In the Old Testament, the patriarch Jacob dreams of a heavenly escalator, along which choirs of angels make their way to and fro between this world and the next. The vision was recalled by the poet Francis Thompson when he described how, just when things are at their worst: "Shall shine the traffic of Jacob's ladder, pitched betwixt Heaven and Charing Cross."
Were it not for the fact that he was writing some 40 years before the invention of the jet engine, one might almost believe that in his mind's eye, Thompson was observing contrails.
To our ancestors, condensation trails, as they are properly called, were something quite unknown. The elongated streaks of cloud, frequently seen etched against a clear blue sky in the wake of high-flying aircraft, are a 20th-century phenomenon, a product of the modern jet engine developed in the 1940s. They are a consequence of interaction between the exhaust gases from the planes' engines and the surrounding air.
The combustion of hydrocarbon fuels in a jet engine releases water as by-product; it is ejected in great quantities in gaseous form as water vapour in a stream behind the aircraft. This moisture raises the relative humidity in the wake and if conditions are right, the air in the affected zone may reach its saturation point. If it does, condensation takes place, water droplets form, and the result is the familiar, elongated tube of cloud.
Saturation, and hence the exhaust trails, can normally occur only when the temperature of the outside air is below a certain critical value, depending on the altitude. At the cruising height for modern aircraft, 35,000 feet or thereabouts, this temperature is about 35C; at sea level it is around 24C. We therefore never have condensation trails in these parts when a plane is taking off or landing, because the surroundings are not cold enough. Moreover, since the process of condensation cannot take place until the exhaust gases have been cooled sufficiently by the air in the vicinity, which takes a little time, the trail only begins to appear some hundreds of feet behind the aircraft.
Condensation trails vary greatly in persistence. If the air is calm aloft, and the relative humidity in the vicinity is high, the trail tends to persist for a long time, because the atmosphere is reluctant to re-absorb the suspended water droplets. If on the other hand, the winds are strong and the moisture content low, then dry, surrounding air vigorously infiltrates the cloud, facilitates evaporation and makes the contrail break up and dissipate with great rapidity.