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.