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Contrails: What’s Left Behind Is Bad News

By Nick Onkow
March 4, 2006

Nick Onkow offers an informative and illuminating exposé on the detrimental effects of contrails to our environment. Contained herein is an undeniably important article not just because of its content, but because it breaches a topic so commonly overlooked, and so consistently regarded as harmless.

“Our ideals, laws and customs should be based on the proposition that each generation, in turn, becomes the custodian rather than the absolute owner of our resources and each generation has the obligation to pass this inheritance on to the future."

There is some irony in that statement which defends the argument that it is the obligation of the living population of Earth to leave an environment in the best condition it can for the succeeding generation. Why the irony? It was said by world famous aviator Charles Lindbergh, the first pilot to cross the Atlantic Ocean alone. He was concerned about the way the environment was beginning to be affected by humans on an increasingly larger scale and at the time, his aerial view of land development was rare. Today the aviation industry is larger than many people would have ever imagined it would be and it is only going to grow more as the population of Earth climbs past six billion and national economies grow with it. The nature of commercial aviation includes some detrimental results to the environment. Fuel is burned and the exhaust fills the atmosphere, but cars, trains, and ships do the same. Jet aircraft, however, have a unique form of harming the environment that is associated exclusively with them: contrails. They are the long, thin clouds that are blasted out of the exhaust nozzle of jet engines at high altitudes. Some days they fade away within a few minutes and they pose no threat. It is the days that perfect conditions exist when they do their damage, drifting and expanding to several thousand square miles and blanketing the lowest atmosphere of Earth through the night, unnaturally trapping heat. Some argue that contrails have no effect on the environment but evidence indicates this opinion is not valid. Several solutions to the problem exist. Jets could fly at different altitudes, or engine standards could be raised so that insurance rates are less for those that are friendlier to the environment. Knowledge from the military stealth aircraft program could be incorporated into civilian aviation to avoid contrails, or contrail forecasts could be incorporated into flight planning process so that contrail-prone routes and altitudes are avoided. Contrary to some opinions, contrails have indeed helped raise the temperature of North America and the entire planet since the start of the jet age and continue to do so, making a long-term plan to reduce them a plan that needs to be initiated.

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Photo © Josef P. Willems


Contrails are essentially clouds and are the same effect as seeing one’s breath on a cold, damp day. The narrow bands of ice crystals gradually expand into a cirrus type high altitude cloud if conditions are just right. Just how often are the conditions conducive to their formation? “At flight altitudes, conditions that support contrail-generated cirrus exist 10% – 20% of the time in clear air and within standing cirrus”4. Although this is a small percentage, the diverse weather of North America coupled with the staggering number of commercial flights in the air results in at least some part of the United States being good contrail weather on any given day. Worldwide, contrails are estimated to cover .1% of the Earth’s surface area and that number is forecast to rise to .5% by 20502. There is some debate over just how effective this cirrus cloud coverage is at raising the average temperature of the land it covers. A NASA study conducted in the USA between 1975 and 1994 found the average temperature to have increased by 1°F 7. Though a single degree may seem trivial, the incredibly large scale that it applies to makes it significant since just 9°F separates our current average temperature from the last Ice Age. In one study conducted by meteorologist Keith P. Shine, data from satellites was used to prove that only one percent of the increase in clouds throughout the world have been from aircraft. There are also inherent flaws in some of the research performed by NASA. One problem is the difficulty that scientists have distinguishing a suspected contrail cloud from a natural cirrus cloud in satellite images. Skeptics of the theory that contrails do not have an impact on weather argued this theory with some success until a significant event occurred in North America, the main testing grounds of contrail research.


