A nickname for contrail clouds left behind by planes that refers to a theory. It is believed that these contrails serve as pollution, reducing the earth surface's exposure to sunlight. This causes global cooling.
The theory though, is that there was and possibly stil is a government conspiracy, in which they are flying military planes that purposely create these chemtrails to cause global cooling (slowing down or lessening the effects of global warming).
j: hey it's actually snowing this winter.
p: yeah, i think those chemtrails are really helping maintain normal weather conditions