why not?

it's documentation, proof of their existence. i wonder what those planes do besides leave trails. i mean, what's their purpose? what are they doing up there? and are they really small, or just really high up?

They don't usually do it on purpose. It's because jet exhaust is hot and the air is cold. Really.

There's a little bit more going on than you think.

http://www.rense.com/politics6/chemdatapage.html

http://www.rense.com/general30/deby.htm

http://www.carnicom.com/contrails.htm

http://strangehaze.freeservers.com/

How is easy. Jets burn hydrocarbons for pushing force. Water and heat are a major waste products of burning hydrocarbon fuel in air. So the waste product is mostly hot, wet air. The air cools as it leaves the engine, and it soon can't hold so much water- hotter air can hold more steam in it. The water liquefies, and hangs as droplets. A big pile of droplets look white from far away. They eventually fall and disperse untill they evaporate.

Why?
Because engineers like me found a propulsion solution that just worked out that way.

To mark the passage of the plane, if only for a little while.

To help us remember that we leave a mark on this world.

Because the world is an interesting place