THE "green flash", described in yesterday's Weather Eye, is very rarely seen. But it is one of nature's most unusual and beautiful displays, when the last sliver of the solar disc momentarily turns a vivid, very brilliant green, just as it disappears below the horizon. The phenomenon is caused by a combination of refraction of the sun's rays by the atmosphere, the filtering of sunlight by the air through which it passes, and an unusual thermal structure in the upper air, all occurring simultaneously.
The flash invariably leaves a lasting impression on those lucky enough to have observed it - and this is not surprising, if Jules Verne can be believed. His romantic novel, Le Rayon Vert, was set in Scotland and published in 1882, and sets out the benefits of a sighting as perceived in those parts.
"It was one of the numerous inexplicable legends of the Highlands, which avers that this ray has the virtue of making him who has seen it impossible to be deceived in matters of sentiment; as its appearance all deceit and falsehood are done away, and he who has been fortunate once to behold it is enabled to see closely into his own heart and to read the thoughts of others." Or more poetically, if less explicitly:
Only those who have seen it,
Can know the peace without compare,
That comes to those touched by its greenness,
The mystique of le rayon vert.
Those who would reap these benefits themselves can improve their chances of seeing the green flash by bearing in mind a few guidelines. Firstly, watch the sun set over the ocean, which will provide a straight, uninterrupted horizon. Secondly, choose a cloudless evening when the changes of temperatures with height in the lower layers of the atmosphere are likely to be greater than usual, thus promoting greater variations in the extent to which the different constituent colours of the sun's rays are refracted; the middle of an anticyclone will do nicely.
Thirdly, conduct your vigil near the summer solstice, when the path of the setting sun makes a relatively shallow angle with the horizon; a green flash, if it occurs, will last longer in these circumstances, and be more easily perceived, than, for example, around this time of year, near the vernal equinox, when the setting sun shoots straight down behind the horizon with a minimum of delay. To this same reason, the further north you are the better.
Most important of all, however, be oblique and circumspect as you observe, since looking directly at the sun, even the setting sun, may cause your eyes some injury.