Why do some planes leave white trails, but others don't?

22nd July 2009

Asked by: Rosemary Whittington, East Lothian

Known as ‘contrails’, the white trails are caused by water vapour ejected from the back of jet engines. Their visibility depends on whether the water vapour condenses into droplets or ice crystals. This in turn depends on the humidity and temperature of the surrounding air – the drier and colder the better. For aircraft cruising at altitudes of 10,000m, temperatures have to be below around -50°C to guarantee contrails.