Contrails – What are they

It’s a clear, cold winter’s day and you step outside. As soon as you exhale, you see the exhalation form in the air. That’s similar to a contrail, just on a smaller scale.

You boil your kettle. Immediately above the spot the air looks clear, but it’s not. That’s water vapour. Then you see the steam, that’s the water vapour condensing into the air. That’s similar to a contrail, just on a smaller scale.

It’s a winter’s day and you are out in your car. Some of the cars you see are emitting white “smoke” from their exhaust pipes. That’s similar to a contrail too. As a hydrocarbon fuel (like petrol, diesel, and aviation fuel) combusts in an engine some of the hydrogen (H) (that’s the hydro in hydrocarbon) chemically reacts with oxygen in the air (O) to produce H2O – which is water. When really hot, it’s dissolved in the air and invisible (water vapour) but as it leaves the exhaust pipe the surrounding atmosphere may not be able to keep the vapour in solution and so the water condenses out into visible steam. Guess what, that’s similar to a contrail too, just on a smaller scale.

Why similar, because at 30,000 feet the water almost instantly forms ice crystals because of the very low temperature.

But why doesn’t it persist, do I hear you ask?

Well, conditions at 30,000 feet, where the airliners fly, is a bit different from ground level. The temperature, for example, is between -40 to -57C – so, it’s a lot colder, the humidity levels are different and the air pressure is a lot lower too.

No chemicals, no grand conspiracy, just science.

When a plane is flying high, exactly the same thing happens as described above, just on a far greater scale.

When a plane’s jet engine burns its hydrocarbon fuel hydrogen is produced at great temperature. The hydrogen (H) combines with Oxygen (O) to produce water vapour. When that water vapour hits the atmosphere, it rapidly cools. If the air is dry, (low humidity so having plenty of capacity to take on water), it’s absorbed invisibly into the air.

However, if the temperature is low (as it generally is at 30,000 feet it’s -40C to -57C typically) and the humidity is high (i.e. the atmosphere can’t absorb any more water) the vapour condenses in to droplets that freeze almost instantly – and your contrail is born. If there’s no wind it’s highly likely that the contrail will persist for a long time.

And the conditions that allow contrails to form are very similar to those that allow clouds to form, so contrails don’t make clouds, but the similar conditions mean that clouds are likely to form.

And for a final verification, buy a cheap set of binoculars and observe the aircraft making the trails. 95% of them will be civilian airliners and the contrails emanate from the engines, not from some secret valve hidden in the fuselage.

Oh, and if chemicals were dispersed at altitude, it’s highly unlikely that they’d make it to the surface and even if they did, they’d be so dispersed as to be useless.

When were Contrails First Seen?

During WW2 which was when hydrocarbon fuelled aircraft could first reach the altitudes that contrails form.

And remember, temperature, wind direction and wind strength at altitude will be very different to temperature, wind direction and wind strength at ground level.