• What are contrails:

    Or why airplanes leave a white trail behind them.

You’ve most likely seen the white trail some aeroplanes leave behind and you’ve most probably wondered what they are and how they’re formed. It’s quite peculiar because where various airways meet, large wefts similar to gigantic spiderwebs, can be formed.

In today’s post, we discuss contrails or condensation trails, the ‘mysterious’ white trails planes leave in the sky. Interested in this topic? Continue reading!

What are airplane’s white trails?

Although there are many conspiracy theories generated by this phenomenon, the explanation of what they are is rather mundane: contrails are water vapour. Yes, it’s that simple. Following, we expand a little.

The first thing we need to take into account is that clouds are formed when a mass of air condensates, that is, when its humidity reaches 100%. In order for this to occur, temperature plays a major role: the lower the temperature, the easiest it is for air to reach the condensation point.

To give an example, at sea level, a mass of air at 75% humidity and at 15º C will not condensate, however, if temperatures were to descend to 5ºC, it would. As we know commercial aeroplanes fly in the highest layers of the troposphere at temperatures around -56ºC, there is no need to explain further, is there?

The other essential factor for the formation of contrails is the way turbine engines work. In the same way as cars do, aeroplanes use combustion engines which generate thrust force burning fuel and oxygen.

As a result of this process, a number of gases are generated during combustion, amongst them, water vapour. Along with low temperatures these go on to form great water vapour trails or contrails.

And why are they called contrails?

In English, contrail is the contraction of condensation and trail. In Spanish we have adopted this anglicism due to its simplicity and briefness, although the correct term is `estelas de condensación´.

Not all planes generate contrails

On some occasions it’s possible to see two aeroplanes flying at the same altitude but only one of them generates contrails or condensation tails. Why is this? It’s because of engine efficiency.

A turboreactor’s efficiency, η, is measured by the coefficient between the work carried out by the engine and the chemical energy produced. To put it in other words, the more efficient an engine is, the lower the altitude it will start to generate contrails at.

At present, aviation is using the most efficient engines there are, therefore, the most efficient aeroplanes generate contrails at increasingly lower altitudes.

However, as there are still aircraft of a certain age in service, two aeroplanes may be flying at the same altitude and only one of them will be generating condensation trails.

What are the ‘chemtrails’ then?

Chemtrails arise from the incorrect belief that contrails or water vapour trails are ‘chemical trails’ spread by aeroplanes for mysterious and evil purposes which are kept hidden from the general public.

It’s a term that contracts chemical and trail and there is a wide variety of supposed reasons or speculations about why they exist.

Some people believe there are substances in them for the mental control of the population or to alter meteorology, there are even some who think they contain ultra secret spying messages.

This couldn’t be further from the truth and the scientific community has been publishing reports for years, always stating the same information: contrails are made of water vapour generated by engine combustion.

Colour trails are not contrails

You may now be thinking: What about the colour trails that can be seen in air shows? Well, these aren’t contrails.

We need to take into consideration that this type of acrobatic flight is performed at low altitudes so they can be enjoyed at a glance and to add to that, they usually take place in summer, when temperatures go up and the weather’s good.

Colour trails of acrobatic airshows are achieved through intentional processes which are controlled by the pilot according to the manoeuvre they are performing. The singularity and surprise element that colour trails bring and that is characteristic of these events, is obtained through mixing colourings.

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