The Sky Bow, also known as a full circle rainbow can only be seen when the viewer is elevated high above the horizon, such as when in an airplane or possibly on a roof.

In theory, every rainbow is a full circle, but an observer on the earth can only sees the upper half. Since the center of a rainbow is diametrically opposed to the position of the sun in the sky, more of the circle comes into view as the sun approaches the horizon, meaning that the largest section of the circle normally seen is never more than 50% during sunset or sunrise.

Viewing the rainbow's lower half requires the presence of water droplets below the observer's horizon, as well as sunlight that is able to reach these water droplets. These requirements are not usually met when the viewer is at ground level, either because droplets are absent in the required position, or because the sunlight is obstructed by the landscape behind the observer. From a high viewpoint such as a high building or an aircraft, however, the requirements can be met and the full-circle rainbow can be seen. Also, it is possible to produce the full circle when standing on the ground, for example by spraying a water mist from a garden hose while facing away from the sun.

A circular rainbow should not be confused with the spectre, which is much smaller in diameter and is created by different optical processes. In the right circumstances, a spectre and a (circular) rainbow or fog bow can occur together. Another atmospheric phenomenon that may be mistaken for a full circle rainbow is the 22° halo, which is caused by ice crystals rather than liquid water droplets, and is located around the sun (or moon), not opposite it.

Like a partial rainbow, the full circle rainbow can have a secondary bow and supernumerary bows as well.

One way to view a sky bow is from the sky, in an airplane. However, in most cases, passenger windows are not large enough to see the full circle. Another way is to climb to the roof of a high building during a rain shower which will provide a better opportunity for seeing a full circle rainbow.

Once, while pressing washing the roof of a two story building, I was able to create a full circle by spaying the water up into the sky in the direction above where the shadow of my head was on the ground, the antisolar point. However, during that very wet operation, I did not want to bring my camera to photograph the circle rainbow and risk getting the camera wet.