Every rainbow is a full circle. The ground just hides the bottom half.
A rainbow forms at a fixed angle of about 42 degrees from the point directly opposite the Sun. From a plane or a high mountain, with rain below you, the whole circle is sometimes visible.
Why is this true?
Each raindrop bends and reflects sunlight, splitting it into colours, and only the drops sitting on that 42-degree cone send light to your eye. That also means no two people ever see exactly the same rainbow.
More facts in this category
More about science & nature
Science and nature supply facts that feel like they break the rules until you see the mechanism. Hot water freezing before cold, glass that flows, lightning hotter than the surface of the Sun. Each entry keeps the physics or chemistry short and readable, states plainly where the effect is still debated, and links to a source for anyone who wants the full derivation rather than the headline.