Why do Jets leave smoke trail?

"Smoke" from "Jet"

"Oh, that's just a jet producing smoke as an indicator for other planes" - one of my dear colleague (senior engineer for airport special ancillary system) explained as a matter of fact. That's the most bizarre statement of my entire life. 

First of all lets make something clear - it's not smoke. It's pure ice. Yes, very very small droplets of ice. And this phenomenon or behavior is not even aviation related.  It is just moisture getting cooled to ice. 

It is the exact same principal as you blow out cold breath; just like the dude in the picture below: 

Random Dude

You see the water droplets in our mouth are exhaled with carbon dioxide. The water droplets are instantly cooled down to ice crystals which are so small that they have negligible mass and thus float in the air.

But we are talking about watching aircraft marking its territory in summer season. I am writing from Saudi Arabia, one of the hottest place on the planet. We see the same phenomena here.

Well, it is because of the height. As the height increases the temperature decreases. Ever saw Mount Everest in bright sunny day? Mount Everest will always have ice on it because it is situated 8,848 m from sea level. Aircraft cruise nearly at 10,000m from sea level.

The turbofan engine of commercial aircraft produces thrust at an exuberant rate. It contains water vapour which is cooled to ice crystals.  By the way this "smoke" is scientifically called as "Contrails". 

How it works.

So its all about temperature. This is also the same reason why we cannot see contrails in ground. Although, in cold countries we can see it in the ground:

Contrails viewed in the ground.

Also notice the gap between the engine and contrail. The water vapour will take some time to cool down. It's not instantaneous. This is the reason why you cannot see it when you are are flying. 

When it comes to military jet aircraft, they don't want to show you their contrails. Think about it, what is the use of 5th generation stealth aircraft if its contrails can be seen. They have special anti-contrail methods.

That being said, I think contrails are really cool. It reminds us how awesome aircraft really are and the stuff they can do.

Aviation Awesomeness

I hope this article has been useful and informative. We welcome your valuable comments and suggestion. 

Until next time guys, this is Chan signing off.

-Chan CK