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Of the major cloud families, the "cumulus" type (including stratocumulus, altocumulus and so on) are generally formed by convection in unstable air, leading to their "lumpy" or "heap-like" forms. The Stratus clouds, form in stable air, by advection (a layer of stratus or altostratus in advance of a warm front), or direct radiative cooling (fog). The absence of convection results in their layered form.

Altocumulus lenticularis does not seem to fit this pattern. It develops in stable air, that is being forced to rise perhaps as a result of upwind topography. Unlike other cumulus clouds, the flow of air remains laminar and not turbulent. There is no convection within the cloud, and the only reason that an extended layer of cloud doesn't form is that downwind the air descends, again as a result of the landforms. Lenticularis clouds are known to remain in one place and not move with the wind.

Lenticular clouds are often similar to cap clouds and pileus, which are classified as stratiform clouds.

So, is convection a defining factor for clouds to be classified as "cumulus" or "Altocumulus"? Is the classification of Lenticularis clouds as a species of Altocumulus based mainly on historical precedent, rather than consideration of their mode of formation?

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  • $\begingroup$ @gansub wouldn't a significant horizontal wind also push it up the topography mechanically even with stable conditions? $\endgroup$ Apr 10, 2023 at 19:20
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    $\begingroup$ For me this poses the question, what causes the rise and fall of air in lee waves. Wikipedia suggests that they are a gravity waves, the rise and fall is not convective and heat driven, but due to a parcels of air being pushed from their position of neutral buoyancy and sinking, but overshooting due to momentum, and the rising again. This tends to suggest that lenticular clouds do indeed form in stable air, and aren't convection driven - but the physics is now getting beyond me. perhaps they are still technically convection for reasons that aren't clear to me. $\endgroup$
    – James K
    Apr 10, 2023 at 22:41
  • $\begingroup$ @gansub it's all been too long for me, so I'll have take your word for it! But that said, I can certainly get why many people who aren't scientists or dabble in the topics would be confused! Perhaps a more thorough full answer (though I know how it goes with finding time... so many answers I've never ended up following through on through the years) $\endgroup$ Apr 13, 2023 at 5:21
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    $\begingroup$ @JeopardyTempest I meant if the velocity is over the critical level then there is chance of a hydraulic jump and no lenticular clouds would be formed youtube.com/watch?v=BoFMd-QFLck $\endgroup$
    – user1066
    Apr 13, 2023 at 9:30

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