Rays leaving raindrops after three reflections produce a tertiary rainbow. Unlike the primary and secondary bows which are opposite the sun and centered on the antisolar point, the tertiary appears sunwards and centered on the sun. With a nominal radius of 42.5º it is similar in size to, but very much broader than, the primary bow.

Why can we not see it?*   Its total brightness is 24% of the primary bow and we might think that it ought to be easily visible. There are two snags, its light is swamped by that of rays that pass through raindrops without any internal reflection. These generate an intense glare around the sun, the zero order glow that masks the tertiary. Another factor is that the tertiary is much broader than the primary and secondary, thus its luminosity per unit solid angle of sky is correspondingly less.

But perhaps not enough people have looked hard enough.**  Next time there is a rainbow, shield both your eyes very carefully from the sun and look for a 3rd order bow - of course, rain must be falling in that direction.

The 4th order bow (four internal reflections) is close to the 3rd order but with reversed colours. Its brightness is only 15% of that of the primary.