The Science of Rainbows

Yesterday, the boys and I made this collage rainbow. It was a nice sensory activity and younger son enjoyed wrapping himself in the different fabrics.

Last night I could not sleep and it occurred to me that the rainbow seems to have become the symbol of lockdown 2020. I am slowly reading “The walker’s guide to outdoor clues and signs” by Tristan Gooley. I had happened to be at the section which explains how and why rainbows occur. Every time I pick this book up, I am just blown away with his observations and it is by far the most interesting book I have read in years. I thought I would share some of the science with you.

We all learn from childhood that we do not get a rainbow, without the rain. But two other elements are required: the sun and an observer standing between the sun and the rain. The time of day is also part of the magical equation so it is not surprising that rainbows are a fairly rare occurrence.

Rainbows don’t appear as a constant size and this is to do with your altitude. For a rainbow to occur, the sun needs to be behind you and the rain cloud in front of the. The light reflecting through the raindrops acts as tiny prisms, splitting the light. If the rainbow were a whole circle, the centre of this circle would be exactly opposite the sun and this is called the antisolar point. Not only is the centre of the circle predictable, it will have a radius of exactly 42 degrees. In laymans terms, this is four extended fist widths.

Diagram curtesy of Tristan Gooley

This information tells us a few things. Firstly, rainbows cannot occur in the middle of the day. When the sun is high in the sky, the centre of this circle would be under ground. Therefore, you will only see stunning rainbows towards either sun rise or sunset.

If you can fee which way the wind is blowing, the rainbow gives you one of two weather signals: it is either about to start hammering down, or it is about to rapidly improve into glorious sunshine! If the wind is coming towards you from the rainbow, get some weather proofs on. It the wind blowing towards the rainbow, enjoy the sun.

Rainbows can also appear very vibrant in colour, or quite pale and watered down. This is to do with the size of the raindrops.

“Very bright violet and green bands, with a clear red band, but very little blue OR the top of the bow appears less bright – Raindrops are big, over 1mm in diameter.

Red is noticeably weak in colour, but still visible – medium sized raindrops.

The bow is pale, violet is the only bright colour, you see a distinct white stripe or red has disappeared – Raindrops are small.”

(The walker’s guide to outdoor clues and signs, Gooley,T, 2014)

And what about this? The double rainbow. You often get a bright conventional rainbow, with a paler inverted colour rainbow and a very dark section of sky between them. Why would this occur? This, if you were lost out in the wilderness could give you an excellent clue as to a significant land mark as these can only occur where there is a large and still body of water such as a lake, or calm sea.

All the science aside. Who does not love a rainbow? It seems to be a universal human truth that we see them and stare in awe. They are special and exciting and rare. So during our lockdown time, I am trying to view all this family time in a similar way: special, exciting and rare.

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