AS CHARLES Lindbergh approached the coast of Ireland on his historic solo flight across the North Atlantic 70 years ago, he descended several hundred feet to get below a layer of cloud. He described the experience in his book, The Spirit of St Louis "The plane's shadow rushes in to meet me as I nose down closer to the waves. I last saw it centred in the rainbow, high up in the morning clouds, such a small shadow, skipping from crest to crest, but losing itself in the troughs, seemingly fearful it won't catch up before I reach the surface."
Now when Lindbergh says of the shadow that "I last saw it centred in the rainbow", he was not indulging his undoubted romantic streak, nor was he imagining a faint silhouette beneath an illusory multicoloured are over the distant Irish hills. He was referring to a glory.
A glory can be seen by any airline passenger whenever an aeroplane flies in sunlight above a layer of relatively low cloud a cloud composed of water droplets. The phenomenon appears as a series of concentric coloured rings centred, target fashion, around the shadow of the aircraft projected on the cloud below. The glory is not in any sense caused by the plane's shadow; the glory and the shadow are two distinct phenomena, but it so happens that both occur in the same place.
The glory is caused by a process called diffraction. The mechanism is rather complex, but in essence what happens is that the water droplets of the cloud interfere with the direct progress of the tiny waves of sunlight, and split the light into its constituent colours - the familiar colours of the spectrum. The diffracted light, a little further on its way, is then reflected inside individual droplets of the cloud, in such a way that it returns, more or less, in the direction whence it came.
It is because the light is reflected back along its original path that the colours of the glory can be seen only at that point on the cloud marked by the continuation of an imaginary line drawn from the sun through the aircraft. And of course it is also in that precise area that you will see the aircraft's shadow.
Glories vary considerably in size, depending on the radius of the cloud droplets. Indeed as the aircraft proceeds along its track, the glory may be seen to expand and contract, as it reacts to the changing composition of the cloudlayer below. The smaller the water droplets in the cloud, the larger the diameter of the glory.