photo #6942346

Rose Ann Rowlett spotted this red rainbow peeking through the monsoon downpour beneath a Cumulonimbus cloud over the Peloncillo Mountains in New Mexico, US. Rainbows appear when drops of rain refract sunlight as they reflect it back at an observer, separating it out into its constituent wavelengths. They can only show the colours of the sunlight shining onto the raindrops, and this is the secret behind a red rainbow like Rose Ann’s.When the Sun is low in the sky, just before sunset or after sunrise, the sunlight has to shine sideways through the low, dense atmosphere to reach a rain shower like this. Our atmosphere tends to scatter away more of the shorter, blue-looking, wavelengths of light than the longer, red-looking, ones, and so it’s only the red and yellow light that’s left to illuminate the monsoon and form a bow. Not only is Rose Ann’s rainbow red, but it is also rather straight. When the Sun is low in the sky like this, it can form a rainbow that is almost a complete semicircle – but only when you’re looking at raindrops falling through enough of the sky. When one forms in a distant shower like this, just one end of the bow can appear and look almost vertical – a ruby rainbow fragment, radiant against the monsoon’s sweeping showers.