Sometimes people mistakenly call them chem trails and assume that planes are dumping chemicals. What the planes are actually doing is disturbing cold moist air in the atmosphere and the moisture (water vapour) condenses out and leaves a trail in the sky behind the plane. They regularly criss cross the sky above where I live as I live quite close to a couple of airports. Obviously they seem more frequent in the summer. If you want to find out more about clouds there is the cloud appreciation society. They have a page on Facebook.