Hm, I find this video very interesting:
I don't find it interesting, especially when the guy assumes they're at the "same height". Of course...because the the difference between 30,000 and 40,000 feet is readily apparently from the ground.
The properties of air at any given location could be vastly different than any other location. It changes even moreso with altitude, and not always in a way that one would expect.
I'll use only temperature as a basic example. The temperature drops as altitude increases, right? Wrong. The troposphere extends up to about 36,000 feet at the equator (its ceiling gets lower at higher latitudes) and this is where most of our weather originates. Average temperature drops in this layer of the atmosphere.
The temperature stops dropping abruptly at the tropopause and then throughout the stratosphere it
goes up, nearly equaling ground temperature at the stratopause, 160,000 feet up. This is because the ozone layer is in the stratosphere and it absorbs solar radiation, increasing its temperature.
Temperature drops rapidly throughout the mesosphere, up to about 280,000 where the coldest temperatures in the atmosphere are. Above that, in the thermosphere, "temerature" as a measure of molecule's kinetic energy is virtually nonexistent and it becomes a function of solar radiation exposure instead. Obviously things exposed to the sun have extreme temperatures.
Anyway, enough rambling. Nothing above the lowermost stratosphere matters to airplanes, but the point is that air is complex, and even in the troposphere where air
generally gets colder with altitude, that is certainly not always the case. That's why they came up with the International Standard Atmosphere to make prediction somewhat doable. This 600 page textbook only covers the basics but there's enough in there to discount any of these ridiculous chemtrail theories.
This one's also quite intersting, could of course have a logical explenation.
Again, not interesting at all. A simple case of the airplane flying into and out of air where conditions were right for cloud formation. They're most likely exhaust contrails. If that pocket of air were below a certain temperature at that certain altitude (I don't know the altitude so I can't tell you the temperature) then the moist engine exhaust combined with the very cold surrounding air would cause clouds to form.