"SOMETIMES they last a long time, and at others they spread out and dissipate quickly; or then again they may hold their shape but break away in fragments. What can they tell us about the weather apart from the fact that it is a nice sunny day?"
My correspondent could, of course, have been asking about ice creams, or garden parties for the meteorological fraternity or even about a flight of starling's in the sky at evening, lit by a slanting sun. But in fact he was curious about condensation trails, those elongated streaks of cloud that form a wake behind high flying aircraft.
"Contrails", as they are known in aviation jargon, result from interaction between the exhaust gases of an aircraft's engines and the air in the surrounding atmosphere. The combustion of hydrocarbon fuels in such engines releases great quantities of water vapour in a stream behind the aircraft.
This moisture increases the relative humidity in its wake if conditions are right, the air in the affected zone may reach its saturation point so that 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 minus 35 C. 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 aeroplane.
So what can contrails tell us of the weather? Nothing really but we can make certain deductions about conditions in the upper atmosphere. Firstly, the presence of a contrail tells us that the air temperature at the level of the aircraft must be very low indeed. Secondly, if the contrails are persistent we can infer that the relative humidity of the surrounding air is high, since it is reluctant to re absorb suspended water droplets.
If on the other hand, the contrails quickly dissipate, it suggests that the winds aloft are strong, facilitating rapid infiltration of the cloud by dry surrounding air. And if the contrails spread, or are fragmented, we can deduce the presence of wind shear - significant variations from place to place over short distances in the speed and direction of the wind various parts of the cloud are being carried along at different rates. But about tomorrow's weather, contrails are always resolutely silent.