Chemtrail Controversy

In the November/December 2000 issue of Weatherwise,
James Rowland of San Diego, California, asked. . .

As a dedicated Web surfer, I've become aware of some recent controversy concerning contrails. Some think there is something sinister going on above our heads in broad daylight and go so far as to describe certain patterns as "chemtrails." Can you please define what a normal contrail is and distinguish them from "chemtrails," should there be such a thing?


Before visiting Web sites concerning "chemtrails," I assumed that "chemtrails" was a new name given to contrails by environmentally conscious people who are concerned that subsonic jet traffic injects water vapor, carbon dioxide, other gases, and soot into the atmosphere at high altitudes. These substances can alter the chemistry and radiative properties of the atmosphere, the high-altitude cloudiness, and perhaps even the regional climate. I was very wrong. In visiting these sites, I learned that ³chemtrails² is the name given to sky tracks left by the deliberate release, either by spraying or mixing with jet fuel, of substances not found in "ordinary" contrails. The releases are allegedly occurring in many countries without the knowledge or consent of the populace, and they are being blamed for respiratory and other health problems.

I consider these claims unfounded. There is no evidence in the meteorological literature or photographs on these web pages to support them. I will summarize what is known about contrails (last treated in the August 1981 issue of Weatherwise), discuss some of the hallmarks that are supposed to distinguish chemtrails from contrails, and list some of the potential hazards of jet exhaust at flight altitudes.

Natural cirrus clouds form high in the troposphere, usually at temperatures below -40 degrees F, when there are sufficient ice nuclei (tiny particles that ice crystals can grow on) in the air and when the water vapor content is high enough. Upward motion in the high troposphere causes the cooling that brings air to saturation and forms the ice crystals in natural cirrus clouds. Contrails (an abbreviation of condensation trails) form when jet aircraft inject water vapor into the high troposphere or lower stratosphere, thereby bringing the air to saturation.

During this year, aircraft will burn over 200 million tons of fuel. The major byproducts of combustion are carbon dioxide (CO2) and water vapor (H2O). There are other substances in the engine exhaust, notably, carbon monoxide, oxides of nitrogen, sulphur dioxide, methane, nonmethane hydrocarbons, carbon soot, and other microscopic particles.

For every pound of fuel burned, about 1.26 pounds of water vapor is created but less than 0.0004 ounces of soot because aviation fuel burns cleanly. Even so, the soot provides plenty of ice nuclei, and the water vapor can usually saturate a large volume of air at the low temperatures where commercial jets fly. The result is the immediate formation of a contrail, consisting of tiny ice crystals in large concentrations. At temperatures lower than about -76 degrees F, contrails almost always form, regardless of relative humidity. The higher the ambient temperature, the less likely that contrails will form. At temperatures above -40 degrees F, contrails are not expected.

The persistence of contrails depends upon temperature, relative humidity, and the vigor of mixing between the exhaust plume and the ambient air. At low temperatures, with high humidity, and with stable temperature stratification (which inhibits vertical mixing of the air), contrails persist for many hours.

Below are some misconceptions about contrails taken fromWeb sites. After each, I describe why they are misguided.

Normal contrails usually dissipate very rapidly and are short in length. As noted above, under the right conditions, contrails can easily persist for hours. If the outside air temperature is very low and the humidity high along the flight path, the contrail can be one hundred miles long.

Chemical plumes (chemtrails) linger for hours and gradually merge to occlude clear skies. Contrails have these properties, and they are easily explained. The longer a contrail persists, the more it spreads. Measurements made of contrails two to seven minutes old (roughly 16 to 56 miles behind the aircraft) indicated widths from 0.25 to 1.25 miles but depths of only around one thousand feet. The vertical expansion of the contrail is controlled by the stability (how quickly the temperature decreases with height); the horizontal spreading is controlled by the vertical shear (how the wind changes with height) and small-scale turbulence, which inexorably disperses the plume.

It is not unusual for contrails to spread and merge so that they cover a good portion of the sky with a filmy veil of ice crystals. Sometimes the moisture and ice nuclei injected into the air by the jet engines can support the formation of natural cirrus, even if contrails didn¹t form initially, provided that a gentle uplift causes enough cooling for clouds to form.

Chemtrails are laid by aircraft flying for hours in dangerous crossing patterns and in illegal proximity to each other. Parallel furrows, Xs, and grid patterns are tell-tale signs. At least for now and unless they have to divert around thunderstorms, commercial jet aircraft fly on specific routes--wide highways in the sky. For safety reasons, every aircraft is tracked, vertical and horizontal separations are maintained, planes traveling in opposite directions never fly at the same altitudes, and FAA Air Traffic Control notifies pilots of crossing paths. Often, pilots have visual contact with each other and, even in the clouds, radar aboard the aircraft paints a clear picture of other planes in the vicinity.

