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What Causes a Rainbow If There Isn’t Any Rain?

Ellen Gutoskey
A circumhorizontal arc, or "fire rainbow," in Japan.
A circumhorizontal arc, or "fire rainbow," in Japan. / Lanapapa/iStock via Getty Images
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When the sun comes out in the wake of a rainshower, the appearance of a rainbow wouldn’t surprise you—those are, after all, classic rainbow conditions. What’s happening is that light waves from the sun first get bent and change speed (known as refraction) when they enter rain droplets; then they bounce (reflect) off the opposite sides of the droplets and leave the way they came in, refracting once more as their surroundings change from water to air. Because the colors refract at various angles, they form a streak of distinct layers.

Knowing that, it seems like a rainbow without any rain would be impossible. So how does it happen?

As New York-based meteorologist Rachael Witter told 96.1 The Eagle, it’s possible that the rainbow you’re seeing actually did form due to rain—but the rain evaporated before hitting the ground. This type of precipitation is called virga, and it’s sometimes visible as a blurry patch extending beneath a cloud.

A rainbow above virga.
A rainbow above virga. / mdesigner125/iStock via Getty Images

However, rain isn’t the only type of water known to cause a colorful streak or arc in the sky. There’s a whole host of phenomena that occur when light refracts and reflects off ice crystals in the atmosphere. Some of these so-called halo phenomena aren’t rainbow-colored—like the shaft of light, known as a luminous pillar, that often extends above a rising sun. But others do manifest colors, and which subcategory they fall into primarily depends on angles and the sun’s location. A 22° halo, for example, is a circle of light with a 22-degree radius that forms around the sun (they can also form around the moon). Sometimes, rainbow spots called parhelia—also known as “sundogs” or “mock suns”—can appear along this halo.

Sundogs around a winter sunset.
Sundogs around a winter sunset. / gretanrk/iStock via Getty Images

A circumhorizontal arc or “fire rainbow” is a type of colorful sun halo that runs parallel to the horizon and can only occur when the sun is above 58 degrees. Its counterpart is the circumzenithal arc, which occurs when the sun is lower than 32 degrees.

Though none of these are technically rainbows, they have refraction and water in common with them. They’re also all similarly brilliant to behold.

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