Cloud on Wing

[ChrisM]

Hi everyone,
I was looking at some photos of some 747's and concords and there seems to be a cloud or spray coming off the wings.  What is it???

Thanks...Chris

[SilverFox441]

Condensation in the airflow. Air pressure changes cause water vapour to condense back into droplets, which become visible. Some planes are very prone to the effect, others will rarely exhibit this phenomenon, F/A-18s will frequently show condensation trails from the wingtips, just inboard of the missile rails. A vortex forms in this location that amplifies the normal condensation effect.

[beaky]

By way of elaboration, I'll add to silver's explanation:
As a fluid (such as air) increases its velocity (as it does when going up and over a lifting surface), its temperature decreases airplane . Air holds more water in vapor form the warmer it is; if that moist air cools, the water is forced to condense into heavier droplets, often to the point where it's visible. This is why carbeurators sometimes collect ice, also: as the air slips into the narrow opening, it speeds up, the water condenses and clings to the rim, and if it gets cold enough in that zone, ice begins to form.

[Rifleman]

By way of elaboration, I'll add to silver's explanation:
As a fluid (such as air) increases its velocity (as it does when going up and over a lifting surface), its temperature decreases airplane ..

Ah, ...I get it now, the plane is really stationary and the air moves past the wing  ???.......well, that really explains why it takes so long to travel by air.........

[SilverFox441]

Airplanes don't actually move, the Earth moves under them. Planes are just a place where a predictable amount of Earth rotation can be experienced,

After all, we all know that the world revolves around pilots.

[beaky]

Ooh- a buncha wiseguys, huh?
Not to get into the "Bernoulli vs. Newton" thing, but air certainly goes a bit faster over the top of the wing than anywhere else on an airplane (in normal flight). That's why you don't see this effect on the whole airplane.

[ChrisM]

Thanks for clearing that up for me.  Two more questions.  What are the thin white streams that come from the wingtips or the edge of the flaps

example

Also how are contrails formed??

example

Thanks...Chris

[beaky]

The "thin streams" in that picture look like vortices that are visible because of the same thing as the "wing-clouds", but I've only seen that off wingtips. A vortex like that is formed because the air underneath the wing is "trying to get out from under" and a great deal of it travels out towards the tips as well as straight to the trailing edge. Once it reaches the end, it tends to curl up and inward. It's a lot like a breaking wave in the ocean- and in the case of a very heavy airplane moving slowly (such as during takeoff climb), they can be almost as powerful.  Needless to say, after breaking free of the bottom surface of the wing, that airflow is going to speed up for a bit as it releases its energy, which could produce that acceleration-induced cooling I mentioned earlier, as well as a lot of energy. They're usually invisible, but that factor (with high humidity) dust, or passing through smoke or clouds can render them visible.  There's a very well-known air-to-air picture of a bizjet skimming clouds and leaving lovely big vortices behind; unfortunately I don't have a link.
Contrails are due to water vapor and possibly other stuff in the exhaust, but the condensation in that case is due to the chilling effect of high-altitude air, as opposed to acceleration. Next time you see a jet laying down contrails, watch them for a while... sometimes they "turn into" clouds. They're the same thing: water vapor condensing (and sometimes freezing) due to a drop in temperature.

[beaky]

Ah, ...I get it now, the plane is really stationary and the air moves past the wing  ???.......well, that really explains why it takes so long to travel by air.........

Ah, I see my typo now; yes, I am in fact a jackass. Or at least, I can't type or cut&paste as well as a jackass...
  In honor of the good it often does me to be humiliated, I will not fix it...

[Saratoga]

It creates some very beautiful effects doesn't it?

After all, we all know that the world revolves around pilots.

Isn't that the truth? Too bad it isn't true.

[ChrisM]

Thanks Rottydaddy

[Jared]

lol, always a good laugh..:-)

[Saratoga]

lol, always a good laugh..:-)

Indeed he is.

[beaky]

Buncha nitpickers, that's what you are.  >:(. But laugh while you can-  I've just about run thru the list of possible errors involved here; probably won't repeat any of them. Pretty good learning curve for an analog-bred, three-finger-typin' caveman...

[Saratoga]

LOL! Typing is a tricky thing, isn't it?