Contrails or Cirrus Clouds?
Newfoundland, 7 May 1999


The terrorist attacks of September 11, 2001 was the aforementioned event, and it was likely to have excited meteorological researchers involved in contrail impact studies. The national airspace was shut down for three days, something that had not yet occurred since the jet age began in the 1960s and is not likely to occur ever again. Scientists took advantage of this unique three day period in history that lacked contrails. What they learned was shocking and is enough evidence to effectively silence any counterargument to their case. One measure of climate is the average daily temperature range (DTR). For thirty years this had been recorded and extra cirrus clouds in the atmosphere would reduce this range by trapping heat. “September 11 – 14, 2001 had the biggest diurnal temperature range of any three-day period in the past 30 years,” said Andrew M. Carleton1. Not in three decades had there been such a large temperature spread between the daytime highs and the nighttime lows. Furthermore, the increase in DTR during those three days was more than double the national average for regions of the United States where contrail coverage was previously known to be most abundant, such as the Midwest, northeast, and northwest regions. The specific increase in the range was 2°F, which in three days was twice the amount the average temperature had increased by over thirty years time1. This is evidence that contrails do alter the climate of the land they drift above.


Northeastern U.S., 11 Feb. 1999


There are several methods that can be explored that will help reduce the role that contrails play in global warming. The easiest way to avoid this global warming through contrail cirrus clouds is to have jets fly at different altitudes. Flying higher than the typical 30,000 to 40,000 feet would usually stop contrails from forming, as would flying lower. Each of these options is, unfortunately, made unrealistic by consequences associated with them. Besides performance limitations of the aircraft above the normal cruising altitudes, airplanes begin flying in the lowest layer of ozone that is found in the tropopause (the dividing line between the lowest two layers of the atmosphere). As for flying lower, the decrease in altitude results in denser air and higher air resistance. This increases fuel burn, which increases the amount of carbon dioxide emitted, negating any benefits from eliminating contrails2.

Ruling out drastic changes in altitude, another option might be to increase the emission standards of jet engines and with that only insure airplanes with the newer, cleaner engines. Tests were performed with a NASA jet aircraft examining the effect of sulfur levels in jet fuel exhaust. During the airborne test one engine was run on normal jet fuel and the other engine was run on fuel that emitted exhaust with a lower sulfur content. The high sulfur engine, representing most jet engines on modern commercial aircraft, produced a contrail that lasted through a larger range of temperatures and formed faster out of the engine. The low sulfur engine did the opposite. “Aircraft generate an invisible aerosol trail which enhances the background level of condensation nuclei, in particular regions with dense air traffic at northern latitudes and near the tropopause”6. This condensation nuclei is the tiny matter that gives water vapor the ability to form. The International Civil Aviation Organization is in favor of making polluting, obsolete aircraft uninsurable. While this option would not completely eliminate contrails, it would narrow the window of conditions needed to form them, making them less common.

Click for large version
Click here for full size photo!

Photo © Josef P. Willems


A third solution to avoiding the large-scale creation of contrails is just that – avoidance. Partly through military research, new methods of forecasting the formation of contrails have been learned. This was a result of stealth aircraft that are not detectable by radar but are easily spotted from the ground if a contrail is following it. A program was initiated by the Air Force Weather Agency with the goal of improved contrail prediction techniques by closely examining the weather that was conducive to their formation. The program, run in 2000, used radiosondes (weather balloons) to measure water vapor content and temperature at different altitudes compared to actual observations of aircraft in the area3. The end result was a success: “The statistical model produced a correct diagnosis of contrail occurrence or nonoccurrence for 85% of the observations”3. Statistical contrail forecasting, then, is the easiest way for this problem to start being dealt with. Returning to the fact that only 10% to 20% of the country’s airspace is conducive to forming contrails at any given time, that leaves at least eighty percent available for use, and that is not even accounting for the third dimension of altitude to be used in avoidance. One way to do this would be to equip each aircraft with a device that detects the conditions that were confirmed in the Air Force study as being conducive to contrail formation3. The Federal Aviation Administration or Environmental Protection Agency could monitor these from the ground to see when an aircraft is flying in one of these areas. Incentives to use other airspace or altitudes could be put in place to reduce the number of jets flying there, such as reduced taxes on fuel or airport fees, or an extra tax or fine on aircraft that fly through the airspace that will leave a cirrus cloud drifting behind.


