John Ivens (October 8) is the latest in a long list of uninformed people paranoid about a perfectly normal phenomenon – water vapour contrails left by aircraft.

These trails have been a regular feature in the skies ever since World War I, when the first aircraft started producing them. Any engine that burns hydrocarbons produces steam, and when steam meets freezing air, it forms a cloud of ice crystals – a long white cloud.

Whether a contrail forms or not, and whether it dissipates after minutes or several hours depends on the atmospheric conditions – pressure, temperature and relative humidity – at the plane’s altitude. In fact you can use an Appleman chart to predict whether contrails will form, and whether they will be persistent or not.

The air in summer tends to be too warm to form contrails, which is why we haven’t been hearing much about them for the last few months. In winter, if the sky is overcast, the contrails will be there but will be above the clouds and so out of sight.

On those days where the weather conditions are just right, contrails will form, and will last for ages – and this is the right kind of weather right now. An article on the science of contrails is available at www.maltahumanist.org/node/150.