Sunday, August 18, 2013

Chemtrails versus Contrails: Do Conspiracy Theories Make Sense?

Several times a month I get an email about chemtrails:  the belief by some folks that white lines in the sky are the result of a secret U.S. government program to alter the climate of the planet (also know as geoengineering).  They suggest that the lines are the result of noxious chemicals being deposited by aircraft and that they pose a dire threat to mankind.  This is a classic conspiracy theory, but it does offer an educational opportunity.

Most of you are aware of contrails (short for condensation trails):  clouds that form behind jets flying high in the troposphere or in the lowest stratosphere.   The picture below shows a typical example.
Contrails occur because combustion in jet engines produce a substantial amount of water vapor and when the vapor escapes into the cold upper atmosphere it condenses rapidly into little water droplets that rapidly freeze, thus producing an ice cloud.  Why does the water vapor condense?  Because the upper atmosphere is cold and cold air can "hold" less water vapor than warm air.  Thus, if you inject water vapor into very cold air, it will rapidly condense, particularly if the air is already close to saturation.  This condensation is aided by the small particles produces by combustion that serve as condensation nuclei to aid the process.

Most of you already are pretty expert in producing clouds in cold air:  it happens during very cold days--you can see your breath!  These clouds are generally made of cloud drops.
Human contrail?

Contrails can be produced by both jet aircraft and high-flying propeller driven planes.  Contrails were first observed in the 1920s and was a great concern in WWII, since they allowed the enemy to see high-flying bombers.  Also contrails could make it difficult for such bombers to stay in formation!

Sometimes contrails are hard to see, while other times they fill the sky as shown in the figure below.  Why?    The chemtrail folks think this is due to enhanced military or geoengineering activity.  The truth is that it all depends on how close the atmosphere is to saturation (100% relative humidity).  Sometimes the upper atmosphere is very dry, thus you need to add a lot of moisture from aircraft to

get saturation.   Other times there is sinking motion.  Both are bad for cloud formation.  But in other times, there is rising motion (perhaps due to an approaching weather system) and this brings the atmosphere close to or to saturation.  How can you tell the upper atmosphere is at saturation?  You see lots of thin cirrus clouds (see below).  If the atmosphere is near saturation, then the addition of moisture readily produces clouds (contrails).  If the atmosphere is at saturation, the extra moisture makes thicker clouds.

A sky with a lot of cirrus clouds is a good sign there is upward motion and an upper atmosphere at saturation.

If you have a situation where the atmosphere is at or near saturation and a region with a lot of aircraft flying in the upper troposphere, you can have HUGE numbers of contrails.  Here is an example from the NASA MODIS satellite.  Look carefully and you can see the cirrus clouds as well--just as one might expect.

 In such situations (where the upper atmosphere is moist) the contrails can not only spread across the sky, but they can persist for hours.    The longevity of contrails is a big deal to the chemtrails folks...they think it is proof of the "chemicals" be inserted by the top secret project.

Contrails, as most clouds, have a noticeable effect on the radiation balance of the planet.  On one hand they reflect solar radiation and thus cool.  On the other, they absorb and re-emit infrared radiaton, both upwards and downwards.  This tends to cause warming.  Right now the best estimates are that the warming effect will dominate and thus contrails will heat the planet by a small amount.  This fact shows why the chemtrails idea of U.S. government geoengineering to stop global warming is such nonsense---contrails warm the planet and thus would NOT be something you would enhance to stop global warming.


richard583 said...

I stiil say, "they're" out there.

weatherfan said...

I knew it... the whole thing is a goverment conspircy to make us even more parinod..

Bob said...

Come on folks, this is high school chemistry. Whenever a hydrocarbon is combusted in air, the result is always carbon dioxide and water. Did ya ever notice the water dripping from a car's exhaust pipe when is is first driven? Yep, the result of combustion! You don't see drips after the muffler becomes hot because water then exits as steam, which you do see in cold weather as a car's "contrail".

Bob said...

Oh yes, a grand experiment was done on this subject! In the 3+ days that airplanes were grounded after 9/11, satellite photos showed a very noticeable clearing of the atmosphere over the US. Yes, the plethora of contrails left by jets does alter the albedo reflection of our atmosphere. Here's a link describing the 2002 study noting the warming caused by the absence of contrails:

Marc Hager said...

I saw an interesting and possibly related phenomenon today as an aircraft passed through a thin cloud. A short trail of condensation appeared behind the entire wing of the plane. Possibly the result of melting and then refreezing of ice crystals as the air is compressed (heated) and then rapidly cooled?

ElenaW said...

During and after 9/11 I was working near MSP and for the several days after when the airport was shut down we had severe clear weather. No contrails.

Bob said...

Airplanes fly because the higher air velocity over the wing top creates a lower pressure there than at the bottom. As Cliff has pointed out many times in this blog, when you compress air, it heats. Likewise, when you drop its pressure, it cools. When you cool the humid air here at the edge of the ocean, you get fog! You will frequently see this happen when airplanes take off here in Seattle, in San Francisco, and in Los Angeles. During take off acceleration, the wings are set for no lift so speed can be gained quickly, then, when the pilot pulls back on the yoke, large white clouds instantly form on top of the wings as the plane is lifted into the air by the low pressure on those wing tops! Of course, this is most frequently seen in early morning when the air is most humid.

David B. said...

Ah, yes, "chemtrails". Frankly, this one (and those who buy into it) are so boneheaded that any attempt at logical conversation is basically pointless. You'll just be accused of being a "government agent" sent to discredit their "information" with "propaganda."

What's particularly amusing is how the chemtrails crowd will first try to pass themselves off as know-it-alls about what happens 35,000 feet up, yet when one mentions things like radiosonde skew-T plots you get a big "Huh? You must be a government plant!" In other words, "why should factual data get in the way of my pet conspiracy theory?"

Red Jetsen said...

I have seen planes flying at similar altitudes with different contrails. I have seen video from planes at altitude leaving different trails. Is the fuel mixture the same for all jets? Some tend to evaporate more quickly while others tend to linger and spread out not evaporating at all. I have observed both happening at the same time and altitude does not seem to matter.

I have seen low flying planes in air that I know is too warm to form ice crystals make dense streams that do not evaporate, while at the same, planes flying much higher in colder air have short trails that evaporate. Something is up but what?

AnniShar said...

I'm still in awe that humans can fly! I look at those contrails and feel pride and even disbelief at the wonder of our technology. I guess I'm pretty lame, eh?

DC Collyer said...

Anyone that knows me well knows that I really hate a good conspiracy theory. However, my view on most theories is that 99% of the ones you find on the Internet are absolute, total bunk produced by people that should be seeking out counseling rather than distributing their delusional ramblings across the Internet. Thanks to a paying customer of mine I currently run a blog about conspiracy theories. My BS meter is constantly aware of so many gullible people who are willing to believe in reptilian overlords and other far fetched stories. Anyways take a look here and you will see what I meen Lukes message conspiracy blog!

Cory Dane said...

I don't know you, but thank you. Anyone that complains about the government's inability to pass laws but thinks they're gassing the atmosphere with chemicals needs to read this.