The North Sea, 15 May 1998


Unfortunately, aviation will always have some detrimental impact on the environment. What is most important, then, is reducing those impacts to the extent practicable. Through studies it has become apparent that contrails expanding into cirrus clouds do have some impact on the weather and the environment. Global warming is already a concern, and although the extent to which contrails are contributing to global warming is debatable, it cannot be argued that they have no effect. Using weather forecasting to predict areas where cirrus clouds will form from contrails should eventually be used in combination with devices on aircraft and cleaner engines with lower emissions (especially of contents such as sulfuric acid) to actively reduce the negative effects of contrails. As the aviation industry grows, limiting its negative impact on the environment will be a difficult challenge, and reducing the amount of heating that has already taken place as a result of high-flying aircraft will be an even greater challenge.

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Click here for full size photo!

Photo © Jeffwell


Notes:
1. Carleton, Andrew M. “Climatology: Contrails Reduce Daily Temperature Range.” Nature. 8 August 2002.
2. Graham-Rowe, Duncan. “High Flyers are Scourge of the Skies.” New Scientist. 19 October 2002, Vol. 176, Issue 2365.
3. Jackson, Artie. “Statistical Contrail Forecasting.” Journal of Applied Meteorology. February 2001, Vol. 40, Issue 2.
4. Minnis, Patrick. “Contrail Frequency over the United States from Surface Observations.” Atmospheric Sciences Research. NASA Langley Research Center, Hampton, VA. 12 August 2002.
5. National Oceanic and Atmospheric Administration. Operational Significant Event Imagery Image of the Day, February 11 1999. http://www.osei.noaa.gov/OSEIiod.html.
6. Schumann, U. “In Situ Observations of Particles in Jet Aircraft Exhausts and Contrails for Different Sulfur-Containing Fuels.” Journal of Geographical Research. 1996, Vol. 101, Issue D3.
7. Watson, Traci. “Plane Trails in Sky Turn Up the Heat Below, Study Suggests.” USA Today. 29 April 2004.

Written by
Nick Onkow

Nick Onkow is a pilot, a flight instructor, and a photographer, whose photographs can be found here at airliners.net. Based on the amount of study invested in this topic, we consider him an expert on contrails and their environmental effects.

12 User Comments:
Username: IsuA380B777 [User Info]
Posted 2006-03-10 04:49:33 and read 32768 times.

Dear Nick

An excellent and well researched article.
Regards

Username: N62NA [User Info]
Posted 2006-03-12 04:29:52 and read 32768 times.

Very well put together. I hope other a.netters will take the time to read through your excellent article.

Username: AFEaviator [User Info]
Posted 2006-03-14 03:18:54 and read 32768 times.

Very interesting article! I know you article is directed specifically at contrails, but you mention the wide temperature variance during the no fly days post 9/11. I am curious if the margin was less or more in large commercial traffic cities. There has been some very interesting studies about large cities producing their own weather effects and I am curious how this no fly time period may have affected them.

Username: KLM685 [User Info]
Posted 2006-03-18 00:36:51 and read 32768 times.

Congratulations for this piece of excellent work! I used to have geography classes with this teacher from Alaska. She always condemned everything she considered as pollution...that means everything. So one day she talked about how contrails damaged the atmosphere, etc... Thanks to this article I'm now able to put the puzzle together. Amazing research

Well done!

Alonsou

Username: Xjramper [User Info]
Posted 2006-03-26 21:05:22 and read 32768 times.

Very interesting read. There are two things I would like to argue. One was weather. That information seems to be lacking, to prove that the drastic change in the temperatures were not caused by a massive frontal system. The other thing that I see is that it was a 3 day observation. What this is telling me is that the earth reversed 40+ years (14,600+ days) of jet polution and showed that great of a climatic change in 3 days. Seems a little unrealistic to me.

zach

Username: Goboeing [User Info]
Posted 2006-03-26 23:02:30 and read 32768 times.

There are two things I would like to argue. One was weather. That information seems to be lacking, to prove that the drastic change in the temperatures were not caused by a massive frontal system.

Evidence showed that the temperatures dropped at various points around the country, scattered far enough apart that a frontal system would not be the only cause.

