Why Do Rainbows Appear As Arcs Instead Of Full Circles

Rainbows appear as arcs because the ground gets in the way, blocking our view of the full circle.

Here is the process:

1. A rainbow is a full circle of light centered on a specific point. This point, called the antisolar point, is located exactly 180 degrees opposite the sun, with the observer's head as the vertex.
2. If the sun is high in the sky, the antisolar point is low (below your feet or below the horizon). If the sun is setting behind you, the antisolar point is rising in front of you.
3. From a typical viewing position on the ground, the horizon cuts off the lower portion of this light circle. We can only see the raindrops in the air that are above the ground.
4. Therefore, the "arc" we see is just the upper section of the complete circular rainbow. The higher the sun is in the sky, the lower the antisolar point, and the smaller the arc of the rainbow will be. When the sun is high (above 42 degrees), the entire rainbow circle is below the horizon, which is why rainbows are usually only seen in the early morning or late afternoon.