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Jet planes´ tail

  1. Jun 18, 2005 #1

    Sometimes when a jet plane is flying above you you can see the white tail from the motors, and sometimes not.
    What is the reason for this?
    Why isn´t this visible all the time?

  2. jcsd
  3. Jun 18, 2005 #2
    Vapour trails are caused by jet planes expelling hot water vapour into the cold air due to the combustion they use. If the air is cold enough, ice crytals form. These crystals are the 'vapour trails'.

    Thats why you see them in a lot of jets above 30,000 feet (its ultra cold up there), however not always. For example, over the Sahara you would not see them because it would be too hot for the crystals to develop.

  4. Jun 18, 2005 #3
    However, even where I live (Hamburg) I see planes both with and without these vapour trails...

  5. Jun 18, 2005 #4
    Clouds caused by jet engines are an example of contrails (con)densed water vapor (trail)ing from the exhaust of a turbine engine. There are many variables (temperature, relative humidity, atmospheric pressure, engine load etc...) that make contrails form. Hamburg, Germany must be somewhat similar to the weather here in Maine, USA. We mostly see contrails but in the warmer months there may be planes without them.
  6. Jun 18, 2005 #5


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    Temujin. Those factors listed by pete depend drastically on altitude. A higher flying plane is more likely to produce contrails than one flying at less altitude.

    This issue was discussed here recently (last 2 weeks or so), have a search for it!
  7. Jun 19, 2005 #6


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    How about your holidays?

    I don't think he will be able to find it, specially if you are referring to my last thread about "Water <Weaks> behind aircrafts". :rofl:
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