What Causes a Triple Rainbow?
A triple or tertiary rainbow was a rare event.
In fact, up until 2010, there were only 5 reported cases of a triple rainbow in the previous 250 years.
Then, American meteorologists Raymond Lee and Philip Laven started asking people to report them, as there really should have been a lot more common than previously thought.
Sure enough, people started sending in photos of triple rainbows, and there was even one reported case of a quaternary (quadruple) rainbow.
This rare phenomenon may be more common than previously thought.
Personally, speaking, I have never seen one, but will be looking out in future for this unusual light display.
How tertiary or quaternary rainbows are formed
Rainbows are optical illusions caused by sunlight refracting off raindrops as it passes through them.
Sometimes, we see not just one, but a second, fainter rainbow lying beyond the first bright one. This is caused by light entering the raindrops and striking off their inner surfaces a second time, before coming back out.
Theoretically, this could happen a third, fourth or even fifth time, or ad finitum.
But what actually happens is that each 'copy' gets fainter and fainter, and the human eye simply can't see them.
What are the best conditions for seeing triple rainbows?
Keep your eyes skyward when the sun peeks through during a heavy thunderstorm.
If the clouds are thick, black and heavy with moisture, these are the optimum conditions for seeing a multiple rainbow.
You may see one rainbow, then it's second reflected one.
Then look towards the sun for the third or fourth rainbows - about 40ο towards the sun, where you'd least expect to see them.
This can be difficult if the sun is shining brightly, so take care to not look directly at the sun.
Here are some photos from those who looked out for them.