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General Science => General Science => Topic started by: neilep on 31/08/2007 19:13:37

Title: What causes aeroplane Vapour Trails?
Post by: neilep on 31/08/2007 19:13:37
I've never seen so many vapour trails in my little corner of sky before (this is all within 15 minutes or so)

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See this last one ?...you can barely see it but why is there a break in this vapour trail ?

How do airomobiles make vapour trails ?...do they become bona fide clouds ?
Do you think it was the weather condition that made them ?

Title: Re: What causes aeroplane Vapour Trails?
Post by: daveshorts on 31/08/2007 23:11:27
The vapour trails which you can see from the ground are mostly because plane engines burn hydrocarbons for fuel, when the hydrogen in these burns it makes water vapour, as this cools down it will eventually condense into water droplets again, unless the air is so warm and dry that the added moisture can be absorbed directly by the air. the reason they take a while to form behind the plane is that it takes the gasses a while to cool down enough for ht water to condene.

The one with a gap in it is probably because in the gap there is some warmer drier air, so either the trail never formed or possibly more likely it has evaporated more quickly than in other areas.

The  other possibility is that the trail is in the dhadow of a real cloud. It only shows up well because it is reflecting lots of sunlight, if it is in shadow it would be very hard to see.
Title: Re: What causes aeroplane Vapour Trails?
Post by: ukmicky on 01/09/2007 15:10:09

I thought vapour trails were created by the engine however whilst i was flying to the Caribbean i noticed one being created from the 757 i was flying in and it looked like it was being created by the metal work surrounding the front of the engine due to it cutting through the air, it was cool as it was flowing in a tight stream over the wing like a scene from a wind tunnel.
Title: Re: What causes aeroplane Vapour Trails?
Post by: paul.fr on 01/09/2007 15:23:58
Another way to look at this is the exhaust pipe from your car during the winter, you see a visible plume of water vapor as the exhaust from the car cools.  You will also see this happening in the exhaust from a power plant chimney. The principle is the same as what you see with the airplane contrails (vapor trails)

Just to add to what Dave sad: The temperature of the air is more important than the altitude or the speed of the plane. The fuel contains water, which is vaporized by the heat of the engine.  This is expelled into the air behind the plane and mixes with the colder air.  As the jet contrail cools to the dew point the water vapor turns into water droplets forming the contrail.
Title: Re: What causes aeroplane Vapour Trails?
Post by: Monox D. I-Fly on 31/10/2020 02:35:50
Wait, that's vapor? Not smoke? That means it's cold instead of hot?
Title: Re: What causes aeroplane Vapour Trails?
Post by: evan_au on 31/10/2020 06:13:51
Quote from: Monox D. I-Fly
That means it's cold instead of hot?
Yes, at the cruising altitude of a jet plane (around 10km or 30,000 feet up), the air is very cold: often -40C to -60C.
- If the humidity is such that water vapor in the jet exhaust forms visible droplets, they tend to freeze into ice crystals.

Some long-distance flights have an information channel on the in-flight entertainment system that provides a map, and statistics on the flight like altitude and ground speed. This often includes the outside air temperature.

The following Wikipedia article comments that in the troposphere, the temperature of the atmosphere declines by around 6.5C per kilometre altitude. The temperature is fairly constant at this low temperature throughout the stratosphere (the layer above the troposphere).
See: https://en.wikipedia.org/wiki/Atmospheric_temperature#Temperature_versus_altitude
Title: Re: What causes aeroplane Vapour Trails?
Post by: Colin2B on 31/10/2020 08:53:53
Wait, that's vapor? Not smoke? That means it's cold instead of hot?
Yes, thats why they are called condensation (con) trails.
Asdescribed by @evan_au
Title: Re: What causes aeroplane Vapour Trails?
Post by: alancalverd on 31/10/2020 10:34:42
You can also get vapor trails from propellor tips and wingtips (even from gliders) where adiabatic rarefaction reduces the temperature below dewpoint or turbulence nucleates the crystallisation of supercooled vapor. Problem at low speeds (propellor aircraft and gliders) is that ice sometimes nucleates on the airframe quicker than it is blown away.
Title: Re: What causes aeroplane Vapour Trails?
Post by: Monox D. I-Fly on 03/11/2020 04:13:57
Quote from: Monox D. I-Fly
That means it's cold instead of hot?
Yes, at the cruising altitude of a jet plane (around 10km or 30,000 feet up), the air is very cold: often -40C to -60C.
- If the humidity is such that water vapor in the jet exhaust forms visible droplets, they tend to freeze into ice crystals.
That's cool! Can people make snow with that?
Title: Re: What causes aeroplane Vapour Trails?
Post by: evan_au on 03/11/2020 09:22:13
Quote from: Monox D. I-Fly
That's cool! Can people make snow with that?
The easiest way to make snow is on the ground, out of water pumped from a lake. All the water gets turned into snow, and it lands where you want it, on the ski slope.

