Contrails: The Chemical Truth


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Contrail illustration

Contrails maybe seen during early morning hours. Image by National Weather Service.

If you awake in the morning, just prior to dawn, look eastward and you may see a jet airplane leaving a trail of condensed ice and vapor (a contrail) in its wake.

To some, the phenomenon is evidence of a vast government conspiracy -and it’s a ‘chemtrail’ not a contrail… However, the ice-vapor trail is a physicochemical phenomenon – a demonstration of physical chemistry.

The contrail is the formation of an artificial cirrus cloud.

Researchers have been studying contrails since WWII – especially when the Allied and Axis forces were sighted prior to their targeted arrival – but what have scientists found out about this very visible phenomenon?

Illustration of phases of water. Contrails form by going from gas to solid (skipping  the middle liquid phase).

This is an illustration of the phases of water. Clouds form by going from gas to solid (skipping the liquid phase). Copyright image by John Jaksich. All rights reserved.

What Are the Physical Conditions of Contrail Formation?

Contrails form because water undergoes distinct phase transitions in the Earth’s cold, troposphere.

At approximately 25,000 feet (cruising altitude for some commercial jetliners) the ambient air mixes with warm jet exhaust and the mixture then quickly cools to form a contrail.

Contrails are, in essence, man-made cirrus clouds (ice crystals).

Traditional forms of matter are solid, liquid and gas. Clouds (ice crystals) form from water vapor. The formation of contrails happens only at temperatures and pressures at which water vapor can condense directly to crystalline form.

The dirty mixture of jet exhaust and water vapor that is a contrail skips the liquid phase at high altitude because of the ultra-low pressures and low temperatures.

Understanding Contrail Formation with Physical Chemistry

Using the Ideal Gas Law as a first approximation gives an intuitive feel for contrail formation from jet exhaust and the surrounding milieu.

Because the pressure and volume of contrail formation (or the amount of artificial cirrus cloud) are directly proportional to the temperature, experimentation reveals that one can plot the results on a Pressure vs. Temperature diagram.

Understanding that different engine exhaust conditions affect contrail formation, the slope values from the graph allow you to directly interpret the temperatures. Critical examination of the research literature reveals that the temperature at which most contrails form is near 40 degrees below zero (Fahrenheit).

Illustration of Ideal Gas Law

The Ideal Gas Law gives a first approximation to contrail formation. Copyright image by John Jaksich. All rights reserved.

The effective temperature of ice formation comes from the type of jet exhaust and the humidity surrounding the aircraft.

Thus, minus 40 degrees is not a magical number, but it is a limitation of contemporary technology.

The Chemical Make-up of a Contrail

Because contrails are nothing more than artificially-produced cirrus clouds, the question arises, do we understand the molecular details of contrail formation?

The physical explanation of contrails was formalized nearly half a century ago by Appleman(1953). However, clouds are formed as heterogeneous species of condensed water or ice crystals.

The examination of contrails reveals that oxides of sulfur species are the most likely to aid in the formation of ice crystals in the troposphere. A primary reason for sulfurous molecules is their ‘water-seeking nature.’ It is the sulfuric acid, from the jet exhaust that is an important contributor to contrail formation.

Sulfuric acid is a nucleating species. Ice crystals form around microscopic droplets of sulfuric acid.  The selective association of water and sulfuric acid adds  stability-or makes the ice a likely species at the characteristic temperature (40 degrees below zero Fahrenheit).

Criss-crossing contrails

Criss-crossing contrails in South Eastern US. Image by NASA

Research on Contrail Dynamics Important for Climatology

The circumstances of man-made clouds added to the global warming puzzle. The tragedy surrounding September 11, 2001 allowed climatologists to study the effects of extra cloud cover on the temperatures around the globe.

For two days, air traffic was at a near standstill, and National Ocean Atmospheric Administration(NOAA) studied how man-made cirrus clouds affected temperatures.

Researchers then realized that contrails raise the effective global temperatures by approximately one degree. The man-made cloud cover acted as if it were an extra blanket enveloping the Earth.

Contrails, Clouds, and Science

Each morning, as we awake from our  slumber, our thoughts may turn to the sunrise and sky with wonder. Thanks to science and skeptical examination, even the most inscrutable natural and artificial phenomenons become clear.

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