Regarding the July 9 letter, ” ‘Chemtrails’ left by planes need study”: Please allow me to attempt to correct a misunderstanding by the writer and many others who believe contrails contain mystery chemicals and exhibit strange and exotic properties.
Her description of contrails is generally quite accurate and graphic. However, her concern that they contain substances that make them “linger” longer than natural contrails and produce an “oily” appearance is misplaced.
The continued growth, spread and appearance of contrails following the passage of high-flying jet aircraft is strongly dependent upon the relative humidity and turbulence of the air through which the plane flies. At high altitudes where the temperature is colder than freezing, ice crystals that form from the moisture in the exhaust of the engines will grow and spread across the sky when the relative humidity is more than 100 percent relative to ice. Because ice crystals can grow and last for hours under such conditions, they produce a thin cirrus cloud layer which partially obscures the sun.
This growth of a cirrus layer is more likely when a storm is approaching because the air is being slowly lifted and humidified ahead of the storm. Contrails in clear air behind a storm are more likely to dissipate quickly because the air is drying in descending air. The so-called “oily” appearance is simply optical effects produced by ice crystals in the thin cirrus clouds. Various colored patterns which encircle the sun are produced by the refraction of light from the ice crystals.
Unfortunately, the growth of cirrus clouds in mid-latitudes has increased by about 10 percent since the advent of high-flying jet aircraft in the mid-20th century. However, the chemicals contained in contrails are not lethal or poisonous as sometimes alleged, but are simply the moisture and combustion gases from burnt jet fuel.
The use of the term “chemtrails” was introduced by those who put too much faith in conspiracy theories.
LARRY VARDIMAN
Atmospheric scientist
Camano Island
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