Thread: Jet Exhaust

Usually, there are jets around that seem to go really slowly and practically straight upwards and they have smoke that comes out of them. I always thought that the smoke was just water vapor, but why doesn't the smoke diffuse into the atmosphere? The smoke lines stay there for an hour or so after the jet leaves. Does it have to do with the pressure up there?

Originally Posted by Nima Rahnemoon

Usually, there are jets around that seem to go really slowly and practically straight upwards and they have smoke that comes out of them. I always thought that the smoke was just water vapor, but why doesn't the smoke diffuse into the atmosphere? The smoke lines stay there for an hour or so after the jet leaves. Does it have to do with the pressure up there?

Well, I guess you're talking about "condensation trails" or contrails, that white trails that jetplanes leave in the sky.

They are formed as the hot air from the exhaust holds water vapor, and this vapor condenses in tiny ice drops upon contact with the cold air surronding it. Anyway, the reason why the contrail is so coherent and looks like a line it's because it actually is a vortex, a sort of horizontal twister, and while it rolls over itself then ice drops can't go away. Once the vortex fades, the ice drops will remain roughly in the same place until they're scattered or evaporate.

Wow, that's really interesting. I figured that at that high altitude there isn't much pressure, so the water vapor should just become water and diffuse, but your vortex explanation makes sense too. Thanks!

So what keeps clouds together? I don't think clouds are vortexes... I'd figure anything at such an altitude would have such low pressure that it would diffuse.

Originally Posted by Nima Rahnemoon

So what keeps clouds together? I don't think clouds are vortexes... I'd figure anything at such an altitude would have such low pressure that it would diffuse.

Actually there is a similarity betwen clouds and contrails, once the vortex vanishes (a contrail vortex will last only for seconds, btw. Its role is that the exhaust gasses are built into a "column" rather than a "cone").

A cloud is not an "object", but an "area" where water condensation happens. It usually is a "pocket" of slightly colder or more humid air; it also may be caused by a column of warm air, as warm air can hold less water than cold air; thus the warm air goes up and the colder air above it becomes saturated with mist, which in turn condenses on little particles tansported by the air -this are the all-well-known thermal "fluffy cotton" clouds or cumulus. Anyway, water vapor is continuosuly entering and exiting the "cloud", and the "condensation area" may move, shift, vanish or grow stronger as the balance is restored or unbalance is feedbacked.

Yet a cloud it's just a place where condensation happens, not a coherent obejct, and thus what causes clouds are local variations of pressure, temperature and humidity. As logn as the unbalance exists, there will be a cloud. What jets do is to add ther own unbalanced area, of ultra-hot and humid air; sometimes the environment is so dry that the contrail disappears instnatneously, sometimes there is such wind that the contrail vanishes as soon as the vortex vansihes, an sometiems the air is so still that the contrail remains there for hours... or may slowly spread in a thinner and thinner layer until condensation is overcome by evaporation.

Actually clouds and contrails are larger versions of when your breath "smokes" on cold days -you add heat and humidity to the air and they condensate if the outer air is already humid.

Originally Posted by Nima Rahnemoon

So what keeps clouds together? I don't think clouds are vortexes... I'd figure anything at such an altitude would have such low pressure that it would diffuse.

Most water clouds do not quickly disperse like dust or smoke does. A cloud is kept together mainly due to the tendency of one water molecule to stick to another water molecule due to van der Waals forces. This is why a cloud has a temporary shape and structure. Van der Waals forces between water molecules are also responsible for cloud formation. Dipole-dipole intermolecular bonds between water molecules result in long curved strands of water molecules in the atmosphere. When clouds get dark, the strands get bunched together tightly like spaghetti in a bowl. Eventually, the strands become wound so closely together that the water vapour condenses and it rains.

'warm air can hold less water than cold air'

doesn't it depend on the temperature? Really cold air cannot hold water as it freezes and drops to the ground - which is why exhaling in the arctic dehydrates you.

'Once the vortex fades, the ice drops will remain roughly in the same place until they're scattered or evaporate.'

also - won't the centrifugal forces of the vortex cause the droplets to scatter?