Not – I hasten to point out – the refraction-through-a-raindrop kind. No, these are rainbows that have autumn as their root cause.
As the amount of daylight decreases and the atmospheric temperature drops, one class of trees protects itself from excessive water loss by shedding its leaves. Prior to shedding, though, the leaves undergo an amazing transformation – one that everyone knows and that everyone looks forward to as a sort of reward for putting up with the loss of summer (assuming there was a summer to lose).
Through the spring and the summer, trees are actively capturing the energy of sunlight using chlorophyll, which colours the leaves various shades of green. As the daylight lessens and the temperature drops, though, maintaining large leaves becomes less cost-effective, so deciduous trees undergo a change. Chlorophyll is broken down and various nutrients are pulled out of the leaf and stored in the tissues of the tree. The loss of chlorophyll leaves behind other pigments in the leaves – carotenes and xanthophylls – giving them a yellow-orange hue.
Additionally, the leaves may start to produce anthocyanins, which colour the leaves red. It’s thought that this is a protection mechanism – apparently aphids are attracted to yellow, so making the leaves red may help prevent an aphid infestation.
Eventually, the leaf is shed and the tree can lie dormant over the winter months, waiting for spring.
Why am I relating all this information? Two reasons: firstly, I firmly believe that understanding a natural phenomenon adds immeasurably to one’s appreciation of it, without in any way detracting from the simple awe of observing it (two for one – bargain).
Secondly, sometimes you get to see all the stages at once. I was out for a walk on a lovely sunny October day, and all the trees seemed to leaves in each stage of transformation – green near the trunk, yellow a bit further out and orange-red at the tips of the branches. So here we have some rainbows – of a kind – caused, not by sunlight, but by autumn.
(Why don’t all trees shed their leaves? Shedding is not without cost: the need to make new leaves in the spring. Evergreens take the water loss – usually minimised because they mostly have small, or glossy, leaves – and offset it against the cost of making new leaves. Deciduous trees balance the books the other way. That’s the beauty of natural selection: if there are multiples strategies to survive in a given environment, evolution will probably explore all of them.)