The other thing that I see is that it was a 3 day observation. What this is telling me is that the earth reversed 40+ years (14,600+ days) of jet polution and showed that great of a climatic change in 3 days. Seems a little unrealistic to me.

The jet pollution from the preceding 40 years of flying did not dissapear. What did dissapear for three days was the jets that produce the contrails that form cirrus clouds. The cirrus clouds expand, drift, and trap heat at night. That results in less cooling. During the three days, there were no contrails over the U.S. and therefore no jet-made cirrus clouds to trap the heat.

Nick

Username: Xjramper [User Info]
Posted 2006-03-27 02:46:49 and read 32768 times.

Evidence showed that the temperatures dropped at various points around the country, scattered far enough apart that a frontal system would not be the only cause.

I wasn't arguing that, in research of this caliber, you need to cover all of your extraneous variables. That way one, like myself, can see both sides of the argument.

The jet pollution from the preceding 40 years of flying did not dissapear. What did dissapear for three days was the jets that produce the contrails that form cirrus clouds. The cirrus clouds expand, drift, and trap heat at night. That results in less cooling. During the three days, there were no contrails over the U.S. and therefore no jet-made cirrus clouds to trap the heat.

First, I did not say they disappeared.

Secondly, I would like to see numbers that were recorded during this time. Is there a link that these are available? Because I randomly looked at 5 cities around the country and noticed little to no difference to the change in temperature.

Thanks...zach

Username: Goboeing [User Info]
Posted 2006-03-27 04:41:32 and read 32768 times.

I would like to see numbers that were recorded during this time. Is there a link that these are available? Because I randomly looked at 5 cities around the country and noticed little to no difference to the change in temperature.

The fourth source at the bottom of the article has the numbers I think you are looking for. It is fairly lengthy but they are in there. Picking five cities at random does not give an accurate idea of a change in the usual daily temperature range (DTR). You'd have to compare the min and max temperatures in 24 hour periods and frontal systems and local weather can affect that.

Nick

Username: Tornado82 [User Info]
Posted 2006-04-06 00:10:44 and read 32768 times.

Sorry Nick, nice editorial opinion article but scientific evidence isn't gathered in 3 days of records to research something as long-term as climatologic facts. Your sampling is simply way too small, and is heavily affected by the unrelated climatology of that three day span. This is why most of us in the meteorological community never embraced the "findings" of the September 11th tragedy timeframe as anything but a loose hypothesis.

Just prior to 9/11 was one of the first pattern-shifting frontal passages of the transition season for that fall. Meteorological/climatological fall begins Sept 1, regardless of the equinox. The atmosphere was relatively dry across much of the nation, with crystal clear skies (even before the tragedy occurred) thanks to the strong high pressure over that period, and still the relatively high Mid-September sun angle. The day time temperatures soared with a well mixed, dry atmosphere across much of the country, especially the Northeastern quadrant where contrails would typically be most prevalent, and where the most observing stations are. A nearly 1030 mb high pressure is quite strong for that time of year, and was anchored over our country bringing a shot of polar air with it. Of course, as will almost always happen in one of these types of atmospheres, decoupling occurred at night. No more mixing is occuring in the atmosphere, the wind becomes very calm, and cold air quickly sinks to the surface with no vertical motion (mixing) to support it. This happens nearly every time you get one of these types of atmospheric setups, it is just that people looked at it with more interest due to the lack of contrails. Additionally, with the upper-level atmospheric setup over much of that time, contrails would not have been very prevalent anyways.

The larger-than-usual diurnal range was caused simply by a very well mixed atmosphere in the daytime, with a very decoupled atmosphere at night, and the magnification of this type of atmospheric setup occuring over a large portion of the nation at the time. Aiding to the huge diurnal range is that this occurred when the SST's and Great Lakes, and any other water-body surface temperatures are near their annual peaks, eliminating any "sea breeze" or "lake breeze" effect to moderate the temperatures. Based on the surface dew points, sky conditions, and decoupling leading to lack of nighttime winds in the time period studied, the low temperatures are right where they should have been. You would have needed much more time to sample effect, or a very sophisticated computer modelling system to replicate the event. So far, neither has happened, and I pray to God that there is no more chance of another catastrophe closing our nation's airspace in a similar manner.

