# Why does this contrail diverge and then converge?

I observed this contrail above me today, and I noticed that the two inner contrails diverge outwards to merge with the two outer contrails. Then, as the contrails dissipate after several seconds they converge again and become closer.

I doubt it's a matter of viewing perspective because I observed it when the airplane was flying both towards me and away from me.

Why does that happen? My attempt to find an explanation online failed, unfortunately.

• Perhaps it simply appears to converge due to dissipation into the atmosphere. The further the contrail is from the aircraft, the more time for dissipation. – reevesii Jun 23 '17 at 22:16
• Interesting question. To me I honestly don't really see them diverging in the pictures, just maintaining the separation size between the engines at first before then merging. They definitely do then converge though, and I've been captivated by this phenomena as well many times. I'm thinking there's some sensible reasons behind it to with the heat/velocity of the air in those two streams, and how they'd inspire air motion between them. Even sort of in line with merger of vortices in drains or low pressure areas, except straightline instead of rotating. – JeopardyTempest Sep 10 '18 at 6:46
• But it's definitely interesting how at first there doesn't seem to be significant change between them, and then they begin moving (I can verify seeing this too). I wonder if it's just because the interactions amplify as their approach, so that at first they'll only nudging closer imperceptibly, and then gradually increase rates. Or if there's some sort of critical length, perhaps related to the flow velocity, and so it requires them to slow to a certain speed before the interaction takes. – JeopardyTempest Sep 10 '18 at 6:49
• And finally, instead of seeing divergence at first, the other thing I was thinking I may've noticed (though I'm not sure) is that the merger point wasn't equally between them, but one flow shifted less than the other. Always wondered if this was related to the flow disruption caused by the plane itself interacting with the two jet flows. Great question! – JeopardyTempest Sep 10 '18 at 6:51
• This question actually works out to be a duplicate of an earlier question at Physics Stack Exchange: Atmospheric phenomenon - what causes condensation trails to converge?. Looks like they don't give an entire thorough writeup, but do provide a very quick summary plus references for further reading, indicating it is indeed due to vortices. [I also remove my questioning of whether they diverge based upon their great picture] – JeopardyTempest Sep 10 '18 at 7:07