The patch of sky between two rainbows often looks darker because fewer redirected rays of sunlight reach your eyes from raindrops at those angles. This shadowy-looking region is called Alexander’s dark band. It is not a strip of dark cloud, and it is not a shadow cast by either rainbow. It comes from the different ways sunlight travels through raindrops to form the brighter inner bow and the fainter outer bow.
What is Alexander’s dark band?
When a primary and secondary rainbow appear together, there may be a visibly darker region of sky separating them. The primary bow is lower and usually brighter; the secondary bow sits above and outside it. The intervening dark zone is named for Alexander of Aphrodisias, an ancient writer who described the optical effect. It is easiest to spot when sunlight shines through rain against a relatively even, dark cloud background.
See the dark band between two real rainbows
In this photograph, the brighter primary arc is on the right and the more subdued secondary arc is on the left. Notice how the sky between them has a different, darker tone. The image records the phenomenon rather than illustrating it with artificially painted bands.
Why are the rainbow colors reversed in the outer arc?
In a primary rainbow, sunlight bends as it enters a raindrop, reflects once inside, and bends again on the way out. Different wavelengths emerge at slightly different angles, creating an arc with red on the outer edge and violet toward the inner edge. In a secondary rainbow, light reflects twice inside the drop before emerging. That additional internal reflection changes the geometry, so the outer, usually fainter arc has violet on the outside and red on the inside.
Why is the area between the arcs dimmer?
The primary rainbow concentrates returning sunlight near an angular radius of about 42 degrees from the point opposite the Sun. The secondary bow appears near roughly 51 degrees. The distribution of raindrop-refracted and reflected light is not uniform: relatively little of that bright rainbow light emerges toward observers from the intervening angles. More light reaches our eyes from inside the primary bow and outside the secondary bow, making the middle region seem darker by contrast. The dark band therefore reflects the directional pattern of light, not a new physical layer in the atmosphere.
How can you spot a double rainbow after rain?
Wait for sunlight to break through behind you while rain or mist remains in front of you. Face away from the Sun and look for the primary rainbow. If conditions allow, scan just beyond its outer edge for a broader, weaker second arc. Compare the sky brightness inside the main rainbow with the band between the two arcs. A dark cloud background can make the contrast easier to see. Do not look directly at the Sun while checking your position.
Why is the second rainbow usually fainter?
Each extra internal reflection sends only part of the sunlight onward along the path that forms the bow; some light is transmitted or redistributed. The secondary bow also spreads its light across a larger angular region. Together, these effects usually make it dimmer than the primary rainbow. Raindrop sizes, surrounding cloud brightness, and viewing conditions affect how strongly the colors show.
Frequently asked questions
Does Alexander’s dark band mean another storm is coming?
No. It is an optical feature of double rainbows. Its presence does not provide a reliable forecast beyond the fact that sunlight and water droplets are both involved.
Can a double rainbow appear around the whole sky?
Both rainbow arcs are geometrically parts of circles centered opposite the Sun. From the ground, the horizon usually hides the lower portions. At a high viewpoint, observers may see more of the circles.
Why do photos sometimes show only one rainbow?
The second bow can be so faint that a camera’s exposure, contrast settings, or field of view fail to reveal it. Even if both bows are visible to your eyes, a photograph may capture only the bright primary arc.
Related reading: How rainbows form in raindrops and Why soap bubbles show shifting rainbow colors. For more detail about the two internal reflections, see the National Weather Service’s rainbow science guide.
Leave a Reply