Odd cloud effect

I looked out of my office window the other day and saw this:

No idea what caused it. Looks like an aircraft contrail in reverse. Not something I’ve ever noticed before.

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4 Responses to Odd cloud effect

  1. Paul Rafferty says:

    It’s a distrail rather than a contrail. A distrail is the absence of water vapour caused by the moist air in the cloud being warmed by the jet exhaust rather than the water in the exhaust showing in an otherwise clear sky.

    • Paul Rafferty says:

      Further… the cloud will comprise of super-cooled water droplets. An aircraft flying through it disturbs the droplets via aerodynamics (vortices and temperature variations) and the engine exhaust provides nucleation centres for the droplets to freeze. The ice drops away from the cloud level, leaving clear air.

    • james says:

      Thank you for the explanation. Makes me wonder why I’ve never noticed one before.

      • Paul Rafferty says:

        The clouds are normally alto cumulus and their altitude is below the usual cruising level of an airliner (37,000 feet ish). Most aircraft spend the majority of flight in the cruise, with say 20 minutes of each flight in the climb and perhaps only 5 minutes of that through the alto level. The descent is conducted at idle thrust and is less likely to cause a distrail. From all of that, you can see that the probability of seeing one is less than that for a cruise contrail.

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