Airplane Contrails

     Contrails are clouds formed when water vapor condenses and freezes around small particles that are present in aircraft exhaust. Some of that water vapor comes from the air in the atmosphere and some is present in the exhaus.
     Contrails were first noticed during high-altitude flights in the 1920's. However, interest in contrails really blossomed during WWII when bombers could be sighted from miles away. In fact, numerous WWII veteran accounts tell of problems to aviation due to massive contrail formations. Planes could not find their targets, and sometimes collided with each other. In 1953, a scientist named Appleman published a chart that can be used to determine when a jet airplane would or would not produce a contrail. 
     The exhaust of an aircraft contains both vapor and solid particles and both are important in the formation of contrails. Emissions include carbon dioxide, water vapor, nitrogen oxides, carbon monoxide, hydrocarbons such as methane, sulfates, soot and metal particles.
     Contrails exist for different lengths of time and are divided into three groups: 

short-lived - these look like short white lines following along behind the plane, disappearing almost as fast as the airplane goes across the sky, perhaps lasting only a few minutes or less. The air that the airplane is passing through is somewhat moist, and there is only a small amount of water vapor available to form a contrail. The ice particles that do form quickly return again to a vapor state. 
persistent (non-spreading) - these look like long white lines that remain visible after the airplane has disappeared. When these are present it indicates that there is a large amount of water vapor available. They appear as long, narrow white pencil-lines across the sky.
persistent (spreading) - these appear as long, broad, fuzzy white lines. This is the type most likely to affect climate because they cover a larger area and last longer than short-lived or persistent contrails. Because contrails are formed at high altitudes where the winds are usually very strong, they will move away from the area where they originated. 

     Contrails are always made of ice particles due to the very cold temperatures at high altitude. Unlike other clouds which can form at a range of altitudes, from very close to the ground, such as fog, to very high off the ground, such as cirrus clouds, contrails only form at very high altitudes (usually above 5 miles) where the air is extremely cold (less than -40 degrees F). 

     Contrail have cousins. Under the right conditions you will see vapor trails form from the wingtips of a jet on takeoff or landing. This phenomenon results from a decrease in pressure and temperature in the wingtip vortex. If conditions are right, liquid water drops form inside the vortex and make it visible. These evaporate very quickly after they form. 
     An interesting question that arises is, “Why do some planes leave contrails, but others don’t?” One reason might be that they have different engine though this is not common. Some engines will leave a contrail in air where another engine will not because the exhaust of the newer engines is a lower temperature. Power settings will also have an effect, especially if it affects the exhaust temperature. 
     However, the main reason why you see trails on some planes but not on others is because they are at different altitudes and they are flying in different regions of air which can vary in tempertaure and humidity even within a few feet even in seemingly clear air. 

Broken contrails and gaps 
Contrail Chart