Okay, here’s the question expanded with more context:
“I understand that rainbows are formed by sunlight refracting and reflecting off raindrops, and that their shape depends on the observer’s position relative to the sun and the rain. I’ve seen countless rainbow arcs, but never a complete circular rainbow. I know rainbows are fundamentally circular phenomena, so what are the specific atmospheric conditions and observational obstacles that prevent me from typically seeing a full 360-degree circular rainbow? Is it simply a matter of being too low to the ground, or are there other factors like terrain obstruction, raindrop distribution, or limitations in the angles at which sunlight can effectively create the rainbow effect that usually result in seeing only an arc?”

Answer

You can’t usually see a full circle rainbow because the ground typically obstructs the lower half of the circle. Rainbows are formed when sunlight strikes raindrops, causing the light to be refracted, reflected, and dispersed. This process separates the white light into its constituent colors, creating the familiar arc we see.

The center of the rainbow’s circle is always directly opposite the sun from the observer’s perspective, also known as the antisolar point. When the sun is high in the sky, the antisolar point is low, often below the horizon. This means that the lower portion of the rainbow, which would complete the circle, is blocked by the earth.

To see more of the rainbow’s circle, you need to be at a higher elevation, such as on a mountaintop, in an airplane, or even on a tall building. From these vantage points, the antisolar point is higher in your field of view, and you’re more likely to see a larger portion of the circular rainbow, possibly even the full circle.

Another factor is the availability of raindrops. A rainbow only forms if there are raindrops in the air opposite the sun. At ground level, the distribution of rain is often uneven, which can further obscure the complete circular shape.

While rarely seen, full circle rainbows are indeed possible, especially under optimal conditions where the observer is at a high altitude and there is sufficient rainfall distributed in a circle around the antisolar point. In such cases, the entire 360-degree rainbow can be observed. Rainbows observed from aircraft often exhibit a full circular appearance.

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