Username: Bwood [User Info]
Posted 2006-04-17 23:29:46 and read 32768 times.

I find this article hard to believe. The idea that the clouds that form from contrails are trapping in heat to me sounds ridiculous. Now before you attack me I know that clouds do trap heat and can keep temperatures higher at night but the clouds we are talking about are at 30000-50000ft. They are so thin you can see through them. For the heat to be trapped at ground level they would have to be lower and only a few hundred to thousand feet off the surface. Also over half the world's entire atmosphere is between ground level and 8000 feet. If these contrails were lower then maybe but not at thirty to forty thousand feet. I know that the ozone is high up in the upper atmosphere as well but this is a layer of gas that is supposed to block in radiation from the sun. Clouds do not have the same affect. The sun's radiation goes through clouds and that is why you can get sunburned on a cloudy day. The radiation also can bounce back up through the clouds to the ozone layer. The radiation is what harms us and the atmosphere. It is not that heat itself bounces back from the ozone layer it is the suns radiation that bounces back that heats up our atmosphere. Tell me how warm it is on a winters night when it is totally clear and 10 degrees below zero then tell me how extra warm it feels when there are some cirrus clouds at 50000ft and it is supposed to be ten degress below zero. It will feel exactly the same. I think the real danger comes from the jet exhaust itself and not water vapor 6-9 miles off the ground.
I also feel that there are far greater polutors in this world. Read March 06's National Geographic that came out on coal plants. There is where we are going to kill our planet. That and cars. Coal plants are the real danger. Jets burn jet A which is basically kerosene or a slightly modified diesel fuel. This has its advantages since it produces only hydrocarbons as pollutants and emits no sulfur or nitrogen pollutants like cars or power plants, however it does produce carbon dioxide. That is a study that should be done. What are the affects of carbon dioxide in the upper atmosphere. I think that the study (or I should say so called study) done for the three days after 9/11 is not anything that you can come to a conclusion from. It is only three days of data and you can not call something like that a fact after only three days of data. There are too many factors that can cause interference with this so called tested and true study. You would need years of data in different conditions and weather to figure out if this was true. I really cannot accept the idea that contrails that are just water vapor are covering the sky, blocking in the heat from the ground and warming our planet by several degress. If that was the case then everyday that we have clouds the temperature of the earth should rise and cause global warming. People have to remember that there is more than one factor that goes into our climate and weather patterns.

I agree that airplanes are pollutors and that something should be done about it. However aviation right now does not have the technology to make "hybrid or alternative fuel planes.'' Ethanol is being tested for smaller piston engines but is only in the earliest experimental stages. You are not going to see an Airbus A380 running on corn or hydrogen anytime soon. I think that more reasearch should be done to try and eventually turn planes away from petroleum and to another clean source but it is decades away. We should try to focus our "energy" on making cars and powerplants (by far the greatest pollutors on our planet) carbon dioxide free within the next ten to twenty years. It is possible but the red tape and political issues are enourmous. We all need to write our congressmen and women along with the president to get more funding and support for zero emission power plants and cars. Which by the way as of right now we know how to make coal plants and cars zero emisson but no one is doing it on the large scale. Lets focus our interests on the big and correctable pollutors first and then expand out from there.

Username: Mdgg2009 [User Info]
Posted 2006-06-23 20:36:07 and read 32768 times.

Chemtrails not contrails

Username: Darcyj [User Info]
Posted 2011-01-13 20:10:42 and read 21905 times.

Driftnetting the articles list brought me to this one. Interesting hypothesis and data, but does it occur to anyone that clouds also reflect? If the entire planet was covered in high-altitude cloud for a number of days, the temperatures would fall, not rise. Contrails might trap heat at night (and that is open to question, given their altitude and the paucity of atmospheric pressure) but it is definite that they reflect solar radiation at visual and longer wavelengths and so their effect on global temperatures is at worst balanced, but probably negative (ie, cooling), overall.

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