Do chemtrails Alter the weather significantly. What is the purpose?

Your second video, what happens is the sun is at a low angle so the "chemtrail" (which is really a contrail as pegminer tells you) "disappears" because the plane flies through a region where the sun is blocked by a cloud. Look closely at the video and you can still see it, it's just not bright white because there's no sunlight. Good god it must be depressing to be you, literally scared of shadows. I'll bet you have a nice gun collection though. You climate skeptics ought to form a usenet group alt.globalwarming.weaponry.orgasms. I would lurk on that because it would be absolutely hysterical.

God damn you people are so clueless. Really. Do some basic research on exchange of mass between the stratosphere and troposphere in the mid-latitudes where this was shot. It's virtually zero, meaning no mixing. Couple that with the presence of high levels of both ozone and UV light, both of which, golly gee, are used to sterilize things and remove organic contaminants. So your theory has the government injecting stuff into a region where it won't reach the surface and will just rapidly decompose. Brilliant. Utterly brilliant.

There are no "chemtrails." They are an unfounded conspiracy theory.

What you see are contrails. They are apparent when water vapor from the exhaust from jets condenses into water droplets or ice crystals behind the plane. They are actually what is called a "mixing cloud," neither the jet exhaust nor the ambient air has 100% humidity, but when mixed they do (actually > 100%) and you see condensation as a result. They come about because if you look at a plot of the Clausius-Clapeyron relation that governs the saturation vapor pressure of water you'll see that it is nonlinear.

The reason that you see them stop and start like in the video is because the plane is passing between air masses with different humidity. If the air is very dry contrails will not form, because even the mixed air will have humidity below 100%. These air mass boundaries can occur when the plane is climbing or descending (quite common) or even when it is in level flight.

They actually DO affect climate. Contrails can expand and cover large portions of the sky because under the right conditions the cloud droplets that form can act as nuclei for more condensation. Contrails don't block much incoming visible light from the sun, but they do radiate IR light downward and prevent nighttime cooling. Nights are warmer with contrails than without them, so the daily temperature range is less. This was demonstrated after 9/11, when the government shut down air traffic for several days, so there were no contrails. Scientists compared the average temperature range before and after, and found that on nights without contrails there was more cooling and greater temperature range.

EDIT: Just to be clear, since some people seem confused about this, this is NOT weather modification, it is NOT cloud seeding. Attempts at weather modification through cloud seeding are fairly uncommon these days because (1) there is no convincing evidence that it does much, (2) conditions have to be favorable for it (clear skies are NOT favorable), and (3) if you actually do affect the weather, you become open to lawsuits.

Paging Dr Lewandowsky....

<<Is it illegal in this country to deliberatly alter weather? Only radar disruption.

Or something sinister?

http://www.youtube.com/watch?v=LrDHr4YCOG4%3E%3E

It is a (purposely?) bad translation of the original German text, which does not speak of chemtrails at all. I checked it. It's about radars giving false hits and the problems this causes.

The only thing sinister here is the Chemtrail Conspiracist who posted this including the misleading translation.

<<Is there a real explanation? does this confuse you?>>

Nope. I understand basic science and so should you. Wanna see 'chemtrails' in your very own house? Go to you kitchen and open up your freezer. There! 'Chemtrails' right in front of you. Now you can go hide under your bed afraid black helicopters will start hovering over your house at any time now or you can be intelligent and try to understand what you just observed in your very own kitchen.

There is a direct correlation between chemtrails/UFO sightings by individuals and the lack of basic scientific comprehension of those individuals. Some contrails are even taken for falling meteors or UFOs. This one even made it to the local TV news: http://www.youtube.com/watch?v=z1RqLbYfBKM but, unfortunately, there's lots more like that on Youtube.

These experiments have been going on for a long time with no real significant advantage. Waste of money and resources. Besides just like trying to micromanage any part of nature their are the unintended consequences. Trying to force weather in one part disrupts weather patterns in other parts.

Sigh.

Edit: Starting off my morning giving Pegminer, who is a known alien ( It's true because I read it on the internets), a thumbs up. What next?

It's not illegal. It is expensive. It must be licensed or permitted by the appropriate jurisdiction, much the same as crop dusting or any such chemical or pesticide dispersion.

Some might. Some don't. We really don't know enough about it to truly get an honest answer.