Everyone has seen them: High in the blue sky, a pencil-thin plume slowly draws a white line. If you have good eyesight, you can just make out the plane that heads it, and if there is not too much noise around you, you can hear the distant roar of engines. For as long as man has built and flown high-altitude airplanes, this sight has existed. In the recent decades as even the high skies become congested with traffic, it is a common sight, and on some days the sky is virtually criss-crossed with contrails.

When air cools, its ability to contain water vapor is reduced, quite drastically. This is the reason for clouds, mists and fog: As air cools, some of the invisible water vapor condenses into tiny droplets that hang in the air and scatter the light, giving a whitish look. On a cold day, you can see your own breath: The warm, moist air you exhale cools and the water vapor condenses and becomes mist. An engine "exhales" water vapor too; fuel is a compound of carbon and hydrogen, and when it burns, it is turned into carbon dioxide and water. Normally, the water vapor in engine exhaust stays invisible because it is quite hot and it disperses in the surrounding air before it cools off sufficiently to condense, but under certain conditions of (low) temperature and humidity, it will condense into a white mist.

Such conditions become more and more frequent with increasing altitude. At 10km (30,000ft) where jet-liners fly, conditions for condensation are the norm: On most days, a plane traveling at that altitude will form a contrail. Sometimes, the mist disperses quickly, the tiny droplets forming it evaporate back into the surrounding air and the mist disappears a short way from the plane. All we see is a small white "comet" that moves across the sky. On other days, the droplets freeze into ice particles and stay visible for a long time: We see a persistent contrail that forms a long line in the wake of the plane and may sometimes hang around long after the plane left, drifting with the wind, sometimes spreading. On such days, if one of the trafficked air corridors is overhead, the sky may get zebra-striped with contrails. In special conditions, when the high air is super-saturated with moisture, we may see a single contrail trigger wide-spread condensation forming a continuous layer of high clouds or mist covering parts of, or the entire sky.

There is a second kind of contrails. You may have seen photos or movies of jet fighters performing some abrupt maneuver. Small contrails come from wing-tips and other places. When air is compressed, it gets hotter, and when it is decompressed, it gets colder. An aircraft, even when flying straight and level, creates a complex of pressure-waves that cause local temperature displacements. Under the right conditions, this gives rise to contrails. Since no moisture is added to the atmosphere, such contrails are less dense and have much shorter duration that the ones coming from the engine exhaust. Under conditions where pressure-wave contrails are created, exhaust contrails will be very prominent, and we rarely notice the other type.

I have often thought that people who did not know about aircraft, if such exist, might find the contrail phenomenon an awesome, fright-inducing sight. However, the fright seems not to be limited to people ignorant of aircraft. A whole movement has sprung up who have developed a sinister conspiracy theory around contrails. Of course proliferating on the internet.

These people have named them chemtrails and they claim that the evil government (who else? Only surprise is that aliens seem not involved this time) have decided to poison the population. The reasons why the government should want to do this are a bit vague, but it is claimed that it is done by letting high-flying planes spread chemicals over the countryside, in a sort of grand crop-duster operation. The evidence for this? Well, haven’t you seen all those planes drawing a trail over the sky?

This has to be one of the more whacky ideas around, but thanks to the internet, it can be spread around quite a lot, so let us dissect it, bit by bit.

Well, understandably, the chemtrail people tend to get a bit vague here. Any government, democratic or not, good or evil, thrives on its population. It may want to be loved by the people or be feared by the people, it may want to tax the people, suppress the people, exploit the people, but it is very difficult to imagine what reason a government would have to kill the people. History confirms this. Even governments that appeared to be headed by evil madmen, like Hitler, Stalin, Pol Pot, or Saddam Hussein, have not attempted to destroy their population at large. They may have tried to exterminate certain demographic groups, but only minorities.

To make the idea even more silly, those pesky contrails show up over any country, not just one. So it would seem that practically all governments in the world have been able to reach an agreement on this scheme. Of course, we know how great the probability is that all governments would agree on ANYTHING.

Now, let’s assume for a moment that the government does want to poison the population. Perhaps they do not want to kill them, just pacify them, or some other chaotic evil intent. Would having high-flying aircraft spread chemicals over the countryside be a useful tool for this? Well, not exactly! At altitudes of 30,000 ft, some severe conditions exist, among other things winds of over 200mph. So, before those chemicals can drift to the ground, they may have been blown hundreds of miles away! Not exactly useful if you want even the slightest control of where you spray or how much.

But it gets worse: Just think of the logistics. To regularly spray an area like the USA, and the chemtrail people claim it happens most every day, you would need hundreds of planes in constant operation. Specially built or modified planes. Each plane requiring pilots, ground-crew, maintenance crew. Thousands of tons of fuel and chemicals would have to be purchased and distributed.

Think of the risks: Any government or government department caught red-handed in such an operation would be crucified, riots would ensue, political opponents would be handed landslide victories on a silver platter. And how could it be kept secret? In a country like the United States, a president has had to resign after fiddling a bit with papers at the head-quarters of the opposing party; only a handful people knew about it, but it got out, and hell broke loose. Another president had a little fun with a female intern in his office; only three people knew about it, but it got out and… you know the rest. The chemtrail operation would involve thousands of people, many of which would have to know exactly what went on, others would have an excellent chance to put two and two together. People whose friends and families would be among the victims. How long would you guess it would take for it to leak? …Well, how long does it take to make a phone-call?

And it is not only the people directly involved who could blow the whistle! Flight controllers and radar operators all over the countries would see these mystical aircraft and begin to ask awkward questions. An air traffic control tracks EVERY plane in its area.

- Basic idea of chemtrails is highly unlikely: No government would undertake such a project.

- The project is impossible: If a government wanted to poison people, this would not be a feasible method.

- The thesis in unnecessary: The explanation for the observations is simple and straight-forward.

In other words, chemtrails are fiction, and not very good fiction at that.

So, contrails are harmless? Not quite. Apart from being the visible clue to some rather massive pollution, they are actually man-made clouds. It has long been speculated whether the massive concentration of contrails over some areas had an effect on the climate. On September 11^th, 2001 a unique and tragic opportunity arose to test this. For several days after the terrorist attack, all regular planes over the United States were grounded, the only flights venturing into that vast airspace being absolutely essential military flights. So for three days that September, practically no contrails were generated.

Later, meteorological data were analyzed, and it was shown that temperatures were actually different in areas where the sky is normally full of contrails. The nights were a little colder and the days were a little warmer. The differences were only a degree or two, and the average temperature over 24 hours did not change measurably. Does this mean anything? We don’t really know, but we now know that contrails do influence the climate in the more congested areas.

Below are a number of putative frequently asked questions. Nobody sent me those questions, but after perusing several websites of chemtrail believers, I find that these are some questions they might ask: