Jet condensation trails

[Khandro]

If condensation trails from jet airliners are the product of water vapour entering a cold environment, why yesterday, planes travelling south towards Switzerland against a clear blue sky did not produce any. Could it be that the air temperature was high due to a Foehn wind?

[webbo3]

I asked this a while back and Chris answeres.

http://www.theanswerb...l/Question440269.html



Dave.

[Khandro]

I looked that answer but I don't think it applies as it is a main corridor south towards Zürich and Basel and beyond, and I think virtually all aircraft will be jet propelled and flying at a similar height.

[webbo3]

Ill ask my friend who is a pilot and see what he comes up with.



Dave.

[jomifl]

What isn't apparent to the eye is that the atmosphere isn't just air all the way up. It is composed of layers of air from different origins and of varying humidity and thickness as can be seen in the following photo.
http://cloudappreciat...009/08/09aug-high.jpg
Whilst driving near the Pyrenees earlier this year I saw a stack of 9 layers.
Jet airliners do not all fly at the same altitude so as to avoid collisions and for routing purposes, as they fly in different layers they have vapour trails of different lengths or none at all.

[jomifl]

and a pretty one
http://wattsupwiththa...ta_lenticular_red.jpg

[Khandro]

jomifl; Beautiful cloud formations, and I have saved the photos; Buddhist stupas in the sky? But I was talking about an absolutely clear sky and over a several hour period, something I have not seen here before.

[gen2]

If the air is very dry, you wont get a vapour trail because the added moisture remains as an invisible vapour/gas. You will therefore see nothing.

If the air is already loaded with moisture, then the extra cannot remain as vapour and condenses out as a visible trail.

If the air is super-saturated, then the additional vapour from the exhaust can trigger cloud formation and the trail will spread into a larger formation.

If the air is partially saturated, the exhaust may initially form a visible trail, but as it mixes with surrounding dryer air it evaporates so the trail only forms for a short distance behind the plane.

[webbo3]

here is what my pilot friend sent back to me.

Hi Dave

Just to clear something up, it will never be to do with a Foehn wind. A Foehn wind is invariably a low level wind that travels down the Lea side of mountains. Moist air starts at sea level, travels along, hits a mountain, rises, and in doing so it will cool at the std lapse rate of 2deg per 1,000ft. Whilst cooling the air shrinks, this makes it more saturated, it keeps cooling until it reaches its Dew point (the temp at which it becomes 100% saturated), it then releases that moisture in the form of rain, then the wind continues to blow over the mountain and as it descends the other side it cools at the dry adiabatic lapse rate because its dumped all its moisture. This wind descends, warms and accelerates as it reaches the bottom of the mountain and can blow very hard indeed.

As for the vapour trail, there's a couple of variables, how much moisture is in the exhaust gas, and how cold it is. Again the temperate lapse rate in the std atmosphere is actually 1.98 deg per thousand feet. You will only see vapour trails if a) there's enough moisture content to saturate the air, and b) if its cold enough for that parcel of air to reach its Dew point.
Let's take 2 aircraft as an example in the same bit of sky, exhausting the same moisture content. For a given parcel of air to be saturated let's say the temp has to be -40. A plane flying at 36000ft might be flying in temps as low as - 40, whereas the other one flying at 34000ft might be in temps of -38, so 2 deg warmer. This 2 deg temp split is enough for the air to not be at its Dew point temperature. So the higher plane shows vapour trails, and the lower plane does not.

Two things from this..1) the Foehn wind has nothing to do with it, and 2) there is no way of telling just by looking up how high one aircraft is against another.

Hope this helps mate.



Dave.

[Khandro]

gen2; That sounds like a definitive answer, thanks. I think my notion about it being a result of a Föhn wind from the Alps might be right then? http://en.wikipedia.org/wiki/Foehn_wind

[Khandro]

Ah! well I posted before receiving the above.

[Khandro]

Thanks Dave and also your pilot friend.

[webbo3]

No problem.


Dave.

[jomifl]

Khandro, I think you missed the point of the photos, they were to illustrate that the atmosphere is composed of many layers as I said earlier. If the air in the photos hadn't been forced upwards by a mountain thus causing adiabatic cooling which iniated condensation the sky would indeed have been clear and planes at different heights would have had different lengths of vapour trails.

[jomifl]

Planes quite often do not produce vapour trails, they just aren't noticed because they are very difficult to spot even when one is looking for them.

[jomifl]

Absolutely not.

[jomifl]

Khandro, since from your previous posts it appears that you do not understand how the foehn wind is generated and what it's properties are I have thoughtfully added the link below to aid your comprehension as it would take to long to explain it personally.

http://en.wikipedia.org/wiki/Foehn_wind

[Khandro]

jomifl; Thanks, I did know what it is though, if you look above at my previous post you will see I cave the same reference, but what I didn't know was what is its maximum height. According to Dave's pilot friend they do not reach very high.

[jomifl]

So you did:-)