Last weekend I had the fun of talking about scientific space balloon as part of the Manchester Science Festival NERC Into The Blue. Whilst there the talk before me, Park Discoverers (check them out!), got me thinking about plane vapour trails. These man-made clouds are the white streaks in the sky that form as an aeroplane flies overhead.
As the plane travels through the sky, the engines eject particles. Onto these particles water vapour condenses and becomes ice, creating the characteristic white line, or “contrail”. Plane contrails can give away the distance of the plane above the ground but also the humidity (water vapour) in the atmosphere. In a region of increased humidity there is a higher moisture content available to mix with the aircraft jet plume, resulting in more icy particles and a longer contrail.
As planes fly in the troposphere, using very simple rules it is possible to estimate the humidity of this layer of the atmsophere:
- Very Dry, No contrail
- Dry, Short lived contrail, can be long or short but disappears quickly
- Moist, Persistent contrail, thinner than 1 finger held at arm’s length
- Very Moist , Persistent spreading contrail, wider than 1 finger held at arm’s length
These rules only work in the troposphere, if the temperature were to get colder a plane engine could have enough moisture to provide a contrail regardless of the humidity. For the full relationship between temperature, humidity and contrail appearance check out the following plot from NASA, but I am going to stick with the simple rules and have some fun reading the sky!