Vapour....trails that is

[JaXanim]

For several weeks now, our regional newspaper has published letters to the editor on the subject of aircraft vapour trails and how they are formed.

Many readers have submitted their observations about these fascinating phenomena and have put forward their hypotheses as to the physics involved. Many people have said that the trails are created by the jet engines. However, several writers have dismissed that idea. Instead, they assert with apparent authority, that the trails are caused by the precipitation of water vapour in low pressure vortices at the wingtips of planes.

Anyone care to comment on the physics of vapour trails?

Cheers,

JaX

[JaXanim]

Let's have some physics. Water from combustion, high-g flight, etc don't tell us much about the mechanism of vapour trail formation. What conditions prevail when trails are formed? Some days they do, some days they don't. Why?

JaX

[JaXanim]

OK, now we're gettin' somewhere. But what's the physics? Surely, someone knows a bit about water vapour?

JaX

[JaXanim]

Watching films of high speed/high-g flight, it's clear that vapour precipitation from the upper side of the wing is quite transient. You can see a standing wave of fog at the lowest pressure zones. However, this re-evaporates towards the trailing edge, where the upper and lower wing pressures equilibrate. I can't see this being the mechanism for vapour trails. In any case, commercial flights do not create those conditions. It's usually seen in military aircraft. Concorde showed it when landing, but most civil flights do not.

Why do trails form on some days and not others? What's the physics?

JaX

[JaXanim]

Ah, the wet and dry bulb thermometer. That sounds more like the proper physics to me. Anyone care to elaborate on this? I think the truth is coming together here.

JaX

[JaXanim]

@blobrana

Yes, that's the transient precipitation I talked about earlier. The car didn't leave a trail right around the track did it.  If the right conditions were to prevail, it might happen, but that's highly unlikely.

So, what is it about flying that's different from the F1 scenario?

JaX

[JaXanim]

I think blobrana is getting nearer the hub (nub?) of it. I need to do some calculations myself. We need numbers like the volume of water vapour ejected at upper atmosphere conditions to get a better picture of trail generation.

Yes Chris, the Shuttle trails are the transient type caused by wing tip/lifting body precipitation. I'd suggest that you *could* see airliner trails on the same day lasting significantly longer than the Shuttle's, eh?

Another clue. Aircraft vapour trails don't usually form when the atmosphere is warmer than minus 50 Celcius (very approx).

Right, we need to use the gram molecular volume. Anyone got that to hand? I also need the air pressure at say 30-40,000 feet.

JaX

[JaXanim]

@fluffy...

That's right, contrails are caused by the engines. The wing precipitation is transient and appears within a standing wave in the low pressure zones. Once the pressure equilibrates (near the trailing edge) the fog re-evaporates.

There's a lot of useful stuff about this on the Net and we should be seeing a pretty definitive explanation of vapour trails quite soon.

BTW, the gram molecular volume is 22.4 litres, so a 747 produces quite prodigious volumes of water vapour at 35,000 feet. More later.

JaX

[JaXanim]

@QuikSanz

I've managed to find this on the Net HERE. Taking 35,000 feet as an average height for commercial flights, the air pressure is 179mmHg or 24 kPa. This compares with 760mmHg/101kPa at ground level.

In other words, the pressure is less than a quarter of that at ground level.

I'm gonna do some calcs using the Combined Gas Law to determine the volume of vapour expelled by a 747 at that altitude. I'll take blobrana's data as a starter. Maybe this will refine as more data comes in on aviation fuel.

Cheers,

JaX

[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

[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

[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

[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

[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