Circular rainbows

If we were birds, and spent our days on the wing, then every rainbow we saw might be circular. As it is, earthbound, the ground gets in the way, preventing the lower portion forming, and we never see more than the upper half. But mimic the birds, and raise yourself high above the ground, either in a

Antisolar Point
Figure 5.7 Circular rainbow. You can see a circular bow if you are able to look down on a shower of drops and are close to it. The gap at the bottom of the bow is due to the object on which the observer is standing getting in the way of the Sun's light.

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Figure 5.8 Circular rainbow. A rainbow would be circular were it not that the ground prevents the lower half from being formed. However, from a tall tower or a mountain top the lower half may be visible, as this photograph shows. The curvature of the pole on the left is due to the fact that the photographs was taken with a extremely wide angle lens (focal length 16 mm). (Photo Claudia Hinz)

plane or balloon, or at the top of a tower, so that you can look down on a shower illuminated by sunlight, and, with luck, the rainbow you see might be completely circular.

Even if you are well above the ground, you won't see a circular rainbow unless you are close to the rain. With the Sun near the horizon, a completely circular bow will be visible only if you are slightly closer to the rain than you are above the ground. The higher the Sun, the closer you have to be to the rain.

In the following account, a rainbow changes shape as the distance between an observer aboard a ship and the rain decreases.

A rainbow of exceptional brilliance and unusual character was seen. It appeared first, 45° on the port bow, as a rainsquall was approaching, and measured about 20° in arc. As the squall reached the ship, the rainbow, which had been increasing in size and brilliance, became a complete circle, apart from the segment cut off by the hull. It seemed to rise out of the sea on both sides of the ship just forward of the bridge. The brilliance was enhanced by the bow being seen close at hand with the sea as a background.

Marine Observer's Log

5.6 Rainbows at sunset and sunrise

Your best chance of seeing a circular bow is from an plane flying through rain, when the Sun is close to the horizon. I had a fleeting glimpse of a short segment of such a bow, which extended from the seven o'clock to ten o'clock positions on a clock dial, from an aeroplane as it was coming in to land, late one afternoon.

By the way, if you travel by plane you will sometimes see a glory. Glories are frequently mistaken for circular rainbows, though they are not at all the same phenomenon. You will find more on glories is section 6.4.

You can see a circular bow in spray from a sprinkler or a plant spray. Stand close to the drops, within a distance that is less than your height, with your back to the Sun. The resulting bow is almost a complete circle except for a segment at 6 o'clock where your shadow falls across on it.

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