Behind the rare ‘stacker’ rainbow
After just days in Taranaki, newly arrived photographer Andy MacDonald managed to capture two beautiful rainbows in one outing.
MacDonald moved to the region on Friday to take up a new role with Stuff and on Sunday he went for a beach walk near Oakura to see the wreck of the SS Gairloch, a coastal steamer that ran aground in 1903.
When it started raining, a double rainbow appeared. Then MacDonald looked north and saw another rainbow, which had extra bands of colour inside.
Later, a Google search told him it was called a supernumerary rainbow, also known as a stacker rainbow. But wanting to know more, we quizzed Niwa (National Institute for Water and Atmospheric Research) spokesperson Seth Carrier.
SO WHAT IS A SUPERNUMERARY RAINBOW?
When faintly coloured bands can be seen bordering the violet edge of a rainbow, or inside the primary bow, these extra bands are called supernumerary rainbows or supernumerary bands, Carrier said.
‘‘The supernumerary bows are slightly detached from the main bow, become successively fainter along with their distance from it, and have pastel colours (consisting mainly of pink, purple and green hues) rather than the usual spectrum pattern.’’
HOW DO THEY FORM?
These rainbows are most likely to occur when water droplets are small and of a fairly uniform size (generally 0.5-1.0 millimetres in diameter), Carrier said.
The alternating faint bands are caused by interference between rays of light following slightly different paths with slightly varying lengths within the raindrops, he said.
Some rays are in phase, reinforcing each other through constructive interference, creating a bright band; others are out of phase by up to half a wavelength, cancelling each other out through destructive interference, and creating a gap.
ARE THEY RARE?
‘‘By all accounts these rainbows are quite rare, as they require very specific conditions in order to form (particularly regarding water droplet size),’’ Carrier said.
WHAT ABOUT DOUBLE RAINBOWS, HOW DO THEY FORM?
‘‘In theory, all rainbows are double rainbows, but since the secondary bow is always fainter than the primary, it may be too weak to spot in practice,’’ Carrier said.
Secondary rainbows are caused by a double reflection of sunlight inside the water droplets. However, the colours appear reversed compared with those of the primary bow. The secondary rainbow is fainter than the primary because more light escapes from two reflections compared with one and because the rainbow itself is spread over a greater area of the sky.
ARE DOUBLE RAINBOWS DIFFERENT TO STACKER RAINBOWS?
The main difference is the process behind how they form, Carrier said.
Double rainbows occur when sunlight reflects twice when moving through a water droplet (this is actually common but the second bow can often be too faint to see).
Conversely, supernumerary rainbows are a result of rays of light taking slightly different paths through water droplets.
Sometimes these different paths meet up in phase with each other (causing the brightly coloured bands), and sometimes they are opposite to each other (creating the dark gaps), Carrier said.
To see MacDonald’s photographs of the vibrant rainbow, visit NZ.Andy on Instagram, or https://www.facebook.com/ Kiwiplaces.