Condensation/vapour traill are formed by the hot exhaust meeting the cold[er] atmosphere.
"Usually", they are formed between 26-37,000 feet. "Persistant" 'trails ARE of course of great interest to the Military, for obvious reasons. At low level, "pressure" trails or vortecies are formd in quite mild humidity, an a/c landing, or engine runs etc.
The "con trails" during the 'Battle of Britain' were in fact wing tip vortices, whereas the B17s were "true" contrails at only 22-25000 ft
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