<p>Of the major cloud families, the &quot;<a href="https://cloudatlas.wmo.int/en/cumulus-cu.html" rel="nofollow noreferrer">cumulus</a>&quot; type (including stratocumulus, altocumulus and so on) are generally formed by convection in unstable air, leading to their &quot;lumpy&quot; or &quot;heap-like&quot; forms.  The <a href="https://cloudatlas.wmo.int/en/stratus-st.html" rel="nofollow noreferrer">Stratus</a> 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.</p>
<p><a href="https://cloudatlas.wmo.int/en/species-altocumulus-lenticularis-ac-len.html" rel="nofollow noreferrer">Altocumulus lenticularis</a> 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.</p>
<p>Lenticular clouds are often similar to <a href="https://www.weatheronline.co.uk/reports/wxfacts/Cap-cloud.htm" rel="nofollow noreferrer">cap clouds and pileus, which are classified as stratiform clouds</a>.</p>
<p>So, is convection a defining factor for clouds to be classified as &quot;cumulus&quot; or &quot;Altocumulus&quot;?   Is the classification of Lenticularis clouds as a species of Altocumulus based mainly on historical precedent, rather than consideration of their mode of formation?</p>

CC BY-SA 4.0
2023-04-10T19:20:17.177Z
@gansub wouldn't a significant horizontal wind also push it up the topography mechanically even with stable conditions?
JeopardyTempest
CC BY-SA 4.0
2023-04-10T22:41:33.917Z
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.
James K
CC BY-SA 4.0
2023-04-13T05:21:11.037Z
@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)
JeopardyTempest
<meteorology><clouds>
Altocumulus lenticularis - Why is this not considered a "strati-form type cloud
James K
# Altocumulus lenticularis - Why is this not considered a "strati-form type cloud

(Asked by: username_0 on 2023-04-09)

Of the major cloud families, the "[cumulus](https://cloudatlas.wmo.int/en/cumulus-cu.html)" 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](https://cloudatlas.wmo.int/en/stratus-st.html) 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](https://cloudatlas.wmo.int/en/species-altocumulus-lenticularis-ac-len.html) 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](https://www.weatheronline.co.uk/reports/wxfacts/Cap-cloud.htm).

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?

**Tags:** meteorology, clouds

> **Comments:**
> **username_1**: @gansub wouldn't a significant horizontal wind also push it up the topography mechanically even with stable conditions?  
> **username_0**: 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.  
> **username_1**: @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)  

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## 0 answers

