Okay, I know some of you don't trust the government, but there's no need to be ridiculous about this. Instead of believing everything you see on the internet or tv, why not ask a professional who knows what he's talking about?
I work as a professional airline pilot. I fly an MD-80, and I can assure you that there is no such thing as Chem trails.
Chem trails are a myth, and can easily be explained. They are usually mistaken as contrails, condensation trails, or vapor trails.
Lets discusss contrails, condensation trails, and vapor trails.
Contrails are formed at high altitudes from engine exhaust mixing with moisture in the atmosphere. The exhaust freezes up high because of the low pressure it creates and because the atmospheric temperatures at those altitudes are very cold (typically between -20C to -45C). Contrails are made up of tiny ice crystals that do not melt at those altitudes, and therefore they remain visible for long periods of time.
An airplane changing altitudes may create these contrails simply by climbing or descending which can give the illusion that they are "turning on or off a sprayer", however, they are not.
Also, since airplanes flying in different directions are assigned different altitudes, it may appear that one airplane is spraying something while another airplane flying nearby is not. In reality, those airplanes are at slightly different altitudes which can make all the difference in whether contrails form or not.
Even if two airplanes are at the same altitude, exhaust temperatures can vary by airplane, engine manufacturer, and by power settings...so two airplanes flying at the same speed and altitude may not both be making contrails.
Condensation trails are usually formed at lower altitudes and are caused by differential pressure as the aircraft surfaces move through the air. They are formed from the rapid decrease in pressure from air moving around an airfoil (like a wing or a tail). A low pressure area causes the temperature of the surrounding air to lower and once the temperature reaches the dewpoint, visible moisture is formed in the form of condensation trials. They typically dissipate quicker than contrails because they are not made up of ice crystals and the surrounding air temerature returns to normal after the low pressure area dissipates.
Vapor trails are similar to condensation trails, and are often formed near moving propellers. They are also often called condensation trails. They can appear to have a spiral shape to them and are in fact caused in a similar manner as condensation trails.
Each of these are normal phenomena caused by aircraft on a daily basis throughout the world. They are not caused by spraying chemicals, nor by a government conspriacy.
Think about it...Why would the government want to poisong the air? They have to breathe air too!
So, please, before you spreading rumors about ridiculous Conspiracy theories, please talk to an expert and give him/her a chance to research the subject.