Why don't we always see two silver streaks in the sky when a jet plane is flying past?

Question

Emissions from flying jet planes are seen sometimes only. What conditions make emissions visible?

Answer 1

It not emissions you are seeing. It's condensation that you are seeing. As the air is compressed in the jet engine it heat flashes the water vapor in the air. Then when it is expelled from the engine and combines with the cooler air, you see the vapor. when you only see 1 trail, it is likely the plane you are looking at has the engines on the tail section of the plane and not under the wings.

Answer 2

When aircraft fly above the mintra level, (a height at which freezing point temperatures are found) the water content from the exhaust gases freezes and forms ice crystals. These are visible from the ground and are known as contrails.Aircraft which are below the mintra level leave no contrails since the water content does not freeze and dissipates away. We, therefore, conclude that you will see the vapour trails only when the aircraft is flying above the freezing levels.

Contrails relate to the number of engines and sometimes can merge together in the jet wash and appear as a single contrail.

Answer 3

The streaks that you see are called contrails in aviation and mixing clouds in meteorology.

Mixing clouds form when two sub-saturated gasses (i.e. gasses with relative humidity < 100 %) mix to form a saturated gas. When a gas is saturated, condensation occurs, and a cloud forms. While it may be counter-intuitive, the reason why two sub-saturated gasses can sometimes mix to form a saturated gas, is because mixing is a linear process and the temperature/humidity relationship that defines saturation is curved. (to understand this more clearly, see the graph at http://cimss.ssec.wisc.edu/wxwise/class/contrail.html )

In the contrail example, the two gasses are the jet exhaust (hot and humid) and the ambient air (cold and dry). If the mix of exhaust gas and ambient air is saturated, a contrail forms. If the mix is still sub-saturated (when the ambient air is too dry or too warm), contrails do not form.

The white cloud that you often see coming from power plant stacks (it usually isn't smoke), and the cloud that you see from your breath on a cold day are also mixing clouds - the exact same phenomena as jet contrails.

By the way, I think AussieBloke does identify a valid way that clouds can form behind wings, but it is not the way most contrails form. In this case, pressures and temperatures inside the vortices coming off the wings are low enough to cause condensation, and hence, a cloud. But this would occur most often with high performance aircraft, whose wings produce the strongest vortices. Also, such clouds would not extend very far behind the wing, because the vortices wouldn't maintain sufficiently low pressures and temperatures at any great distance behind the wing. I think you can sometimes see such clouds in close-up video of fighter planes making abrupt maneuvers, and I think I've also seen it a few times when landing or taking off in an airliner in very humid conditions.

But, nonetheless, nearly all of the contrails that you see from the ground are of the mixing cloud type.

Answer 4

As said, these are not emissions, but no one is clear as to how the contrails are formed. There are 2 kinds of contrails. Exhaust and aerodynamic. The only thing necessary is an almost saturated layer of air.
When an aircraft passes through a layer of air that is almost saturated with moisture, the air is heated by the exhaust and becomes saturated with moisture. The moisture freezes into ice crystals and becomes visible.
Aerodynamic contrails are formed in almost saturated air by aerodynamic pressure reduction by airfoils, engine nacelles and propellers, which cool the air and the moisture becomes visible.
If you watch enough WWII movies, you will see it often forming behind the gull-winged F4-U Corsair.

Answer 5

The "silver streaks" are contrails or condensed vapor emmissions. These are caused by warm air from the engine meeting freezing air and condensing into ice crystals. The reason you don't always see them is the plane is flying under the 25,000 feet ceiling necessary to cause this effect. At 25,000 feet the air is 40 below 0- If you put a bare hand on metal the skin comes off.


Dr. Tommy Skelton

Answer 6

I'm not sure where Aussie Bloke gets his info but its way off. You get wingtip vortices's off the tip of the wing but the differential pressure isn't enough to see a "cloud". sorry. the reason you usually only see two trails is because most commercial jets have two engines out on the wings. this wide spread is far enough apart to get two contrails. I've seen b-747s go over and you see 4 contrails.

Answer 7

What you've seen is not emissions. These are contrails - vortices of air coming off the wing tips that squeeze the moisture from the air, causing a thin "cloud" to form where it passes. This has nothing to do with the engines at all - otherwise, you'd sometimes see 1 trail, and sometimes 4. However, there are only ever 2 (occasionally, there will appear to be only one trail. Give it a moment to spread and dissipate a little, then look again - it will show as 2 again.)

Answer 8

Simply put:

The exhaust expelled from the engines DOES contain water vapour. At high, cruising altitudes, the air is so cold that the water vapour freezes, forming ice crystals. Eventually they disperse.

Cirrus clouds (the ones that look like whispy fluff or milk and are high-level clouds) work by the same process. They are actually ice crystals in suspension.

Answer 9

Cold air at altitude.