Aviation never ceases to amaze! One of the wonders that aviation offers is contrails(condensation trails). Yes, you must have seen them or continue to see them. Beautiful lines drawn in the sky when an airplane passes high above. Sometimes the lines remain for hours together or disappear after few minutes. What are these trails and how are they formed? Well, they are something similar to vapour formed when we exhale through the mouth on a cold winter morning. Contrary to the popular belief, contrails that we see are not the hot jet exhausts. Commercial jets typically fly at altitudes in excess of 25000 feet. Jet exhaust basically contains carbon di-oxide, water vapour, soot and unburned fuel. All these components are released at high temperature and pressure into the atmosphere. The ambient temperature at those altitudes is typically between -41 to -44 degrees C and the pressure is around 0.37atm.
The water vapour and some impurities in the exhaust like soot in extremely fine form act like condensation nuclei facilitating formation of a linear cloud. However, this isn’t always the case. The formation of contrails depends on a few factors. When the air at the altitude at which the aircraft is flying is wet and cold, the chances of contrail formation are more and they can stay around for a longer period of time. Sometimes the trails spread and stay around for longer time assuming different shapes resulting in what is known as “chemtrails”. Now chemtrails is a conspiracy theory but for now let us not venture in there. Not all aircraft flying in the same airspace create contrails. The factor that is in play here is also the type of engines. The aviation industry has gradually moved away from the turbojet engine to the turbofan engine. Turbofan engines are more efficient. They have a higher bypass ratio which releases cool air around the hot exhaust gases through the core. A study by Ulrich Schumann has shown that such engines operating at higher propulsion efficiency reach conditions which result in higher relative humidity for the same ambient temperature resulting in formation of contrails.
To test this theory, two large commercial jets, a A340-300 and a Boeing 707 were flown together, wing to wing, at designated altitudes during ascent and descent. The A340 is powered by CFM56-5C4 high bypass ratio(6.8 – more efficient) engine whereas the Boeing 707 is powered by JT3D – 3B low bypass(1.4 – less efficient) engine. The ambient conditions and the contrail formation was observed from a research aircraft. It was found that the A340 created contrails in many more instances than the B707. Since early days of aviation, military planners were interested in the formation of contrails. If an aircraft is forming contrails and if it flies over enemy territory, it can easily give away itself and would thus help the enemy take counter measures. A scientist named H.Appleman devised a chart that could predict whether a jet plane would or would not produce a contrail. This chart named as the Appleman chart is still used widely across the world to predict the formation of condensation trails.
Another factor that affects the contrail formation is the distribution of wet and dry air in and around the clouds. Inside the cloud, the concentration of wet air is much higher than outside. However, this is not just limited to the clouds. The variation in wet and dry air can be found throughout the clear air. So a jet flying through wet air to is lot more likely to produce contrails than the one flying through dry air. This is the reason why a jet flying at a particular altitude leaves behind contrails while another one in the same vicinity does not. Sometimes the wet and cold air can co-exist within few feet of each other. Sometimes, rockets flying vertically up have been known to demonstrate this. as soon as it hits wet air, it can be seen tracing a contrail. As it enters dry air, the contrails disappear.
So next time you see a jet streaking by with a beautiful white trail, just take a few minutes out to savour the sight.