Contrails along parallel paths or criss-crossing contrails are merely indications of busy flight corridors. Many hundreds of aircraft are in the air over the United States at a given time, and many of them leave contrails. It is usually obvious at flight level but not often from the ground that the contrails are at different levels. Even if the contrails cross at the same level, the aircraft that created them were never less than a few kilometers apart.

When struck by certain angles of sunlight, chemtrails display bars of color, indicative of a chemical signature. Like cirrus clouds, contrails contain ice crystals, grown from the vapor in the jet exhaust. Certain kinds of crystals in the proper orientation, such as columns or flat plates, refract sunlight and produce colored arcs or spots in the sky. Such optical phenomena have been discussed often before in this column; they are covered in depth in Robert Greenler¹s book Rainbows, Halos, and Glories (Cambridge University Press, 1980.)

Older, spreading contrails in which larger crystals have grown can produce refractive effects. There is some evidence that newer contrails may produce aureoles or coronas in the same part of the sky as the sun. These are diffractive phenomena most likely to occur when the crystal size in the contrail is uniform and very small. Several color photos of optical displays caused by contrails are on page 1,893 of the September 1997 issue of Bulletin of the American Meteorological Society.

Ice crystals from H2O and optics are all that are necessary to explain colors in contrails.

Contrails appear as continuous tracks; chemtrails may be suddenly discontinuous. In fact, contrails may be suddenly discontinuous. Especially in winter, westerly winds crossing the Rocky Mountains generate gravity waves, a series of up-and-down motions that sometimes extend from near the crest of the Continental Divide to 30 to 60 miles downstream. Wave clouds sometimes form at the crests of these waves. Sometimes the waves are regularly spaced, sometimes not. Air ascends and cools as it rides up toward the crest of the wave; it descends and warms as it approaches the trough on the other side. In the same way that cloud formation may be aided near the crest of the wave, so is contrail formation.

Even if the sky is clear, enough moisture may be added to the air by a passing jet that a contrail will form just at the crest of a wave but not elsewhere. Discontinuous fresh contrails are not uncommon in the Denver area in winter. They can appear in other parts of the country as well because gravity waves are ubiquitous. Flow across mountains is just one way to generate them.

Sometimes a fresh contrail is visible within a thin layer of cirrostratus cloud, but the contrail ends at the cloud edge. Sharp gradients of moisture and vertical motion exist in the atmosphere. A quick transition--from gently rising motion in moist air to sinking motion in dry air--that is responsible for a cloud edge can be equally responsible for supporting, then preventing, contrail formation.

I contend that there is no visible or other physical evidence of high-altitude spraying, and so I do not believe in chemtrails. The moniker might just as well be applied to contrails, however, for both the visible and invisible (chemical) components of jet exhaust are cause for at least some concern.

The water vapor released into the atmosphere by jet aircraft is estimated to be only 0.04 percent of the amount evaporated from the Earth's surface, and so does not contribute significantly to the greenhouse effect (whereby outgoing radiation at infrared wavelengths is absorbed by atmospheric gases and reradiated back toward the Earth¹s surface). Jet exhaust may be a significant source of water vapor in the stratosphere, however. Ice crystal clouds formed from the water vapor probably have a larger radiative effect than the water vapor itself.

The average contrail coverage over Europe, where air traffic is quite heavy, is about 0.5 percent. The mean global coverage is less, perhaps only 0.1 percent. Researchers agree that increased cirroform cloudiness should affect climate, though they are still debating whether it will lead to tropospheric warming.

Though aircraft inject well over 500 million tons of CO2 into the atmosphere each year, this amount is still only 2.6 percent of the total contributed by the burning of fossil fuels. CO2 is a greenhouse gas, and how to control its rate of input into the atmosphere has been the subject of international political debate.

Aviation contributes about 3 percent of all anthropogenic sources of oxides of nitrogen (NOX) in the atmosphere. Nitrogen oxides tend to increase ozone in the troposphere, where it is not wanted. "Ozone alerts" in Houston made news last summer when the high concentration of ozone near the ground became a health hazard. There is suspicion that NOX also contributes to the destruction of ozone in the stratosphere, but the jury is still out. Stratospheric ozone partially shields the Earth¹s surface from ultraviolet light that causes sunburn and is implicated in some skin cancers. Jet exhaust nearly doubles the background concentration of oxides of nitrogen in the Northern Hemisphere¹s stratosphere.

In summary, legitimate concerns about contrails and other products of jet fuel combustion are that 1) the Earth-atmosphere radiation budget may change because of more high, thin, ice-crystal clouds and the increase in greenhouse gases, principally water vapor and carbon dioxide and 2) oxides of nitrogen alter ozone concentrations in the troposphere and stratosphere.

  Weather Queries are answered by NOAA meteorologist THOMAS SCHLATTER.

Index of