Aviation fuel is far more expensive than water. Lifting it into the air and then burning it is not a cost-effective way to make snow - it makes more carbon dioxide than water vapor. Any snow will probably land kilometres from where you burn it (if it makes it to the ground at all).
Title: Re: What causes aeroplane Vapour Trails?
Post by: Monox D. I-Fly on 04/11/2020 01:49:29
Quote from: Monox D. I-Fly
That's cool! Can people make snow with that?
The easiest way to make snow is on the ground, out of water pumped from a lake. All the water gets turned into snow, and it lands where you want it, on the ski slope.
But I live near the equator. Can a lake nearby from my place be used to make snow?
Title: Re: What causes aeroplane Vapour Trails?
Post by: evan_au on 04/11/2020 08:32:55
Quote from: Monox D. I-Fly
But I live near the equator. Can a lake nearby from my place be used to make snow?
The Himalayas are near the equator, and you can make snow there.

But the general condition for a stable snow pack is temperatures less than 0C, over a period of weeks.

Near the equator, you need to be at high elevation to get temperatures that low (see the Wikipedia link above suggesting that temperatures reduce by about 6.5C for every kilometre in altitude)..
Title: Re: What causes aeroplane Vapour Trails?
Post by: alancalverd on 04/11/2020 09:29:03
There are a few indoor snow ski slopes in the UK. Far better to injure yourself near an NHS hospital than have to be helicoptered to a Swiss clinic and fill in lots of paperwork. As the outdoor ambient can be as high as 40 deg C,  you can get sunstroke, frostbite and a broken leg all in one day. It's even better than sailing!
Title: Re: What causes aeroplane Vapour Trails?
Post by: Petrochemicals on 05/11/2020 00:03:44
I've never seen so many vapour trails in my little corner of sky before (this is all within 15 minutes or so)

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fimg.photobucket.com%2Falbums%2Fv280%2Fwithdrawnmist%2FDSC03252.jpg&hash=4928846cd2dac0fdd89f59b7c3a74865)

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See this last one ?...you can barely see it but why is there a break in this vapour trail ?

How do airomobiles make vapour trails ?...do they become bona fide clouds ?
Do you think it was the weather condition that made them ?


I should say that it is the crystallisation on detritis  released in the engines providing nucleation points. This is a recent thread

https://www.thenakedscientists.com/forum/index.php?topic=79426.0
Title: Re: What causes aeroplane Vapour Trails?
Post by: evan_au on 05/11/2020 08:52:14
Quote from: neilep
How do airomobiles make vapour trails ?
If you are a carefully stealthed military aircraft, the last thing you want is a visible line extending across half the sky pointing to your exact location.

Part of airforce weather forecasting is to tell the pilot what altitude to fly so they don't generate contrails. Often an altitude change as little as 500m can eliminate contrails.

Quote
...do they become bona fide clouds ?
Studies have shown that in areas of Europe with many criss-crossing airline routes, sufficient contrails can build up to measurably reduce incoming solar radiation (it gets bounced back into space as visible light before it hits the ground, after which the energy gets trapped as infra-red radiation, via the greenhouse effect).
Title: Re: What causes aeroplane Vapour Trails?
Post by: alancalverd on 05/11/2020 09:46:40
You can occasionally see what looks like very thin altocumulus forming on a single vapor trail
Title: Re: What causes aeroplane Vapour Trails?
Post by: Monox D. I-Fly on 06/11/2020 01:53:52
Quote from: Monox D. I-Fly
But I live near the equator. Can a lake nearby from my place be used to make snow?
The Himalayas are near the equator, and you can make snow there.

But the general condition for a stable snow pack is temperatures less than 0C, over a period of weeks.

Near the equator, you need to be at high elevation to get temperatures that low (see the Wikipedia link above suggesting that temperatures reduce by about 6.5C for every kilometre in altitude)..

So it is still impossible. In fact, there's only one snowy mountain in my whole country.
Title: Re: What causes aeroplane Vapour Trails?
Post by: Petrochemicals on 06/11/2020 03:36:06
Quote from: Monox D. I-Fly
But I live near the equator. Can a lake nearby from my place be used to make snow?
The Himalayas are near the equator, and you can make snow there.

But the general condition for a stable snow pack is temperatures less than 0C, over a period of weeks.

Near the equator, you need to be at high elevation to get temperatures that low (see the Wikipedia link above suggesting that temperatures reduce by about 6.5C for every kilometre in altitude)..

So it is still impossible. In fact, there's only one snowy mountain in my whole country.
I bet it is, chill the water and air and abracadabra snow. It would probably melt pretty quick though
Title: Re: What causes aeroplane Vapour Trails?
Post by: Monox D. I-Fly on 07/11/2020 02:02:00
There are a few indoor snow ski slopes in the UK. Far better to injure yourself near an NHS hospital than have to be helicoptered to a Swiss clinic and fill in lots of paperwork. As the outdoor ambient can be as high as 40 deg C,  you can get sunstroke, frostbite and a broken leg all in one day. It's even better than sailing!
That's kinda... ironic. To be able to be damaged by heat and cold at the same day...
Title: Re: What causes aeroplane Vapour Trails?
Post by: alancalverd on 07/11/2020 11:11:02
There is no other point to downhill skiing. Gravity wins every time, until you break a leg.