Why is possible to see a circular rainbow when you are in an airplane, yet it is impossible from the ground?

Question

Why is possible to see a circular rainbow when you are in an airplane, yet it is impossible from the ground?

Answer 1

Because of the physics and geometry of rainbows, the center of a rainbow's circle is always in exactly the opposite direction from the sun.

When you're on the ground, if the sun is above the horizon, then the point exactly opposite from the sun must be below the horizon.  That means the center of any rainbow you see must also be below the horizon, and that means you can't see the full circle because it's blocked by the ground.

But when you're in an airplane, the point exactly opposite from the sun might still be in the sky; and in particular that point might be in some clouds or water vapor.  If there are clouds or water vapor on all sides of that central "anti-solar" point, they'll refract the light so you'll see a 360° rainbow.

Answer 2

It's all to do with angles. Each rainbow is unique to you because you're in a different place to someone else. You need to think in 3-dimensions for this next part to make sense...

We can see a rainbow when the light reflects at 42 degrees. At ground level, 42 Degrees down is almost always below the surface of the earth.

The link I've included in the source section can explain this much better than I can... They've got pictures!