Function of saturation old chap. .. .If saturation becomes 100% then the. .exhaust water vapor will condense its. .got nowhere to go . .. .Clearly as one climbs the colder it . .gets and the air is capable of carrying . .less and less water vapour hence the contrail. .is normally associated with jets. .. .You can get it with any aircraft even a . .light aircraft flying high in the winter . .months - Dont tell the GA guys though as they. .will be too proud...