Vapour....trails that is

[JaXanim]

That's a nice picture. Note the fog precipitation in the low pressure standing wave atop the wings. This re-evaporates almost instantaneously and doesn't produce a significant trail. The tips create a short-lived trail.


@QuikSanz

Yes, that's got to be the ultimate contrail! From my reading so far, this vapour almost certainly does freeze into minute ice crystals.


@blobrana

Aviation kerosene is mainly a C10-C12 hydrocarbon mixture, but that doesn't have any significant effect on water volume. In fact I'm taking your estimates on this. The old story that a gallon of petrol creates a gallon of water isn't so far of, is it?

JaX

[FluffyMcDeath]

JaXanim wrote:

@blobrana

Aviation kerosene is mainly a C10-C12 hydrocarbon mixture, but that doesn't have any significant effect on water volume.

Think those were my numbers your refering to. As to the hydrogen content, I was just guestimating what the average H would roughly be for such a mix of hydrocarbons.

[JaXanim]

@Fluffy...

Yes, sorry. I'm using your figures for the water creation rate. Assuming kerosene is a saturated hydrocarbon fraction, its empirical formula will be C(n)H(2n+2). The carbon number(and therefore the hydrogen content) makes relatively little difference to the weight of water created on combustion. It's only important at low molecular weights. So a gallon of kerosene oxidises to create about a gallon of water (as liquid of course). Convert this to vapour at 35,000 feet and you'll have some idea of what vapour trails involve.

At this point, the numbers look astronomical. I'll put them up when I check them again. The vapour rate of a 747 (4 engines) is just mind boggling!

Cheers,  

JaX

[KennyR]

JaXanim wrote:
At this point, the numbers look astronomical. I'll put them up when I check them again. The vapour rate of a 747 (4 engines) is just mind boggling!

Yes, they are. Some scientists are also starting to believe that jets can cause changes to weather patterns because of what they do up there.

[FluffyMcDeath]

JaXanim wrote:
@Fluffy...

Assuming kerosene is a saturated hydrocarbon fraction, its empirical formula will be C(n)H(2n+2). The carbon number(and therefore the hydrogen content) makes relatively little difference to the weight of water created on combustion.

Cheers,  

JaX

When I was looking up jet fuel all I found was the weights of fractions expressed in terms of C. IN addition to the saturated hydrocarbons, jet fuel also has such stuff as napthalene (C10H8), so I jogged the H down a bit.

[JaXanim]

@KennyR

That's right. There is a lot of evidence that contrails affect weather. For example, when flights were grounded for three days following the 9/11 tragedy, US daytime air temperature was one degree celcius above average. For more data on this, do a Google on vapour trails and weather.

@Fluffy...

Yes, it can get a bit complex, but the volume of water generated is gonna be about right whatever the specific composition of the fuel. A gallon of water per gallon of fuel is applicable to almost any hydrocarbon fuel whether its simple hydrocarbons or mixtures with aromatics.

The numbers are collossal! I'll post the results sometime tomorrow. In the meanwhile, I'm interested in anyone's observations/comments on vapour/con trails.

Cheers,

jax

[sir_inferno]

stop being so patronising Jax:

That's right

 :lol:

The Lockhead SR-71 Blackbird and the Stealth Bomber both have pipes which inject cfc's into the exhaust to eliminate vapour trails

/me waves bye bye to o-zone layer over afganisthan and iraq :-)

speaking of which... B-52 Bomber

[JaXanim]

@sir_inferno

Presumably, making the Ozone Hole even bigger!

Sorry....SIR!...?

JaX

[JaXanim]

This is my take on the generation of vapour trails. It's rather long, so ignore it if it's not of interest to you. (Sorry also to Wayne, who has asked us to be brief.)

It's in two parts. The volume of vapour involved and the mechanism of trail formation.

Taking Fluffy..'s data on a 747 allows you to calculate the first part. I've assumed it's flying at 35,000 feet. The numbers vary a bit for other altitudes, but 35,000 seems about average for commercial jets.

