# Why are contrails not visible from the cabin?

If you observe a plane flying whilst on the ground, plenty of times you will see contrails:

Why are they not visible from the plane itself when sitting in the most rear part of the cabin even by trying to look towards the rear of the plane fuselage? They seem to be quite close to the engines.

Knowing that they form due to the heat, they should be seen quite close to the engine.

• The cobtrails is created by water moisture from the engine and moisture takes times to cool down to be visible
– Him
May 30, 2015 at 6:41
• Next time you see a contrail being formed, grab a pair of binoculars and and take a close look. You'll see that there's quite a distance between the tail of the airplane and the contrail. May 30, 2015 at 6:57
• Even in the photo in the question, you can clearly see that the contrail begins some distance behind the plane. May 30, 2015 at 11:11

The distance between the engine and the start of the contrail depends on:

• the engine type
• the temperature of the outside air
• the moisture content of the outside air.

Usually the contrails don't form before the tail of the aircraft, but in extreme cold or humid air contrails can be seen forming just behind the wing.

On larger aircraft you have a better change of seeing the contrail from inside the aircraft. On the 747 you can often spot them for the window of the rear passenger door.

• The last photo illustrates an additional factor: The visibility increases significantly if the contrails are lit almost exactly from behind. From far away and with the sun almost exactly behind the aircaft, heat refraction alone may be enough to show a contrail. May 30, 2015 at 9:08

Contrails do not form due to heat, but in fact the opposite: cold.

They occurs when water from fuel combustion exceeds the saturation point of the ambient air. As it is unable to hold more water it turns into water vapor instead. This freezes at sub-zero temperatures into ice crystals and you end up essentially making a cloud.

Since the saturation point is connected to temperature (think mixing sugar in cold vs. hot water) this happens a bit back from the fuselage when the exhaust cools down. The point where this happens is dependent on the outside temperature and the existing humidity.

Under the correct conditions you can see contrails from the cabin, but usually they crystallize too far back to be seen. See this and this video for it occurring.

• Would it necessarily be ice crystals and not water droplets? Steam from a kitchen kettle condenses into visible water droplets. Would water vapor from an engine exhaust never pass through a condition under which it could condense without already being below freezing point? May 30, 2015 at 17:20
• Either it's water droplets or ice crystals. Water crystals are impossible. Jun 1, 2015 at 16:14
• @AnthonyX My brain switched off there for a bit, you are of course right. Jun 1, 2015 at 20:58
• Water ice crystals. Jun 8, 2015 at 17:29
• Not the terms I would use. Out of the engine it is water vapor. Once it precipitates out (exceeds the saturation limit of the air) then is it liquid water. Once it is cooled below freezing it is ice. A cloud can be tiny water droplets or ice. They small enough they are held in suspension. Jun 11, 2015 at 14:37

This is more a comment than an answer, but since I have a picture, I write it as answer.

It has alredy been written that contrails may form aside or behind an aircraft, so you may or may not be able to see them from the rearmost seats.

But also note that the blast stream can be much lower than what you would expect. In a two-engine aircraft, you have to look down from the upper edge of the window, and even then, the stream will be on the lower border of your field of view. Of course, the stream of the outer engines of four-engine aircrafts will be more centered in your field of view.

If you are flying over a scenery with high optical contrast, you can recognize the blast as a sharply defined region where the view is very blurry, as in the lower right region on this picture:

This was taken from the last seat row of a A340 after takeoff at Chicago O'Hare, and you can see the blast of the outer engine. (And yes, the outer stabilizer is in a little weird condition...)

So, look out for these blurry regions, that's where contrails may form.

EDIT: (just for fun)

Coming home from vacation yesterday, I noticed another sign for contrails from your aircraft: The contrail's shadow: Unfortunately, the clouds became less dense when I made the photo, before I could also see a good shadow of the aircraft itself.

• Sorry for the OT. Is this actually an A340 or an A330? Seems the latter from the wing. May 30, 2015 at 13:54
• Definitively A340, as it had 4 engines. The picture is zoomed. And if it were a 2-engine A330, I had to make the poto pointing more downwards to get the blast on the picture. May 30, 2015 at 14:06
• darn wings, always getting in the way of great pictures... Jun 8, 2015 at 17:05