Why do some jets leave contrails while others leave nothing while flying at roughly the same elevation?

Question

My house is over a major flightpath and I see planes flying the same route every day. Without fail several will have a contrail, then one does not. What gives?

Answer 1

It depends on the conditions at altitude, which changes from day to day, same as on the ground.
Even flying at a slightly different altitude can have a huge difference, look at clouds, they have a bottom layer below which there is no condensation; at that altitude conditions change just so that a cloud can be present, lower and the water remains in the form of invisible vapor instead of water droplets in suspension.

Answer 2

Using a temperature sounding of the atmosphere from a radiosonde attached to a weather balloon, it is possible to determine the altitude at which contrails will form. This was important during WWII. Bombers used to fly just below the level where contrails formed. Any fighters attacking from above would leave contrails which would make them easier to see.

As well as contrails forming from the condensation of water vapour in the engine exhaust, long lasting contrails occur when there is supersaturated air. For water vapour to condense into water droplets or, at those altitudes, into ice crystals, there must be condensation of freezing nuclei present. These are minute particles of dust or salt. If the air is very clean, the water vapour has nothing on which to condense so the air is supersaturated. If you then fly an aeroplane through it, pumping out large quantities of particles in the exhaust, the water vapour quickly condenses or deposits as ice on the particles forming contrails that are very long lived and often spread out.

Answer 3

Very good observation and here is the correct scientific answer. Contrails form when the ambient humidity is high. Jet engines expel a great deal of water vapor in their exhausts. When this vapor enters an area where the humidity is very high, the likelyhood of the humidity reaching saturation is greatly increased. Where the humidity reaches 100% the contrail (short for condensation trail) will develop. Contrails form at one location but not another due to changes in humidity at the flight altitude. Hope this clears this question up for you.

Answer 4

Eventhough planes appear to fly at the same height,each has a flight level and they are allowed to fly at that level only.The minimum difference between two adjacent flight levels is 1000 feet.Enough moisture should be available at that particular level to leave contrails.Hence altitude and moisture content detemine the production of contrails. .The hot air from the plane evaporates the invisible moisture which later condeses into visible water droplets and they appear as contrails.The one you saw without contrails might have been flying at a higher altitude where enough moisture would not have been present.The moisture content may vary from day to day and sometimes from hour to hour.

Answer 5

The atmospheric conditions must be correct in order for jets to leave contrails. Moisture( be it from clouds, other contrails,etc) must be in the air. Even the slightest difference in the location of the jet can determine if it leaves a visible contrail.