YVR74
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Jet Exhaust At Ground Temps Of -40C

Thu Dec 05, 2002 10:30 am

I am not a regular user of this forum, so if this is one of those regularly asked questions or previously discussed, please excuse me and point me in the right direction.

One morning I was returning to Vancouver, BC from Calgary, AB. When we arrived at the airport at YYC it was -37C. When I saw the airplanes arriving and departing from the gates, I had to wonder why there was no vapour produced by the jet exhaust at such cold temps. Then I realised that sometimes I've spotted aircraft passing high overhead with no contrails either.

How and why is this?

Lance
 
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RE: Jet Exhaust At Ground Temps Of -40C

Thu Dec 05, 2002 1:49 pm

Contrails usually only form at altitudes above 25,500 feet. IT has to do something with the Thin Air and Cold Temperatures up there.
Puhdiddle
 
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RE: Jet Exhaust At Ground Temps Of -40C

Thu Dec 05, 2002 2:11 pm

Depends on the moisture content of the air. On very humid days the contrails seem to last for longer and they are really thick. A contrail is simply the jet exhaust condensing into the atmosphere. I think it also depends on the dew point temp but not 100% sure.
 
JetCaptain
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RE: Jet Exhaust At Ground Temps Of -40C

Thu Dec 05, 2002 4:28 pm

Persistent contrails require a relative humidity with respect to ice (RHI) that exceeds 100%, and are also dependent to some extent on the efficiency of the engine. Check out this website for more information, and a graphic contrail forecast application that you can play around with.

http://www-pm.larc.nasa.gov/sass/contrail_forecast/contrail_prediction.html

The coldest static temperature that I have ever personally experienced on the ground was -46C. I witnessed a departing B737-200 leave a big thick white contrail, similar to what you see at cruise altitude, as it rolled down the runway on its takeoff roll. This is the only time I've ever seen a jet contrail on the ground. I don't remember what the dewpoint was, but the visibility was reduced to about 4 miles in ice crystals at the time. The contrail extended from the point the takeoff roll commenced until rotation, and continued during climb, and lingered far a few minutes. I wish I had a camera, it would have made an impressive picture.

JC
 
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RE: Jet Exhaust At Ground Temps Of -40C

Fri Dec 06, 2002 3:30 pm

You are asking why you don't see vapor out of a jet engine at cold temps like you do when you see the car exhaust running right???

The answer is water or lack thereof. That "smoke" is water vapor in the exhaust condensing in the air.

Jet Fuel is run through a series of collescers(spelling?). If memory serves three of them. Each collescer(spl?) is a special type of filter that removes water from the fuel. So by definition jet fuel is very very dry compared to auto fuel or diesel fuel, which doesn't have the water content and isn't run through though those collescers(spl?) before sale.

Any water that is in the fuel goes in the engine and comes out the tail pipe.

Automobiles (gas ones) also have catalytic converters to improve their emissions. The byproduct of this process is oddly enough water also. If you are ever behind a car at a stop light, sometimes when the light turns green you will see a little trail of water coming out of the tailpipe. This is water that has collected in the tailpipe from the exhaust both water that was in the fuel and water that was "made" by the catalytic converter and is only being blown out as the pressure in the pipe increases.

Jet airplanes obviously lack catalytic converters, their fuel is much drier, so their exhaust also contains far less moisture then a comparable car engine, so it is that much less likely that a contrail will be made from that exhaust.
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YVR74
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RE: Jet Exhaust At Ground Temps Of -40C

Fri Dec 06, 2002 4:12 pm

Thanks for your posts guys. It makes a bit more sense to me now.

Lance
 
airtangora
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RE: Jet Exhaust At Ground Temps Of -40C

Fri Dec 06, 2002 6:08 pm

Ahhh haaa, the answers to this very question has answered a question that came to me just today driving from Seattle to Portland. As I was nearing Portland I saw two separate sets of contrails disappearing. At the time, not sure why, but now I think I do...they were descending below the FL to create the contrails as they were getting ready to land at SEA I presume. Would I be correct?
A bad day on the road is better than a good day in the office.
 
Klaus
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RE: Jet Exhaust At Ground Temps Of -40C

Sun Dec 08, 2002 10:51 am

Actually, the water vapour in car exhaust is mostly generated in the combustion process as far as I know. Even if there´s absolutely no water contained in the fuel or in the ingested air, vapour will be formed from oxygen in the air and hydrogen that was chemically bound in the fuel (which contains numerous hydrocarbon compounds).

A significant part of an oil combustion engine´s power is actually generated by burning the separated hydrogen to water, while the carbon will burn to carbon oxides (mostly carbon dioxide, hopefully).

I don´t know too much about the exact mechanisms in forming jet contrails, but my guess is that it´s a combination of a relatively high "base humidity" in the ambient air in conjunction with the additionally generated vapour from the combustion process which pushes total humidity beyond the dew point (it would have had to be close enough to it).

Since the turbine exhaust is hot initially, the actual condensation only starts when the exhaust has cooled off sufficiently.


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BR715-A1-30
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RE: Jet Exhaust At Ground Temps Of -40C

Mon Dec 09, 2002 9:04 am

collescers

You must mean COALESCERS
Puhdiddle
 
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RE: Jet Exhaust At Ground Temps Of -40C

Mon Dec 09, 2002 2:50 pm

Thank you BR.

That word isn't in the spell check Big grin
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Yikes!
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RE: Jet Exhaust At Ground Temps Of -40C

Fri Dec 20, 2002 8:54 pm

Moisture in the air. No question. MY personal best was Peace River, Alberta in 1981: -63 degrees Celsius. People left contrails walking along the street.

Best Regards,
 
Lubicon
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RE: Jet Exhaust At Ground Temps Of -40C

Sat Dec 21, 2002 7:37 am

Ahhh, Peace River, my home town. Last time I was home for Christmas it didn't get warmer than -40° C (and that was the high temp!). Not as bad as S.E. Saskatchewan though, -55° C with a windchill of -86. Nasty. Sorry to hi-jack the topic.
 
airplay
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RE: Jet Exhaust At Ground Temps Of -40C

Sun Dec 22, 2002 7:36 am

Why do people continually make up these tales of Canadian Winters?

The record all time low for Peace River is -49.4 C in 1950 and I couldn't find anything lower than -48 C for SE Saskatchewan. STOP EXAGERATING!

 
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wjv04
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RE: Jet Exhaust At Ground Temps Of -40C

Wed Jan 01, 2003 9:23 am

NO kidding Airplay -38*c in yyc, YA RIGHT what day was this? cause if was december youd probally know that calgary didnt get any snow untill the 29th of december, and we had extremly high temperture for december, some times as hot as 15*c for the entire month.

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