I HAD a letter from an English teacher recently and he wonders why poets so often describe the noon as "purple". He cites Shelley's Stanzas: Written in Dejection above Napies - "the purple noon's transparent might" - and also The Lake Isle of Innisfree, where according to Yeats "midnight's all a glimmer, and noon a purple glow". "Is there," he asks, "some meteorological or atmospheric explanation which is uncommon, yet well known to poets?
Now, Weather Eye, of course, has no personal insight into the poetic psyche. I have always assumed, however, that such references were to the bluish tinge that shrouds the distant landscape on a sunny, hazy day - variations, if you like, on Housman's theme,
What are those blue-
remembered hills;
What spires, what farms are those?
or on Thomas Campbell's shrewd observation that
`Tis distance lends enchantment to the view, And robes the mountain in its azure hue.
The explanation lies in the scattering of light waves by tiny particles suspended in the atmosphere. Waves of any kind are often obstructed by obstacles in their path - just as a boulder interferes with water waves, sending wavelets off in many new directions. Tiny dust particles in the air, and indeed the very molecules of the air itself, are efficient scatterers of the shortest light waves - those at the blue end of the spectrum. As a consequence, the blue light in a ray of sunshine is diverted in many different directions as it passes through the atmosphere; the remaining colours with longer wavelengths - like red and orange - continue on their journey groundwards unaffected.
As you look at a mountain in the distance you see, naturally enough, the mountain itself. But superimposed on this image is another source of light; some of the blue from rays of sunlight passing through the air is diverted in your direction by the process of scattering. It is this scattered blue light which lends the scene its characteristic colouring. Moreover, the greater the distance to the mountain, the more air there is in between to scatter light in your direction, and so the bluer the distant hills will seem to be - sometimes a deep enough shade to merit being described as purple.
We also notice that at the beginning or end of a sunny day, the atmosphere, again because of scattering but in a different way, tends to acquire a pink or orange tinge. It is therefore around midday that the far off hills are at their bluest - and hence, I suppose, this poetic fixation with the "purple noon".