Logan22L From , joined Dec 1969, posts, RR: Posted (5 years 3 months 6 days 1 hour ago) and read 510 times:
I've looked through some old posts on contrails, but couldn't really find the answers I am looking for. The other day I saw a contrail that extended from horizon to horizon (of course, near Boston, that isn't as far as some places), whereas today I just saw one that quickly dissipated about 1,000-2,000 feet or so behind the aircraft. The weather here on earth was pretty similar both days, so I'm wondering what factors in the stratosphere (e.g., humidity) are influencing the longevity of the contrails. Both contrails were well defined, so it didn't appear that turbulence was a factor, and both aircraft were clearly at cruising altitude. Thanks for any help,
WomBat151 From Netherlands, joined Aug 2004, 36 posts, RR: 0 Reply 1, posted (5 years 3 months 6 days ago) and read 453 times:
The contrails are comparible to the window drops at your home. When it's cold outside, and you breathe agains the window, little drops will appear on the window.
An airplane's exhaust gasses are also hot, and this way humidity concentrates in the air. It's a sort of artificial clowd. Clowds dissolve faster when there is less moisture in the air. When there's a lot of moisture, the clowd stays longer because there's no room for the moisture to dissolve in the air.
The short trail is because of the air up there was much more dry as in the other case.
Logan22L From , joined Dec 1969, posts, RR: Reply 3, posted (5 years 3 months 5 days 23 hours ago) and read 434 times:
I guess it's surprising to me that the humidity in the stratosphere could change so significantly, since it starts more than six miles up. Any atmospheric experts know what range of humidity (or relative humidity, as appropriate) is present at this altitude, and what change is needed to drastically affect the length of a contrail as I described in my initial post? Thanks,
Klaus From Germany, joined Jul 2001, 17761 posts, RR: 62 Reply 4, posted (5 years 3 months 5 days 23 hours ago) and read 429 times:
Contrails are formed largely because of additional water vapour introduced into the air by the combustion of hydrocarbon fuel. The hydrogen contained in the fuel is burned with oxygen from the ambient air to ordinary water vapour.
This pushes the relative humidity of the air upwards. And if ambient humidity had been high enough to begin with, water will start to condense (and possibly freeze) and thus form contrails. If the ambient air is moist enough, the contrails will not evaporate/sublimate again for a while.
Humidity is usually measured in percent of the vapour capacity of the air. While the total capacity will decrease with temperature/pressure, air at altitude can still have up to 100% relative humidity.
Goboeing From United States, joined Jun 2000, 2191 posts, RR: 18 Reply 5, posted (5 years 3 months 5 days 23 hours ago) and read 426 times:
Logan22L,
I saw the contrail you might be talking about. Was it Tuesday morning at 7 or 8 AM? It was the longest contrail I have ever seen...it just kept going in either direction!
747 contrails are my favorite; they leave behind curly strands of cloud that gradually get wrapped up and go away. Neat looking.
Logan22L From , joined Dec 1969, posts, RR: Reply 7, posted (5 years 3 months 5 days 22 hours ago) and read 417 times:
Klaus: Thanks for the reply; as a chemist and environmental scientist I understand the chemistry part, but I am surprised that the humidity fluctuations in the stratosphere would be enough to so drastically affect contrail length.
Goboeing: I live in Boston, so it doesn't seem like the contrail you saw was the same as the one I saw, but I believe that it was on Tuesday, only later in the morning.
Klaus From Germany, joined Jul 2001, 17761 posts, RR: 62 Reply 8, posted (5 years 3 months 5 days 22 hours ago) and read 418 times:
Logan22L: Klaus: Thanks for the reply; as a chemist and environmental scientist I understand the chemistry part
...probably a lot better than I do, actually. Chemistry was never really my strongest suit. I just try to get a hang of the fundamentals as far as possible.
Logan22L: but I am surprised that the humidity fluctuations in the stratosphere would be enough to so drastically affect contrail length.
Well, you can have "natural" clouds at all the same altitudes, so the variations are certainly there already. Aircraft only skew them far enough to sometimes get a visible effect.
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