why do airplanes make those white lines in the sky?

Answer 1

They are known as contrails - if you go to this article it will tell you all about them.
http://privatejetcharter.spaces.live.com/

Answer 2

Under the right conditions these are contrails; condensation forming in the atmosphere due to the heat from jet engines. Usually this dissipates quickly. If you're wondering about the white lines that linger, often for hours and slowly spread out in a thin sheets, are chemtrails, reasons and theories for there existence vary, see links below.

Answer 3

Exhaust has water in it. When that water condenses into ice in the cold air, it appears white, like a cloud. They're called contrails, short for condensation trails.

Details:
When you burn fuel in an engine, one of the byproducts is actually water - in the form of water vapor. Did you ever notice that even your car exhaust looks kind of white during the winter when it's cold out? You even get that effect from the air you exhale. To avoid being technical, that's what happens when exhaust mixes with freezing cold air, whether it's from a car engine or a jet engine. And at the height where jets fly, the air is usually way below freezing. The whiteness is due to water vapor condensing into tiny ice particles.

More details:
The size (width), density (whiteness), and length of contrails and also the time which they take to dissipate depend on the air temperature, humidity, wind, and even the plane itself (airflow in the wake of the aircraft). Contrails do NOT consist of chemicals that the government is spreading on the population, though a few odd people actually believe that.

Answer 4

The lines are actually condensation from high pressure. The high pressure from the bottom of the wing is wrapping around the tip of the wing to meet the low pressure on top. This causes wing tip vortices that have a high pressure in the center. During certain atmospheric conditions the air in the center of the vortices condenses into vapor causing the vapor trails that you see.

Answer 5

What you are seeing is called a vapour trail. The hot exhaust from the engines combined with the cold air of higher altitudes creates water vapour which appears as a white trail or cloud like streak across the sky.

Answer 6

Some very good answers here, just thought I'd add another $0.02.

If the contrails last for a very long time the weather is due to change within 24 hours (for the worst)

Answer 7

Its because the air is veary cold up there so the hot and cold air from the engines and outside temperature and create steam and thats what the lines are

Answer 8

Exhaust! Haven't you ever heard of global warming?

Answer 9

burnt fuel and moisture in the air

Answer 10

I think this is a good question, I always wondered the same thing.