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Clouds left by jets spark protests

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Scott Sutherland
Meteorologist/Science Writer

Saturday, April 25, 2015, 12:30 AM - They crisscross our skies on nearly a daily basis, sometimes lasting only seconds and other times expanding into widespread clouds. What are contrails? How do they form? What's behind their behaviour? And why have they sparked a global protests march, planned for April 25?

Marches have been planned around the world, including in several Canadian cities, to spread the message of what the conspiracy theorists call "chemtrails."

The focus of all this attention? The wispy lines of cloud produced by jet aircraft as they fly across the sky, known as condensation trails or "contrails."

While these are trails of frozen water droplets, mixed with some pollutants from burning jet fuel, some have misinterpreted them as chemical sprays being emitted from aircraft, usually attributing them to government programs designed to alter the climate. They've been the subject of advertisements in the Toronto transit system, and an Arizona state senator is being criticized for offering her constituents a public forum to discuss "chemtrails."

So what's really behind this weather phenomenon?

What are contrails?

Recently, we highlighted an exceptional short video by Minute Earth, that elegantly pulled back the curtain on how clouds form in our sky.

While most clouds form due to warm, moisture-laden air rising into cooler parts of the atmosphere, the rising air isn't really necessary. It's just a simple 'delivery' method to get the moisture that goes into making the cloud from point A (a body of water or parcel of land) to point B (a height above the surface where it's cold enough for water vapour to condense).

However, clouds can and do form using other 'delivery' methods other than rising air. Look up into the sky, to a passing jet airliner and you may see a perfect example of this: a condensation trail, or contrail.

Contrails are produced by one or more of a few different processes:

  • Either some point on the jet itself - typically part of the wing or tail - is cold enough to act as a condensation point, and/or
  • The wing tip or tail can produce a pressure void behind it, which lowers the temperature of the air so that condensation forms, and/or
  • Moisture-laden engine exhaust adds warm water vapour to a cold layer of air.

Not all contrails are produced in the same way, nor do they all behave in the same way.

Why do some jets produce contrails and some do not?

Atmospheric conditions can vary quite a bit from place to place, even between two heights above the same point on Earth, or right next to each other. Take the regions just inside and just outside a cloud, for example. 

Thus, one jet producing contrails and another not producing them can be as simple as them flying at different heights or through different areas at the same height. Thus, they each pass through air with different sets of temperature and moisture conditions.

Also, the design of jet engines can vary enough based on model and year that, for the same set of atmospheric conditions, some produce more contrails, while others produce fewer or none at all. For example, an engine that produces colder exhaust will be more likely to leave behind contrails than one that produces warmer exhaust.

How can contrails form in an otherwise perfectly clear blue sky?

Even a clear blue sky can contain at least some water vapour and cloud condensation nuclei. However, since clouds aren't forming, conditions (obviously) aren't right for them to form on their own. Either it's too dry, there aren't enough particles for water molecules to cling too, or it could be just a bit too warm for condensation.

In fact, even in that clear blue sky, conditions might only need the slightest 'nudge' for clouds to form. Along comes a jet aircraft, providing that 'nudge' as it flies through the area, and suddenly you get long wispy clouds left in its wake.

Why do some contrails persist and some do not?

This depends on the atmospheric conditions, and what happens to the water vapour after it condenses.

Since most contrails form at temperatures well below zero, the condensed cloud droplets tend freeze into ice crystals almost instantly. If the water droplets didn't freeze, they would be much more susceptible to immediate evaporation into the dry air. Since it's much harder for ice crystals to evaporate (technically, 'sublimate'), they persist to form the bright contrail. Ice can sublimate directly back into a gas, of course, but it takes a combination of sunlight and very dry air. Thus, the drier the air, the quicker the contrails will disappear, the more humid the air, the longer they'll persist.

If conditions are just right when the contrails form, with relatively clean, supersaturated (and probably supercooled) air, they can even spread and promote the formation of cirrus clouds around them.

Why are more contrails evident now than in the past?

Partly this has to do with the amount of air traffic. There are simply more jets in the air these days, with more daily flights, than there ever were in the past. 

Another part of this, though, is global warming. With temperatures in the troposphere - the lowest level of the atmosphere - on the rise, it means the air is able to hold more moisture. Thus, the atmospheric conditions that are ideal for contrail formation are just more common than they were in the past.

What are 'chemtrails' and 'geoengineering'?

For years, some groups have been claiming that the only "true contrails" are those that dissipate quickly. Contrails that persist, on the other hand, are supposedly something entirely different - chemicals, deliberately sprayed into the atmosphere by aircraft, which these groups call "chemical trails" or "chemtrails."

The apparent reason why this spraying would be taking place differs depending on who is making the claim. Some say it is a "geoengineering" program, designed to deliberately alter Earth's climate, which is having unintended toxic side effects on us, and all other life on the planet. Others believe that this "spraying" is something far more nefarious.

Geoengineering has been studied as a possible method of alleviating the symptoms of global warming, which could be used until we can develop a more permanent solution. However, due to the potential risks involved with these plans, to date, no nation, company or government has instituted a program of this kind.

Small-scale cloud-seeding operations, using substances such as silver iodide crystals, have been attempted in some regions, specifically to influence rainfall patterns and prevent hail damage.

Are contrails harmless?

Contrails are mainly composed of ice water crystals, with most of the water vapour that goes into them coming directly from the atmosphere. Burning jet fuel also produces the standard byproducts of burning fossil fuels, so the ice crystals can contain soot particles, sulphur dioxide and traces of metal particles. Carbon dioxide, oxides of nitrogen and organic compounds are emitted as well.

While these byproducts and pollutants rarely reach the surface in any concentrated form, and if they do reach the surface, it is typically thousands of kilometres downwind, this does add more pollution to the atmosphere.

Thus, it can't be said that contrails are harmless, and they also add to the effects of global warming.

How do contrails affect global warming?

It was mentioned above that the increase in the number of contrails over the years is partly due to global warming. However, research has shown that contrails can a 'positive feedback loop' for global warming, and thus could make climate change worse.

These thin, wispy clouds at high altitudes not only reflect sunlight back into space, but they are also capable of reflecting escaping infrared radiation (heat) back towards the surface, trapping it in the atmosphere. Thus, as more contrails occur, they can trap more heat in the atmosphere and make global warming worse. This produces conditions where contrails are more likely to form, and it goes around the loop.

With air traffic increasing all the time as well, and the threat that global warming and climate change represents to human civilization, this is a very valid and important concern about contrails. For this reason, scientists (including those at NASA) have designed contrail avoidance strategies, such as changing the altitude of flights to fly through drier levels of the atmosphere. However, these strategies naturally tend to be less effective around urban areas, where arriving and departing jet aircraft have fewer options for avoiding the more humid layers of air above our heads.

Sources: NASA | Contrail Science | Nature | Global News

Editor's note: A previous version of this article did not include references to small-scale cloud-seeding operations. This has been updated for completeness, and we apologize for any confusion this may have caused.

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