The Pressure above the wing is lower than that below. Everything wants to equalise itself, thus the air from below seeks to go above (High to Low), thus creating a rapidly spinning vortex, creating an expansion of air, cooling it. Cold air can hold less humidity than warm air, thus condensation occurs.
What you usually see is water vapour from exhaust. The cold, high altitude, low pressure air can not hold the added humidity (Contrails) and it is seen. Same reason you see clouds.
That's it. It's only Water Vapour.
Well, ice crystals seeing as the conditions at cruising altitudes will turn any water into ice. Same thing with high altitude clouds, they are ice Crystals.
"But there are a lot more chemtrails contrails now then in the past"
Well, yes, there ARE a lot more planes in the sky then 20 or even 10 years ago.
Some High altitude Clouds, such as a band of Cirrocumulus:
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Can be confused for a "Chemtrail".
"But why are their some which are much widder than others and linger for a long time? If it is how you say it is, wouldn't there always be lingering long trails of clouds, not the shorter trails?"
Well, not exactly.
Pressure at higher altitudes is much lower and a volume of extra humidity has been injected into the atmosphere. The air wants to equalise itself and thus spreads the added volume, thus the expansion of Contrails.
The reason they can seen for soo long is because overtime there is still too much humidity for how much the atmophere can hold (too cold, too low pressure).
Remember, these are ice crystals. It's like trying to make sand disolve in water. Untill the conditions are right, you will see the ice crystals.
Also, there can be low to no wind, such as in "Cols" which prevent the dissipation of the contrail.
What we are seeing as the aircraft entering the conditions for visible contrails. Thus the "On and Off".
Yes, it can be such a dramatic change. Within the difference of a foot, the numbers can change to make the humidity visible. This the same reason some clouds have a flat bottom. Bellow that line, the air is warm enough to hold the humidty, but above it, the same amount of humidity for a given volume is too much due to the temperature.