Widely touted as a 'conspiracy theory', chemtrails are a phenomenon popping up all over the U.K as well as the States and alot of Western Europe. They are, in basic, the trails of chemicals that are sprayed over us on about a weekly basis. Disguised as 'contrails', it is important to note that contrails are just condensation from a planes engine when it is flying at high altitude and the trails always disappear after 2 minutes at maximum. Chemtrails, however, at first look like they are contrails but do not disappear and can linger in the air for hours, often spreading into thicker natural looking 'clouds' and then blanketing the area underneath them. Another odd thing is that they are always sprayed in grids, or S shapes as the helps them form into a blanket quicker, and are normally over major cities.
Below you can see the perfect example, notice the bottom planes trail disperses shortly behind it, as the top planes trail does not.
Full size pic, here.
The government and such have been confronted about this issue, but have always refused to take it any further than saying 'Ask NASA'. Now the trustworthy people at NASA have never conducted any research on tests on the matter, and have never said anything more than "they are contrails". When asked why some trails disappear and others do not, they have only gone along the lines of "it must be pretty cold up there". When asked about pictures with two planes at about the same altitude and conflicting trails, NASA have only ever said it must be a 'natural phenomenon'.
Without a proper investigation into these chemtrails, I personally won't believe NASA.
There are many many different theories into what exactly is being sprayed into our atmosphere. One is that the chemicals are part of a secret weather control project. Another more sinister theory is that it is part of a mass inoculation, whether good or bad is debated. It's up to you to make up your own mind on this one, here are some more pics, some videos and a few websites for you to look at.
Pictues are here, here, here and here.
There is a full length documentary here and some shorted videos here and here.
Finally some resourceful websites;
Chemtrails, An Introduction
Chemtrails Data Page