Shadows in the sky cause by clouds appear by perspective effects to converge towards the antisolar point, which is also the center of a rainbow. When clouds or dense rain showers shadow the light falling into your rainbow cone the shadowed raindrops can no longer send the rainbow's rays towards your eye.

The result is one or more dark radial spokes centered on the antisolar point and making the rainbow sometimes resemble a wagon wheel.

Bright spokes are similar to anticrepuscular rays except that light is directed into specific directions by large raindrops rather than widely scattered by dust and aerosols.

Look for large dark clouds during a rain shower. The dark clouds are what will block the sunlight and may cause the spoked effect.

Sometimes, when clouds are moving fast across the sky, the rainbow wheel appears to rotate.