Rainbows are circles, not naturally half-circles. From the ground, the horizon and nearby landscape usually block the lower part, leaving the familiar arch. From an airplane or another high viewpoint, you may be able to see enough raindrops below you to reveal the full ring.
Why is a rainbow shaped like a circle?
Sunlight enters a raindrop and bends, reflects inside the drop and bends again on its way out. Different colors emerge at slightly different angles. For the bright primary rainbow, red light arrives at the observer from a direction about 42 degrees away from the point exactly opposite the Sun. Since there are many raindrops all around that direction, the possible colored light forms a cone—and the edge of that cone looks like a circle in the sky.
What a complete circular rainbow looks like
This actual aerial photograph was captured while skydiving. The colorful arc curves down and around to make a ring, something that is usually hidden from someone standing on level ground.
Why don’t we see the bottom half from the ground?
For a person standing on level terrain, the center of the rainbow is in the direction opposite the Sun and often below the horizon. The circle’s lower portion would be in the region obstructed by the ground. A higher observer can sometimes look down through rain beneath their position, where the missing section becomes visible.
The National Weather Service’s rainbow explanation describes why more of a rainbow appears as the Sun gets lower and why the observer needs the Sun behind them.
How to spot a circular rainbow from a plane
Look away from the Sun, toward a region of rain or water droplets below the aircraft. You need sunlight reaching those drops, an unobstructed view and favorable geometry. The circle’s center is opposite the Sun; from an aircraft it may be somewhere beneath the horizon. A window seat on the side facing away from the Sun can help, but airplane windows, glare, clouds and the position of the rain may still hide the ring.
Is a circular rainbow the same as a glory around an airplane shadow?
No. A glory is usually a much smaller set of colored rings near the antisolar point, sometimes surrounding the aircraft’s shadow on cloud tops. It involves different scattering and wave effects from the large rainbows made by raindrops. A full primary rainbow is much wider and has the familiar red outer edge and violet inner edge. The Royal Meteorological Society explains the distinction.
Can a rainbow be a circle in a backyard?
Sometimes. Water droplets from a sprinkler, fountain or waterfall can create a more complete arc if you can look into the droplets from above and keep the Sun behind you. Keep in mind that your own viewing position matters: different observers are looking at light from different drops, so there is no fixed rainbow hanging at one physical location.
Frequently asked questions
Is a full-circle rainbow rare?
The circular geometry is normal; having the right high viewpoint, droplets and sunlight to see the entire circle is less common.
Does a full rainbow touch the ground anywhere?
No. A rainbow is an optical view created by sunlight and droplets relative to your eyes, not an object with ends you could walk to.
Why do some rainbows have a second, fainter ring?
Secondary rainbows come from light reflecting twice inside drops. They appear outside the primary bow and have colors in reverse order.
Can you see a circular rainbow at night?
Bright moonlight and water droplets can produce a moonbow. Under suitable circumstances its geometric form is also circular, though it may look pale to human eyes.
Related reading: How rainbows form in raindrops and Why the sky is darker between double rainbows. NOAA offers a family-friendly illustrated explanation.
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