Rainbows typically appear as half-circles because the ground usually obstructs the lower portion of the complete circular rainbow.
The Circular Nature of Rainbows
A rainbow is actually a full circle. This phenomenon occurs because sunlight is refracted and reflected within raindrops, with the light exiting the raindrops at an angle of approximately 40-42 degrees relative to the incoming sunlight. The observer's position relative to the sun and raindrops determines what portion of the rainbow they can see.
Obstruction by the Horizon
The ground gets in the way in most situations. Imagine the sun is behind you, and you are looking at rain in front of you. The center of the rainbow is directly opposite the sun.
- If the sun is high in the sky, the center of the rainbow will be low to the ground, and most of the circle will be below the horizon, hence only a small arc will be visible.
- If the sun is lower in the sky (near sunrise or sunset), the center of the rainbow rises higher in the sky. When the sun is low, a larger portion of the circle, potentially a semi-circle, can be seen because less of the rainbow is blocked by the horizon.
Circumstances for Seeing More of a Rainbow
To see more of the rainbow, including a full circle, you need to be at a high vantage point, such as:
- On an airplane: From an aircraft, you can often see a complete circular rainbow because there's no ground to block the lower part.
- On a mountaintop: Similarly, a mountain peak can provide a high enough perspective to see more of the rainbow.
- Near a waterfall: The spray from the waterfall can create localized rainbows, and the elevation change might allow a better view.
Summary
Rainbows are full circles, but we usually only see a portion of them because the horizon obscures the rest. The lower the sun is in the sky, the larger the visible arc of the rainbow, possibly a semi-circle. For a complete circular view, you generally need to be at a high altitude.
