Contrail Science

Contrails are generally classified into two types. Exhaust contrails and aerodynamic contrails.

Exhaust contrails are formed by the mixing of the hot humid exhaust of the engines with cold humid surrounding air, creating long streamers of clouds. If the conditions are right then these can persist and spread. These are the most common type of contrail observed.

A typical exhaust contrail. There are initially four, one for each engine, then they mix together. 

Aerodynamic contrails are  formed by the temporary reduction in pressure of the air moving over the surface of the plane, or in the center of a wake vortex. Reducing the pressure of the air means it can hold less water, so condensation occurs.

An aerodynamic contrail on a landing jet – condensation is visible above the wing surfaces, and in the center of the vortices coming from the outside ends of the deployed flaps, but nothing from the engines. This type of contrail is seen in high local humidity, as indicated here by the misty conditions.

I propose a useful new classification for a type of contrail, the Hybrid Contrail, defined as a portion of an exhaust contrail that has larger ice crystals due to wake vortices. A hybrid contrail is formed in a narrow range of atmospheric conditions, specifically with temperature below -40F, and a relative humidity with respect to ice slightly below 100%. When RHI is below 100% then a contrail that forms will not be persistent, and will eventually sublime away. The low pressure in the wake vortex core allows for a longer period of time in which the mixing air is above 100%, and hence the ice crystals in that portion of the contrail will grow larger and/or more numerous.

The entire evolution of a hybrid contrail can be see in this video. Notice the trail starts out as a large dense regular exhaust contrail, then this fades away leaving the hybrid contrail which separates away from dissipating exhaust contrail, breaking up into loops and segments.

Hybrid contrails will not form when RHI > 100%, as the entire contrail, including the vortex cores, is above the threshold for ice accretion, and so will accrete (gain ice) at the same rate. Hybrid contrails will  not form at values significantly below RHI of 100%, as the relative increase from the vortex core is small, and cannot push the ambient RHI over 100% after initial mixing. Hence hybrid contrails will only form in marginal conditions with RHI only slightly below 100%. A similar narrow range may also apply to temperature.

The resultant region of greater contrail densities will initially be indistinguishable from the exhaust contrail. However as the exhaust contrail sublimates (turns from ice back to water vapor) then the hybrid contrail will be revealed as two thin rope-like regions running along the contrail. The hybrid contrail will sometimes sink away from the exhaust contrail, due to the large size of the ice crystals. Usually the hybrid contrail will persist for a few minutes longer than the exhaust contrail. Since the hybrid contrail is much smaller in cross-section than the exhaust contrail, then the effects of turbulence and crow instability cause the hybrid contrail to twist into loops and curls that often resemble chromosomes.

A hybrid contrail below the parent exhaust contrail. The larger ice crystals in the hybrid contrail have caused it to fall quicker than the Exhaust Contrail, leading to considerable separation, even though they were originally part of the same trail.

The reason this new classification is needed is that people frequently mistake these hybrid contrails as being regular exhaust contrail, and they cannot understand why these particular contrails loop and twist in such a dramatic and asymmetric manner. In addition hybrid contrails are often spotted within regular exhaust contrails, and this is presented as evidence of something being sprayed within the cover of the contrail. Hybrid contrails also often look very unusual, and this is taken as evidence of some novel propulsion mechanism.

Hybrid contrails often end up looking like a string of loops or chromosome pairs. This looping and twists would apparent with the much larger exhaust contrail, as it would simply happen within it. The loops are actually wake vortices themselves twisting, and as the hybrid contrail exists in the center of the vortex, then the effect is much more pronounced.

While I’m suggesting a new classification, this is not in any way a new type of contrail. In fact it has been observed for many decades, such as in the 1972 book: Clouds of the World:

This 1972 book, Clouds of the World, discusses the formation of Hybrid contrails. But does not give them a particular name.

The full development can be seen here:

www.airliners.net/photo/Arik-Air-(Hi/Airbus-A340-542/1820435/L/&sid=ecfd72e0685325752848fb2b4cad1867

Development of a the hybrid portions of a contrail are shown from the initial four separate exhaust contrails, though to just the two hybrid contrails and crow instability breakup. The hybrid contrail is probably sinking below the exhaust contrail, but since it’s viewed in line, there’s no visible separation.

The trails that aircraft leave in the sky are called “contrails”, which is short for “condensation trails”. They are formed by the condensation of the water vapor in the aircraft exhaust.

When you breath out on a cold day, you see a little cloud of condensation form from your breath. This is the same kind of thing, your damp warm lungs add moisture to the air, and when you breath out, you get condensation.

But the condensation from your breath quickly evaporates, usually in less than a second. Condensation trails from a jet can last for many minutes, even for hours sometimes. So why is there this difference? Why do jet contrails sometime persist, but your breath condensation quickly evaporates?

The difference is because a contrail freezes.

It’s really that simple. Contrails form at -40 degrees Fahrenheit (which is also -40 Celsius), or colder. At that temperature the tiny drops of condensed water will instantly freeze. Once frozen they can not evaporate. They also can’t melt, as it’s -40. They can however fade away through a process known as “sublimation” – where a solid turns into a gas.

You’ve seen sublimation before. Dry Ice is frozen carbon dioxide. It does not melt, it just sublimes directly into the gas. If you take a bit of dry ice, and just leave it in the sun, it will just kind of fade away. That’s what happens to the ice in a contrail.

Ice will only sublime if the humidity (at that altitude) is lower than around 60% to 70%. So if it’s a bit higher then the contrail can last for a long time, just like clouds do sometimes. If the humidity is low, then the sublimation happens very fast, and the contrail only lasts a minute or so. If the humidity is high (above 70%) then you get reverse sublimation (also called desublimation, accretion or deposition, where water vapor turns directly to ice, but only when in contact with ice), and even more ice will form on the frozen condensation, causing the trail to spread out.

So, the difference can be summed up as:

Contrail = Condensation + Freezing + Sublimation
Breath = Condensation + Evaporation

This isn’t really a property of your breath though, it’s a property of temperature. It you breath out at -40 degrees or colder, then your breath will freeze, and it will not evaporate. Instead of a little cloud that quickly evaporates, your breath at -40 degrees will look like smoke. Like these guys in Siberia, at -52C, you can’t tell the difference between their breath, and cigarette smoke.

Note: when we say a contrail freezes we are generally talking about an exhaust contrail – from the engine. There’s another type of contrail that’s the aerodynamic contrail you sometimes see when a plane is landing, streaming from the wing tips or flaps. That’s actually liquid water condensation, like your breath, and that’s why it quickly vanishes.