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Open House Oct 2007
CONTRAILS- WHAT ARE THEY, AND HOW ARE THEY FORMED?
Contrails are clouds that form behind aircraft. These clouds are easily distinguished by their linear nature,
straight lines happen very infrequently in nature and contrails are very easy to spot because of this.
To begin to understand how contrails form we must understand what is happening with the airplanes.
Airplanes burn fuel, the fuel that airplanes burn are hydrocarbons. below is a depiction of a hydrocarbon chain;
The hydrocarbon molecules all consist of one or more carbon atoms with hydrogen atoms bonded to them.
These hydrocarbons range from one carbon atom with four hydrogen atoms bonded to it, which is known as methane,
to the extremely long chain of carbon atoms with hydrogen atoms bonded along the entire chain that form the bases
of many plastics. Based upon the purity of the crude that the fuel is refined from, or by design to add properties such
as anti-knock for reciprocating engines, there may also be trace amounts of other atoms such as sulfur.
The fuel (hydrocarbons) is converted by oxidation into two major components, carbon dioxide and water as shown below;
The image above shows a hydrocarbon chain next to oxygen molecules with the lines showing the molecular bond.
In the oxidation process the bond is broken, as indicated by the red lines on the bonds, and the resulting freed hydrogen
atoms bonding to oxygen atoms to form water. The carbon atoms bond to the oxygen atoms to form carbon dioxide
molecules. Because of the large percentage of Nitrogen in the atmosphere and trace elements in the fuel as mentioned above,
other oxidation processes occur in the combustion forming Nitrates and Sulfates as well as other trace oxides.
As a result of the combustion, jet engines eject water vapor and carbon dioxide as discussed above.
Depending on the relative humidity of the air through which the jet passes, the addition of water vapor may condense
to form clouds, generally formed of ice crystals which are known as contrails. Below is an image of the upper air
sounding from the 1st of Feb of 2009;
A rough rule for predicting whether contrails will form is if the difference between the temperature ( the jagged red line)
and the dewpoint (the jagged blue line) is five degrees C or less. On this day you can see that this indeed was the case
a flight level of 30,000 to 35,000 ft. The conditions at flight level can change rather abruptly from one location to another
as differing conditions are present and changing in the atmosphere, causing contrails to be present over one portion
of the sky, and absent in another. when the temperature and dewpoint difference is slightly greater, contrails will form
but dissipate as they spread into the less favorable environment.
Parallel contrails are formed by the movement of air at the flight level of the jet. In the example above, winds at 35,000 ft
were around 55 knots. Two aircraft following the same air route, but several minutes apart, will leave parallel contrails.
As more and more flights pass over the same route in the cross winds, more and more parallel contrails are formed.
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www.weather.gov 
