Why when you watch planes fly there is a white line behind them? What is that cloud like line?

When you watch them from land you can always see that line behind big jets. When you watch them from another airplane, however, sometimes that line is not white but black. Can anybody let me know what is that line suppose to be? The cloud like line is called a 'contrail'.

At higher altitudes there is a certain height above which water vapour freezes. This is known as the 'mintra level'. This height varies with the weather/season. Aircraft flying above the mintra level emit exhaust gases which contain water vapour. This water vapour condenses and freezes and forms ice crystals which can be seen as white trails from the ground. Once they drift to lower levels these trails hit warmer temperatures and the ice crystals melt and dissipate and are not visible.
The black smoke you refer to are merely the smoky exhausts from the older aircraft. The new generation engines are clean burning and do not leave behind any such black smoke. It's called a "contrail," and it is actually a thin plume of ice crystals that reflect sunlight to give the appearance of white smoke.

Jet engines burn fuel that produces lots of water vapor at high temperatures. In fact, you can think of a jet engine as a kind of steam engine, in a sense. When the water vapor from the engine combustion contacts the extremely cold air at high altitudes, it condenses and then freezes, producing the white vapor trail that you see.

The internet myth about "chemtrails" is vacuous and pointless. There is of course no such thing. How would this be done without all the pilots, mechanics, fuel crews, and others knowing about it? Not true!

It could be vapour in the air being condensed as it passes through the engines.

Or it could be air being condensed as it passes over the wings, because wings create a low-pressure zone, which reduces temperature. If the conditions are right, this could condense water from the air.

this is called a Contrail its like exhaust from a car but because of the atmosphere & altitude of planes its seen as a white vapor much like a cloud an only occurs at high altitude which is y u dont c it wen a plane takes off

Its water vapor from the engine exhaust.

It is a chemtrail line. They are poisoning the atmosphere on purpose to make people into sheeple. That and the floridated water.