[jimbo]

jimbo  asked the Naked Scientists:
   
I understand how planes make vapour trails but i am just wondering why / how they can start/stop them at certain places in the sky. If you see one there is a good chance you will see other planes make them in the same area in the sky.
 
It is not like there just this special place in the sky it is like they are marking their path for some reason. Can you please get back with why this happens and why it is done at special places? 

I will be waiting to hear what you have to say,

thanks

jim. 

What do you think?

[LeeE]

Well, you can get some places that might be more conductive of contrail production but I wouldn't describe them as 'special'.  These would be where large ground features have resulted in standing high-altitude (contrails generally only form above 8000 metres) 'waves' where the temperature/wind/water-content will be different to the air either side of it: as the aircraft flies into or out of these regions it may start or stop producing contrails.

Mostly though, it's just that the conditions vary from place to place according to the weather in the upper atmosphere.  If the upper atmosphere weather is 'patchy' then, just as with the wave situation, you may see an aircraft start or stop making contrails as it passes through different regions.

These areas of different weather conditions can also be localised in height as well as area, so if you've got good eyesight, you might notice two aircraft, apparently flying close beside each other, but with only one of them producing a contrail.  The reason in this case being due to one aircraft flying higher than the other so that only one of the aircraft flies through the patch of different weather.