What Atmospheric Conditions Cause the Appearance of Rainbows? Credit: NCAR + UCAR - Atmospheric & Earth System Science/NOAA SciJinks

Full-circle rainbow viewed from an airplane. Credit: NOAA
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A rainbow is an atmospheric optical phenomenon that can be seen in the sky right after it rains, when the Sun is shining behind the viewer and there are no clouds blocking the view. Within a rainbow, sunlight is separated into a spectrum of colors that appear to the naked eye after colliding with water droplets within the air. This is why rain is necessary in order to see a rainbow, and the viewer must be in the perfect position in order to spot it.
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Because the air is less dense than water, when sunlight passes through a water molecule, the sunlight travels slower through water than through air. This causes the light to refract or bend as it slowly passes through the water droplet. As light passes through the water droplet and refracts, it bounces off the inside of the water droplet before exiting the droplet in the same direction in which it first entered. When the sunlight reflects within the water droplet, the light separates into wavelengths, or colors. Thus when this light exits the water droplet it exits in this new spectrum of colors, displaying a rainbow.

Sunlight refracting and reflecting through a water droplet. Credit: Australian Government- Bureau of Meteorology
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When viewing a rainbow, the colors vary from violet to red. Because violet is the shortest wavelength of visible light, it will appear to bend the most as light exits the water droplet, and therefore can be seen at the bottom of the rainbow. Red is the longest wavelength on the visible light spectrum and bends the least out of all of the colors, and therefore can be seen at the top of the rainbow. The brightness of these colors displayed within a rainbow depend on the size of the water droplets in which the sunlight is refracted. Water droplets with diameters over a few millimeters create bright and vivid colors, while smaller water droplets with diameters less than .01 millimeters create very faint rainbows in which the colors appear almost white and blend together.

Rainbows are most common in the summer and are quite rare during the winter because in order to observe a rainbow in the sky there must be both sunshine and rain, therefore water droplets in the air. This is a common occurrence during the summer months. During the winter months, water droplets within clouds are often frozen ice particles that are incapable of scattering sunlight in order to create a rainbow. These frozen ice particles however can sometimes scatter and reflect light into very unique patterns. 

Double rainbow in Alaska. Credit: Eric Rolph- CC-BY-SA

Double rainbows are a phenomena that may also appear if sunlight is reflected two times within the water droplet before exiting the particle. This secondary rainbow will appear to have the spectrum of colors reversed, with the red wavelength observed on the bottom, and the violet wavelength observed on the top. All rainbows are created through Geometric Optics, by this reflection and refraction of sunlight within a water droplet. 

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©2019 Weather Forecaster Christina Talamo