'To whom it may concern," began a fax message to The Irish Times some days ago. "As your excellent columnist Brendan McWilliams is currently discussing wake vortices and CAT, can you ask him to write an equally challenging column on jet vapour trails, or 'contrails'?" Well, I suppose it concerns me, and when it is put like that, how could one possibly refuse? Contrails figure prominently on our television screens of late, albeit in unpalatable circumstances. Many of the US bomber aircraft flying to and fro across Afghanistan can be seen to be "trailing clouds of glory", plumes of white brilliantly etched against the clear, blue, Afghan sky.
Condensation trails are a consequence of interaction between the exhaust gases from a plane's engines and the surrounding air. The combustion of hydrocarbon fuels in a jet engine releases water as by-product, ejected 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 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 -35 C; at sea level it is around -24. We therefore never see condensation trails in these parts when a plane is taking off or landing, because the surroundings are not cold enough.
As you may have noticed, condensation trails vary greatly in persistence. If the air is calm aloft, and the relatively 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 the contrail breaks up and dissipates with great rapidity.
The type of aircraft has an impact, too. If the engines are slung below the wing - as, for instance, in the case of a Boeing 737 - the aerodynamics are such that the exhaust gases are drawn into the wing tip vortices - the spirals of air which trail behind an aircraft. In these circumstances the contrail tends to form at the core of the vortex, and is protected by the whirling tube from incursions of drier air. These contrails, therefore, often persist longer than those formed behind rear-engined aircraft.