We all see those clouds that form behind aircraft flying way up in the sky, but what are they and why/how do they form? Well, to begin with they are properly known as condensation trails, or “con trails,” NOT “Chem trails,” as they are incorrectly called by those susceptible to conspiracy theories. Trust me, there are enough real, documented conspiracies that there is no need to invent that kind of unscientific mythology. But on to the science, the facts.
My daughter is studying meteorology, and I asked her to ask her prof to explain the mechanics of the formation of “con trails,” but she sort of didn’t understand it well enough to explain it to me later…sigh. Shortly after, I was later helping out in a homeschool science class, and in the course of the experiment I had an epiphany and I got it(!) even though it wasn’t directly identifying this as an example of the adiabatic (ād’ē-ə-bāt’ĭk) process, which is most simply identified this way:
…When a gas is compressed under adiabatic conditions, its pressure increases and its temperature rises without the gain or loss of any heat. Conversely, when a gas expands under adiabatic conditions, its pressure and temperature both decrease without the gain or loss of heat. The adiabatic cooling of air as it rises in the atmosphere is the main cause of cloud formation. [The bolded bits are important.]
[http://dictionary.reference.com/browse/adiabatic?s=t <—— this link also has the correct pronunciation]
In short: as the is aircraft flying, air is compressed and heated up going into/through the engine, then as it comes out the back end it decompresses, cools and condenses on the fine particles in the air, forming con trail clouds – ta da!
IMPORTANT: When there is little moisture up there and/or there is a strong high pressure system the clouds evaporate quickly; it’s that easy.
Here’s a really cool video demonstrating the effect. Actually watch it; it is very short and entertaining and makes it all so easy to understand.