Tuesday, April 04, 2006


When I was younger my father would point out vapor trails. Those cloud-like formations that followed planes.

Now they have a different name. Contrails. And now I sometimes discuss them with my (13 year old) son. Here's what they are
The condensation trail left behind jet aircrafts are called contrails. Contrails form when hot humid air from jet exhaust mixes with environmental air of low vapor pressure and low temperature. The mixing is a result of turbulence generated by the engine exhaust. Cloud formation by a mixing process is similar to the cloud you see when you exhale and "see your breath".

Note that term "turbulence?" He's been insisting to me that when the contrails widen and dissipate it's a result of turbulence in the atmosphere. This explanation though isn't consistent with what he's told me
If you are attentive to contrail formation and duration, you will notice that they can rapidly dissipate or spread horizontally into an extensive thin cirrus layer. How long a contrail remains intact, depends on the humidity structure and winds of the upper troposphere. If the atmosphere is near saturation, the contrail may exist for sometime. On the other hand, if the atmosphere is dry then as the contrail mixes with the environment it dissipates.

So it would seem that whether or not the contrail expands or simply dissipates is a function of the humidity and wind. Is wind what he means by turbulence?

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