#### derrick06

##### Active Member
Hey all! I couldn't find this specific answer in the search section so I decided to make a quick thread to ask a question about Contrails.

I live next to an airport so staring at aircraft is nothing new to me. Especially their contrails which I have even seen on racecars on humid days! (Check out this old picture from F1 I attached) Now on a race car It's easy to see the factors that effect the airflow around it and the contrails that come off its spoiler but on a plane with it's wings/exhaust It can be tougher so I noticed one thing that I was curious about!

What causes some parts of the contrail of an aircraft to stay present longer than other parts? Looking up at planes I sometimes notice this. The plane will be long gone and the trails closest to it have dissipated while another part of the trail further back can still be present. Can there be a change that much in moisture from one area to the next or wind currents? Also I have seen the contrails behind a plane suddenly stop while flying high, is this because it gained enough altitude where this won't be present? I read articles on here on how they work and such but since this was a specific question related to them I figured maybe I could post it? I apologize if this was already answered on here. Anybody have any insight?

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#### Ross Marsden

##### Senior Member.
Calculations of Aircraft Contrail Formation Critical Temperatures
This note provides a brief explanation of contrail formation theory and presents a simple derivation of the critical temperature of contrail formation for representative jet engine types.
...
Contrails are clouds that form when a mixture of warm, unsaturated, engine exhaust gases and cold ambient air reaches saturation with respect to water, forming liquid drops, which quickly freeze.
Content from External Source
There are two diagrams on the second page that explain why, in certain atmospheric conditions, contrails don't dissipate. Note that the physical gap between point 1 and point 2 is usually much shorter than illustrated in the diagram.

#### Mick West

Staff member
What causes some parts of the contrail of an aircraft to stay present longer than other parts? Looking up at planes I sometimes notice this. The plane will be long gone and the trails closest to it have dissipated while another part of the trail further back can still be present. Can there be a change that much in moisture from one area to the next or wind currents? Also I have seen the contrails behind a plane suddenly stop while flying high, is this because it gained enough altitude where this won't be present?

Two articles specifically:

Broken contrails:
http://contrailscience.com/broken-contrails/

and
Why some parts of a contrail persist longer:
http://contrailscience.com/hybrid-contrails-a-new-classification/

There's a lot more to it, but that should give you an idea. If you still have questions, then a photo example would be very helpful.

The contrail on the car is an aerodynamic contrail. Most of the contrails you see from planes are engine exhaust contrails, which are quite different in their formation.

#### derrick06

##### Active Member
Thank you! I don't know why I couldn't find these. I knew there had to be differences in the conditions at the altitudes. All of your information answered my questions perfectly. Thanks everyone. This community rocks, I learn something new every time I visit!