Chemtrails and Clouds and Missing the Obvious

     A while ago, I was fortunate enough to be assigned window seat on my return flight from Vancouver. I always love having a window seat, because even in this age of ubiquitous photography and access to images from space and the deep sea and everywhere else, the visceral experience of acceleration and ascent while watching the ground in its human scale recede into the distance somehow puts things into a perspective you just can't get from admiring photographs or even videos. But here's one anyway:

     On this particular flight, it was the clouds that kept my attention. We passed through two distinct layers, and as we climbed above the higher one, I was treated to a spectacular view of a vast rippling expanse of cloud tops. And my mind was filled with wonder at how so much structure could emerge from just water dissolved in air. Look at the way the ripples form as the waves of air flow over the mountains!

     Now, this morning, a friend on Facebook posted a link to a website about chemtrails, which opened with the following paragraph:

When I was a little boy, I used to wonder what those beautiful streaks in the sky were. Why would some of them dissipate immediately, and others stayed in the sky all day?

     The implication, of course, is that something suspicious is going on here. Otherwise, we should expect all vapour trails to behave the same way: they should either all dissipate immediately, or all linger a similar length of time.
     On the one hand, I want to applaud the inquisitiveness that drives such a question. And yes, obviously there should exist some sort of explanation for why some vapour trails last longer than others, and why some are so huge and billowy, and why some never appear at all. But on the other hand, I am kind of disappointed that the inquisitiveness stops there, because it completely fails to consider the equally mysterious question of the huge diversity of normal ordinary clouds themselves.

     What is a cloud, after all? Well, we all know (I hope!) that it's water in the air, a mist of tiny droplets that scatter light so you can't see very far inside one. Fog is just clouds at ground level, and when the weather is chilly, we can see our breath: it's the same stuff.
     But here's the question that always troubled me as a little boy, and which still caused me to marvel at the age of 50 from my window seat over the Rockies: Why do clouds seem to have boundaries at all? Sure, the boundaries are pretty fuzzy when you get close to them, but on a large enough scale, the patterns become stunning: that patch of air is cloud, and that patch is not cloud. And you see these amazing ripples and puffy towers and all sorts of complicated fractal shapes that just scream for an explanation: How is it that this volume of air gets all misty and cloudy, while that volume just over there remains clear?

     It's not at all a simple question, but there are a few basic factors that are simple enough to understand. There's temperature, of course; we all know that sometimes air can be hot and sometimes it can be cold. There's also humidity, which is how much water vapour is actually in it; you won't get clouds at all if there's no water vapour there to condense. There's also pressure, which is related to temperature, but is a different concept; differences in air pressure from one place to another are why we have wind. And wind has a velocity, which as a completely relative quantity might be of little relevance but for the fact that it  leads to turbulence. And then, on top of it all, there's particulate content: dust, smoke, pollen and such, which is important because it provides a surface for water vapour to condense onto; without any particulates, you can have extremely humid air and still no cloud formation. And who knows what difference various local changes in chemical composition might make, if the percentage of nitrogen here is a little higher or oxygen there a little lower.

     Now, I really have no idea how all of these factors combine, but the evidence of my eyes from watching clouds drift by is this: it's really complicated, and the atmospheric conditions that form clouds can vary greatly from place to place and time to time and altitude to altitude, locally and globally. And as a result, we can get an astonishing variety of cloud patterns. I also know, from introductory high school chemistry, that burning hydrocarbons (like jet fuel) produces carbon dioxide and water vapour, so it's to be expected that aircraft exhaust would have water vapour in it. (Also, heat and turbulence and probably particles of soot which would help promote condensation, and some engines burn cleaner or dirtier than others.)

     So when I hear chemtrail conspiracy theorists talk about how suspicious it is that one plane might leave a trail while another one doesn't, I am perplexed. If ordinary, natural weather patterns can generate such a huge variety of cloud forms, why should we expect all aircraft vapour trails to look the same? I honestly do not know the answers, because I do not understand the complex processes of cloud formation well enough to predict what conditions will produce what sort of vapour trail, but it seems to me likely that the same variation in conditions that produce varations in clouds should be capable of producing different sorts of vapour trails, without any need to postulate some diabolical and deliberate chemical-spraying conspiracy.

     What am I missing here?