I've never seen any of these myself, but every once in a while I read someone talking about chemtrails on the internet. These are supposedly airplane contrails with unique properties. Now the chemtrails themselves are supposedly caused by special additives in the fuel that get released from the engines, or else they have their own separate container and are supposed to be released from a separate dispenser (much like the "smoke trails" from airshow planes). Regardless of how they are released, rather than just dispersing like normal contrails, they spread wider and wider until they form clouds in the sky. These clouds can even deliver rain like normal clouds supposedly (which sounds a lot like the well known practice of "cloud seeding" which is usually used in dry parts of the world in order to extract what little moisture there, is for the purpose of crop irrigation).
However unlike "cloud seeding" other unusual properties are often attributed to these clouds. One person on the internet once said they observed the resulting clouds, but rather than staying in the sky they floated down to the ground like fog and when they finally reached the ground the material was seen to be a solid powdery substance, not moisture, and when the person took it to it to a lab to have it analyzed, it was found to contain some rare element (I think it was selenium). Someone on Youtube had a video of the grass in a large pasture covered with what looked like spider webs, but it wasn't as no spider could build such thick dense webs over that many acres of land. The explanation gave in the video was that this mysterious material was the result of the special "top secret" chemicals in the chemtrails (think something similar to spiderman's webshooter chemical) reacting with the grass.
Whether or not these specific instances are real chemtrails or not, and in fact whether chemtrails in general as people have claimed are real or not, I don't know.
What do you guys think? Do chemtrails exist? If so, do you think that they are part of some secret government plot, or just a legitimate science experiment involving cloud seeding, or something else entirely?