How and why are two lines of cloud formed by airoplanes. What type of airoplane engines cause this(is it is caused only by war planes or do commercial planes too emit this parallel clouds?

Aircraft engines be it jet or piston produce water vapor in their exhaust. The lines you see in the air is the water vapor condencing (sp) and forming cloud like lines. It usually depends on the hight of the aircraft the higher the more likely you will see the streaks.

Just to give a name to what Raymond47 described---those lines behind high-altitude aircraft are called "contrails."

I have also seen them when aircraft pull high g-loads. From what I remember, the contrails in this case is caused by the water vapor being literally squeezed out of the air even at low altitude.

Just another bit of info on contrails... I live in a tropical and humid part of the world, I have never seen a contrail where I live, even when aircraft fly over at 36,000' or so. They don't form in moist air, where the dew-point is higher ;)

They're called "contrails", short for condensation trails. It is just the water vapors in aircraft engine exhaust turning to ice crystals at high altitudes because of very cold temperatures. Kind of like when you can see your breath on a cold day.

Contrails are water condensation. When the air is compressed, it heats up, when it returns to normal, it cools down. Water will then come out of solution.

I am sure that water vapor from the engines has some effect, but why then are there always TWO contrails regardless of the number of engines the plane has and the placement of those engines. For example, would one see two contrails from tens of miles away if the engines are center mounted?

The number of contrails equals the number of wings and comes about when the air returns to its stable state after having energy apllied to it by the wings.

I have witnessed high G-load contrails on aircraft close to the ground. The contrails clearly come off the wingtips. Because of the relatively high air temperature at low altitudes, these contrails dissipate rapidly.

The next time you are seated in a window seat on a commercial aircraft during takeoff on a humid day, look at the wings shortly after rotation. You may see "clouds" forming on the top of the wing. This is a similar principle at work.

It is interesting how far behind the aircraft a contrail will form.