A 747 produces 15 tonnes/hour of water in the exhaust gas of its four engines. The volume of water vapour this creates can be calculated using Avogadro's Law and the Combined Gas Law (Boyle's + Charles' Laws).

Avogadro says that the vapour created from one gram mole of any substance occupies 22.4 litres at N.T.P. (Normal Temperature and Pressure) which is 0C(273K) and 760mmHg.

A gram mole of water is 18 grams, so every hour the 747 creates:
                      15,000,000/18 g.moles
                    = 833,333 g.moles

The volume of this at NTP = 833,333 x 22.4 litres
                         = 18,670,000 litres

To convert this to 35,000 feet requires the Combined Gas Law:                P1.V1/T1 = P2.V2/T2

At 35,000 feet the atmospheric pressure is 179 mmHg and the temperature is -65.6F(-54C/219K). These data are available
HERE and HERE.

So:            760 x 18,670,000/273 = 179 x V/219

Therefore:     V = 63,590,000 litres/hour
                = 17,700 litres/second

Blimey, I hope somebody will confirm these figures!

The second part is what happens to this invisible vapour?

The atmosphere will support a certain water vapour content. Its capacity to do so is directly proportional to temperature and pressure. Hot air holds more than cold. High pressure more than low.

Under specific conditions, water vapour will condense into liquid droplets (fog). These conditions occur at the Dew Point of the air. If the Dew Point is below 0C, the fog may freeze into ice crystals, so it's called the Frost Point.

Ice formation usually requires a nucleator such as dust, soot, pollen, bacteria, etc. around which the crystals form. Without this, water drops can go down to -40C without freezing.

If the atmosphere is at or near the Dew Point, any excess water vapour added to it will condense into fog/ice crystals. This is when vapour trails are produced. Wing tip/vortex precipitation produces transient trails because no extra water is involved. This fog re-evaporates very quickly.

If the atmosphere is very dry, it will be well above its Dew Point and the extra vapour introduced remains in the gaseous phase. So no vapour trail is produced.

In 1998, NASA flew a jet in circles until its vapour trail created a Cirrus cloud covering 1,400 square miles! See HERE.

Does anybody fancy checking the numbers? All the required data are available where indicated.

Cheers,

JaX

[aardvark]

On a cold day, my car leaves a vapour trail, even when it's moving slowly. The vapour tends to drift upwards though.

This is something so obvious to people living in northern climates, that we barely even think about it. In the middle of winter, at temperatures around -20 to -35 C (or colder :evilgrin: ) morning rush hour traffic produces dense ice fogs that often don't disapate until 10:30/ 11:00 AM.

[sumner7]

Seeing that this has happened, there really must definitely be life on mars. :-)

[JaXanim]

@aardvark

A kind of ground level vapour trail from cars. All the principles involved with airplanes apply to cars as well. You just need the air to be at or near the Dew/Frost Point.

I guess car trails rise because they are slightly warmer, initially at least, than the surrounding air. I think aircraft trails probably rise somewhat, but 'cos the plane is moving so fast, the trail is always way behind it. You may notice that trails can be near or far away from the engines. It depends on how much the gas has to cool before it condenses.  

When it's as cold as you say, I bet the car fog persists at ground level for quite a while eh?

Cheeers,

JaX

[aardvark]

Until the sun burns it off. The car vapour trails do not last long but disperse adding to the general fog levels. You do have a kind of condensation giving a fairy like frosting on trees, ushes and powerlines. (even the prostitues get 'hoarfrost') :-D

[JaXanim]

@aardvark

An occupational hazard, eh?

JaX

[Edit: Hoare frost happens when the air is at/below the Frost Point. This makes the vapour condense and freeze into ice crystals. These stick to any solid surface, trees, bushes (and prostitutes) to form a crystal coating. Beautiful to see.]

Cheers,

jaX