What Are Contrails?
Since the dawn of the aviation age, a new type of cloud has been appearing in our skies. Cirrus aviaticus clouds, more commonly known as contrails, are now found crossing the skies over nearly every populated part of the world.
Contrails, short for condensation trails, are the clouds that form in the wake of a passing aircraft at high altitude. Sometimes these clouds dissipate quickly, and at other times linger for several minutes after the plane has passed. In some cases, contrails can spread, forming wispy blankets in the sky as they mix with other contrails.
Though some observers find these man-made cirrus clouds beautiful, others consider them an unwelcome pollutant spoiling our pristine skies. Climate scientists have taken an interest as well, hoping to better understand the environmental impact of these artificial clouds - and the hydrocarbon-burning airplanes that produced them.
Why Do Contrails Form?
Simply put, a contrail forms when the hot water vapor and exhaust gas from a jet engine combines with water vapor in the extremely cold environment of the upper troposphere. The water vapor solidifies into trillions of tiny ice crystals in a process known as deposition.
A passing jet engine creates an artificial cloud by mixing hot moist air from the exhaust with the sub-freezing humid air it is passing through. You can observe a very similar mixing cloud by exhaling on a cold winter day - warm water vapor from your breath combines with water vapor in the air and condenses into tiny water droplets to form a breath cloud.
Contrail formation is a more extreme version of this mixing cloud since the difference in temperature is far more extreme at airplane cruising altitudes. Generally, contrails form when temperatures are below −40°F (−40°C). Jet engine exhaust emerges at about 1560°F (850°C). As the super-hot air from the jet engine mixes with the super-cold air of the atmosphere, it rapidly cools, causing its own water vapor - and the water vapor already in the surrounding air - to condense into water droplets and then quickly freeze into tiny ice crystals.
This only happens under certain conditions, however. Contrails only form when the air at cruising altitude has just the right mix of air temperature, air pressure, and humidity. Since the atmosphere is not uniform, these can change over different areas and at different altitudes. This is why it is possible to see airplanes forming contrails as they pass through one region of the sky but not in another. It is also why planes travelling in the same direction passing over the same point can have differing degrees of contrail formation - atmospheric conditions can be very different at different altitudes.
Meteorologists began to study contrail formation during World War II when it became a matter of military importance. Since contrails were dangerous to high-altitude missions, giving away the locations and flight paths of Allied planes, the military was keen to understand why these clouds formed.
A National Weather Service meteorologist named Herbert Appleman created the Appleman Chart to forecast the conditions of temperature, pressure, and humidity that would be most likely to cause contrail formation. Well over a century later, we can still use this chart - in conjunction with atmospheric sounding data from weather balloons - to predict whether contrails will form over a given area at a given altitude.
Atmospheric conditions not only determine whether contrails form, but also how long they last and how they behave after formation.
Types of Contrails
High-altitude contrails can be generally categorized into three types. These types form depending on differing conditions of temperature and humidity at cruising altitude.
Short-lived contrails dissipate very quickly after formation, usually a few minutes. These form when humidity in the surrounding air is low and temperatures are warm - by upper-troposphere standards. The mix of exhaust and outside air just barely crosses the condensation curve, forming a contrail. As the mixture continues to cool, the ice crystals in the contrail pass the point of sublimation and begin changing phase back to gas, causing the contrail to dissipate.
Persistent contrails form when temperatures are much colder, allowing the ice crystals to remain in the upper troposphere for many minutes longer. As these contrails age, the ice crystals within them begin to sublimate back to gas, causing them to eventually disappear. However, these can remain for dozens of minutes to longer than an hour.
When persistent contrails form under high-humidity conditions, the ice crystals not only remain in the upper troposphere, but spread as the wind carries them, causing more ice crystals to form. These persistent spreading contrails can remain for many hours, mixing with other contrails to form an artificial cirrus aviaticus blanket over the area.
Are Airplanes Spraying Chemicals Into The Air?
It should come as no surprise that contrails have become the subject of an Internet-enabled conspiracy theory, since the Internet has provided the science-illiterate with a far-reaching platform for congregation. Proponents of the "chemtrail conspiracy" insist that persistent contrails are the result of chemicals being sprayed into the atmosphere by high-altitude top-secret aircraft. Exactly what is being sprayed is not known, of course, but proponents are certain that it is for nefarious purposes ranging from geoengineering to weather manipulation to mind control.
But is it true?
The simplest answer to this is "yes." The two main products of jet fuel combustion are carbon dioxide (about 70%) and water vapor (a bit under 30%). Other by-products such as carbon monoxide, sulfur oxides, nitrogen oxides, and soot are produced in far smaller amounts. All of these are chemicals, by definition. Therefore, airplanes are definitely spraying chemicals into the air via their exhaust.
Could there be airplanes on secret missions from secretive sources filing secretive flight plans and spraying additional secretive chemicals into the upper atmosphere? It's possible, but not likely. And there is currently no evidence to support such an assertion.
Geoengineering is by far the most plausible of the "chemtrail" conspiracists' ideas, and it is an idea that is still highly conceptual. Though there have been some proposed geoengineering schemes that would emit reflective nanoparticles into the stratosphere to reflect solar radiation and combat global warming, these are still hypothetical ideas and are not currently being tested.
Even if such geoengineering schemes were being conducted today, airline contrails would not be an effective method of distribution. In fact, they would be counter-productive. Persistent, spreading contrails have a net warming effect on the land area below them, reflecting heat energy back down to the ground. This is in addition to the carbon dioxide contributed to the atmosphere by the airplane's exhaust. Therefore, assertions that current airplane contrails are part of a geoengineering scheme are not based in fact.
Sources and Further Information
- Contrails - University of Wisconsin
The condensation trail left behind jet aircraft are called contrails. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature.
- EPA: Aircraft Contrails Fact Sheet
This fact sheet describes the formation, occurrence, and effects of “condensation trails” or “contrails.”
- Aviation & Emissions – A Primer
This paper provides a brief overview of important issues regarding aviation emissions.
- Photophoretic levitation of engineered aerosols for geoengineering
Aerosols could be injected into the upper atmosphere to engineer the climate by scattering incident sunlight so as to produce a cooling tendency that may mitigate the risks posed by the accumulation of greenhouse gases.
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