In the summer of 1940, strange patterns like these began to appear in the sky over southern England. Today they wouldn't be thought so unusual (except that they are on the twisty side), for contrails are a common sight now, especially over London. Seventy years ago, however, they were a little mysterious, even to those in the aviation community, and even though similar phenomena had sometimes been seen before. Flight reported in July that year:
Some readers may have observed lately what they at first thought to be sky-writing, and a member of the staff of Flight saw a particularly good example on Sunday afternoon, July 7, over London. The same sort of thing had been seen previously, but this was the best example to date and exhibited some features not observed on other occasions. For the benefit of those who have not seen the phenomenon it consists of a thin line of what looks like white cloud, or perhaps of very white smoke made by a sky-writing aeroplane.
While it was allowed that the clouds might be caused by 'the discharge of white smoke from a military aeroplane for some purpose connected with the war', the explanation ultimately plumped for was pretty close to the mark: so-called 'visible vortices':
The explanation which has been given before as a possible reason for visibility of these vortices is that there is condensation of moisture. Such condensation might perhaps be caused in regions of low pressure which may be those parts of the vortex where the velocity is highest. Perhaps there is significance in the fact that it is at the tip of the airscrew (where the blade velocity is greatest) that the visible ring occurs. A fog formed by reduction of pressure can be seen in tunnelling work under the earth when, in order to keep out water, compressed air is supplied to the working face. The men, to get out, have to go into a chamber where the pressure is reduced before they can go into atmospheric pressure. During this decompression, the whole chamber may be filled with fog.
In the case of the trail behind an aeroplane, the condensation theory might be correct as there is plenty of water vapour in the products of combustion in the exhaust gas. If the atmospheric conditions are right, the condensation would certainly cause a visible trail.
But even though (as we now know) this explanation was essentially correct, there was as yet no proof, and there followed considerable correspondence from readers. (Some helpfully suggested that that the visible vortices might be used to track enemy aircraft, either by fighters underneath during the day, or by searchlights at night.) By September Flight felt it had enough information to tentatively confirm its earlier hypothesis, and also to note that there were two types of visible vortices: long-lived helical ones from engine exhaust ('slipstream trails'), and short-lived ones from wingtips ('wing tip trails'). In 1942 de Havilland published a similar but more technical explanation of both types of contrail, so it seems that Flight's theory had become widely accepted. A mathematical theory of contrail formation was independently formulated in Germany in 1941 and in the United States in 1953.
Science aside, the contrails quickly became part of the Battle of Britain and its memory, tracing out the deadly dogfights overhead, as suggested by Paul Nash's 1941 painting Battle of Britain (IWM ART LD1550):
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