Alexander’s band, also known as Alexander’s dark band, is an optical phenomenon observed in rainbows. It refers to the region between the primary and secondary bows, which often appears noticeably darker to an observer compared to the rest of the sky.
The areas of the sky around a rainbow may appear blue or grey depending on weather conditions and cloud cover. However, these areas outside, inside, and between the primary and secondary rainbows tend to have distinct tonal differences from one another:
The area inside the arcs of a primary rainbow always appears tonally lighter than the surrounding sky.