Unlike a double rainbow that is two separate and concentric rainbows, a twinned rainbow appears as two rainbows that split from a single primary rainbow base. These rainbows are very rare indeed!

The colors in the second bow, rather than reversing as in a secondary rainbow, appear in the same order as the primary rainbow. A normal secondary rainbow may be present as well. Twinned rainbows can look similar to, but should not be confused with supernumerary bands. The two phenomena may be told apart by their difference in color profile: supernumerary bands consist of subdued pastel hues (mainly pink, purple and green), while the twinned rainbow shows the same spectrum as a regular rainbow.

The cause of a twinned rainbow is the combination of different sizes of water drops falling from the sky. Due to air resistance, raindrops flatten as they fall, and flattening is more prominent in larger water drops. When two rain showers with different-sized raindrops combine, they each produce slightly different rainbows which may combine and form a twinned rainbow.

Although very rare, look for twinned rainbows when separate rain shower curtains come together.