Except for those unforgivable mental lapses when we've left the Colombian coffee behind, most of the contrails that I've had a hand in making have been, I'll admit, monotonously straight. No daydreaming child ever conjured up a puppy dog or favorite cartoon character looking at one of mine. But that's flight-level flying for you, where jet airways and great-circle RNAV routings generally require few acute course changes. As an artistic form, contrail cloud shapes are a decided flop. But while they may seem as predictable as the next row of corn, there is a certain mystery about them. Why, for instance, do aircraft produce them some days, and not others? And how come some last literally for hours, while others disappear in mere seconds? There's more to contrails, it turns out, than first meets the eye.
Short for condensation trail, a contrail is a kind of mixing cloud, formed when two parcels of air with dissimilar temperatures and vapor pressures are rapidly mixed. In this case, typically very cold, dry, high-altitude ambient air is mixed with much warmer and moister air from a jet engine exhaust. The resulting state of saturation produces water droplets, which almost instantly freeze into ice crystals. Ice crystals can also form when water vapor molecules attach themselves to tiny particles, or nuclei, made up of various byproducts of combustion. The actual temperature necessary for ice-crystal contrail formation varies with relative humidity, but generally speaking, if the ambient air is minus 38 degrees Celsius or colder and enough moisture is present, contrail formation is likely. At warmer temperatures, ice crystals generally don't form, because there is insufficient water vapor in the mixed air parcels to attain a state of saturation. On the other hand, if the ambient air parcel itself already contains high relative humidity, contrail formation at warmer-than-normal temperatures is more likely.
How much water is found in a jet engine's exhaust? Quite a lot, as it turns out. According to Dr. Rowan Pilie, a scientist who performed early contrail research for the U. S. Navy and Air Force, each gram of jet fuel burned produces one and a quarter grams of water. Of course, jet fuel burns are usually measured in the many thousands of pounds, not grams, which explains why there is no shortage of water to make contrails in the first place. Sometimes this abundance of water can lead to the growth of especially large ice crystals. When they become heavy enough, the largest begin to fall away from the main body of the contrail. This creates the appropriately named fallstreaks, the wispy, feather-like plumes that are sometimes evident in mature contrails.
A contrail's life expectancy depends largely on prevailing winds and relative humidity. Presuming that conditions supported its creation in the first place, a contrail can last for many hours if not disturbed by strong or turbulent winds aloft. It is not uncommon to observe a long-lasting contrail stretched sideways like taffy in light wind conditions, transformed into a very thin, localized cirrus layer a mile or more in width. Choppy winds aloft tend to dissipate contrails more quickly, as does extremely low relative humidity. Sometimes contrails exist for only a few seconds or are produced in short, staccato-like bursts when an aircraft happens to be flying in and out of the exact temperature boundary where they will form. But no matter how long they last, they all disappear for the same reason — eventually they are diluted with the comparatively dry, surrounding air. Relative humidity within the contrail cloud steadily decreases, and the ice crystals disappear through the process of sublimation.
While they last, contrails offer pilots useful clues about traffic and flight conditions. For instance, they can make it easier to spot fast-moving or distant traffic. An otherwise invisible oncoming aircraft can sometimes be quickly discerned by the plume of its contrail at distances of 40 miles or more. A ragged, distorted contrail is a visible warning of possible clear air turbulence at that flight level, while well-formed, lingering contrails are a welcome indication of smooth flying conditions.
While we tend to associate contrails with high-altitude flight, altitude by itself is not strictly a factor in their formation. Ice-crystal contrails can appear in the jet exhaust of an aircraft on the ground if the temperature is cold enough and the relative humidity of the mixed air parcels is high enough. This has been observed in cold climate locations like Alaska, for example, where the resulting phenomenon is more usually referred to as ice fog.
Not all contrails are composed of ice crystals, either, as demonstrated by a low-altitude kind of water-droplet contrail most commonly observed when large aircraft land or take off in extremely humid conditions. In this type, already highly saturated ambient air encounters low-pressure regions above the wing and in the turbulent wingtip vortices. Water droplets form in these low-pressure regions, producing generally short-lived, but highly visible streaming contrails.
With many millions of high-altitude commercial and military flight hours flown each year around the world, scientists have long wondered what effects contrails are having on our global climate. Clouds of all kinds influence climate in various ways. For instance, they reflect some of the sun's energy into space, and also trap heat from the earth's surface that would otherwise be lost through radiational cooling. Presumably contrails play some role in these kinds of processes, but quantifying their effects has so far proven elusive. Among the questions being pondered is whether the various byproducts of jet engine combustion found in contrails are altering the basic chemistry of the upper atmosphere. If so, is the world climate gradually changing as a result? NASA and other interested scientific organizations haven't yet come up with definitive answers to such questions. Some isolated findings are intriguing, however. One study, for example, suggests that there has been a 20-percent increase in historical cloud cover along certain busy jet airways, with contrails suspected as the cause. Other scientists point out that contrails make up only a tiny percentage of the earth's total cloud cover, and they question the overall significance of contrails in the larger scheme of things.
It isn't only jet engines that make contrails, of course. During World War II, Boeing B-29s and B-17s and other high-altitude aircraft regularly produced them, which was not necessarily a good thing from the perspective of maintaining the element of surprise. The realization that contrails were an Achilles heel in aircraft stealth led to postwar efforts to minimize or eliminate contrail generation altogether.
Pilie was one of the researchers involved at the forefront of this undertaking. In 1953, he and fellow scientists proposed to the U.S. Air Force an ingenious method of eliminating contrails. They knew that the amount of light scattered by ice crystals is, to some degree, a function of their size. If the ice crystals in a contrail could be made small enough, very little visible light would be reflected, and the contrail would be difficult, or even impossible, to observe. They reasoned that there are only a fixed number of water molecules available in an engine's exhaust stream. They suggested that by seeding an exhaust with high concentrations of hydroscopic (water-attracting) nuclei, they could create a highly target-rich environment for the water molecules. Faced with an overabundance of nuclei with which to bond, the molecules would spread themselves thin, precluding the formation of the larger kind of ice crystals usually found in contrails. Whatever ice crystals did form would be much smaller than usual.
To test their theory, the scientists devised a system whereby the engine exhaust streams on one wing of an Air Force B-47 jet could be injected with a highly hydroscopic acid solution. The engines on the opposite wing were left unmodified for comparison purposes.
"Every time we threw the switch, the contrails on the modified side of the airplane disappeared," recounts Pilie. "On the other side, they were produced the same as always, of course." It turns out that the modified ice particles had been reduced to a size that reflected only the blue portion of the visible light spectrum. Against the blue sky, what little remained of the contrails was completely invisible from the ground.
Although they had achieved their goal, the results ultimately proved to be of only minor importance to the Air Force. "We were getting ready to instrument a B-52 to perform further studies, when we were told that advances in radar had more or less made visual detection of contrails a moot point." With that undeniable development, the project was terminated.
Perhaps some politically correct politician will decree that Pilie's work be resurrected. That way, every trace of high-flying aircraft could be erased from over our national parks. But I'm glad it didn't work out that way. Even if they make lousy cloud art, every contrail has a story to tell.
