Zartan From , joined Dec 1969, posts, RR: Posted (15 years 4 months 3 weeks 4 days 12 hours ago) and read 4486 times:
I have noticed several times when taking off or landing (and, oddly, only at those times, never in flight or above a few thousand feet), the appearance of a perfect "wind tunnel" appearing condensed vapor stream over and under the wing of the plane I've been on (mostly small to medium jets i.e. 737s). Kind of hard to explain but it's almost as if a smoke wand was being held just in front of the wing the way they do to show where the air is flowing when something is in a wind tunnel. It's a really neat effect - you can see the way the air curves up and over the wing, and then whirls in vortices (?) a few feet behind as it comes together behind the trailing edge. It's oddly perfect looking and well-formed; the vapor is uniformly maybe 9 to 12 inches "thick" and appears to be only over a very limited area of the wing, and it perfectly follows what I know from reading about flight is the airflow over and under the surfaces.
Any idea what causes this? My guess (and only a guess) is that the wing rapidly compressing the air in front of and around it causes air on the verge of being saturated with moisture to "lose" the moisture, condense and form water vapor, because the "smoke" looks exactly like cloud substance...
I know this explanation is a bit hard to follow, but it's hard to explain. Anyway, if anyone has any ideas or has seen similiar, I'd be curious, because it's an awesome effect and I would love to know what's going on.
BigGiraffe From United States of America, joined Jan 2000, 257 posts, RR: 0
Reply 1, posted (15 years 4 months 3 weeks 4 days 10 hours ago) and read 4397 times:
Actually, you pretty much have it. When the air is nearly saturated with moisture, sometimes the low pressure over the top of the wing causes the moisture to fall out and a "cloud" forms, which we see as a trail. It normally only shows when the flaps are down. Watch for it especially when you are about to climb into low clouds are have just descended through them. I've seen the trail on the 737, but the whole flap area and sometimes whole upper wing surface on the 727.
I've never seen the trail coming from under the wing - did I understand you to say one comes from under?
Zartan From , joined Dec 1969, posts, RR:
Reply 2, posted (15 years 4 months 3 weeks 4 days 5 hours ago) and read 4350 times:
I don't know that I've seen it coming from the bottom... I have seen it extending for 2 or 3 yards behind the wing and forming a series of progressively smaller vortices, though, which is pretty damn cool. Overall it's a very impressive effect. Also, I've never seen it near clouds, just in the several hundred/thousand feet prior to takeoff.
Anyway, good to hear some backup to my guess... I'm guessing that it is more likely in high relative humidity, eh?
JETPILOT From United States of America, joined May 1999, 3130 posts, RR: 27
Reply 4, posted (15 years 4 months 6 days 3 hours ago) and read 4292 times:
Actually Zartan you have the process backwards.
As air is compressed it is heated allowing it to hold more moisture in suspension.
As air is decompressed moving over the upper wing surface and losing pressure it's temperature is lowered below the dewpoint. The air becomes completely saturated causing it to condense into visible moisture.
Chris28_17 From United States of America, joined Jul 2000, 1439 posts, RR: 9
Reply 5, posted (15 years 4 months 6 days 3 hours ago) and read 4283 times:
jetpilot has it right, exactly as ive been taught. And yes this is an absolutely captivating effect, i love watching it.
As a side note, its not just on 737's or -27's, in such conditions the vapor will form from wings of just about any decent sized aircraft, even the little props, of course the bigger the wing, the more area there is for this effect to take place... and yes, its quite beautiful.
What is really impressive is when you see footage of a jet fighter transending the sound-barrier in humid conditions. I saw one once where as it broke the sound barrier an almost shell-like dome of vapour was evident in a 30 meter radius around the aircraft - absolutely awsome...