Some say chemtrails don't exist and its just ice crystals. Some say its to curb "global warming" or used for weather modification. Some say it has to do with HAARP. Some say its barium and other poisons that are designed to lower the immune system of the population.. I don't claim to know what it is for sure but if you checkout the CFR Council of Foreign Relations website and look up Geo Engineering there's patents for chemtrails but they are very subtle about what the real reason for it is.