rainbow spokes. Larry Phillips pictured this unusual rainbow
during a storm late on July 17, 2003. ©2003
Larry Phillips, shown with permission. More
in the sky appear by perspective
effects to converge towards the antisolar point, which
is also the centre of rainbows. 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.
Sometimes, when clouds are moving fast across the sky, the rainbow
wheel appears to rotate.
The brightness of the unshadowed areas illustrate nicely just how much of a rainbow's
light is cast inside its rim at deviations larger than the minimum
of the rainbow angle.