizona’s big, clear desert skies make for some truly breathtaking phenomena. From viewing the expanse of the Milky Way to catching a Technicolor sunrise, to a spotting a brilliant rainbow after a summer monsoon, the sky’s the limit when you turn your eyes towards the heavens.

As the summer rainy season approaches, Phoenicians will be treated to gorgeous, intense rainbows that linger in the air. Rainbows have been the subject of mythology and lore for centuries, as well as being a good omen or sign of good fortune across cultures. From ancient Greece to the Wizard of Oz, rainbows have held a magical, whimsical role in human culture.

Although rainbows are certainly majestic, they aren’t made by magic – what causes a rainbow is the refraction and reflection of light, principles of the science of optics.

Rainbows can occur anywhere there is moisture in the air and sunlight in the sky. For the desert, this means that we can often spot a rainbow without ever feeling a drop of water on the ground. Droplets that evaporate before they can fall to earth are still accumulated in the sky and when hit with sunlight create the arc of color above the earth.

You need sunlight to create a rainbow, and to view a rainbow, the sun will be at your back when the rainbow is directly ahead of you. With most Arizona rainstorms happening in the afternoon, with the sun moving towards the west, most rainbows will be spotted to the east.

The same principles behind a prism are also what will cause a rainbow to occur.  Picture (or Google) the cover of Pink Floyd’s “Dark Side of the Moon” – a beam of white light enters a prism and where the material of the prism bends and separates waves of light into different colored frequencies.

White light is a composite of all the different frequencies of visual light, from 400 nanometers to 700 nanometers in wavelength. White light is also known as polychromatic light because of this. When polychromatic white light enters a new substance other than the air it is traveling through, it is forced to bend. This is the optics principle known as refraction.

Different wavelengths of light will bend at different angles in the new substance, separating different colors into a visible spectrum instead of the unified appearance of polychromatic light.

With a crystal or prism, the light entering a clear, faceted form separates the light and produces spectrums of color. With an actual rainbow, droplets of water in the air each act as a suspended prism, with the combined effect producing an arc of vivid color.

While it is most common to spot a rainbow just after the sun emerges from the clouds of a rainstorm, other conditions can produce rainbow effects as well.

Sun Dogs are an unusual phenomena produced by ice crystals in the air. When the sun is refracted through the ice, two rainbow arcs are visible, one on either side of the sun, like an incomplete halo. Sometimes a sun dog is also produces reflections of the sun itself in the sky, with the visual effect of seeing three suns.

Water and moisture close to the ground can produce an effect known as “dewbows”. Dewbows are frequently spotted over irrigated fields or watered golf courses. They can also sometimes be seen as a low fog clears. The dispersion and evaporation of moisture in the air breaks the light in a similar way that water particles in the air do. Seeing a rainbow in the spray of a hose or sprinkler is an example of a dewbow.

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