<p>Lenticular clouds don't form because "rising air comes in contact with cooler air" but rather from moist air cooling to its dew point as it is forced to rise over the terrain.  Unlike most clouds that move with the wind, clouds like these are "anchored" to the terrain and the air moves through them.  If you observe them closely you can see new cloud forming as air flows upward and into the cloud on the windward side of the slope and evaporating as it exits descending on the lee side.</p>

<p>As air rises, pressure decreases and the air expands (volume increases).  This results in a temperature decrease in the parcel.  When the rising air cools to its dew point the cloud will form.  In this case air is only rising because it is forced over the mountain.  Once past the peak, the air descends the lee slope and is compressed and warmed.  Once it warms up and relative humidity is below 100% the cloud evaporates.  If the streamlines parallel the contours of the peak, this results in a roughly circular region where the air is suitable for the cloud to form.   </p>

<p>If the terrain was a long ridge instead of an isolated peak, the lenticular cloud will elongated as a line over the ridge tops.</p>

<p>You generally won't see the cloud elongated downwind, as the air is forced down the lee slope.  What you might instead see are clouds at regular intervals downstream of the mountain if the mountain (and atmospheric stability) has caused a standing wave to form. In this case you might see clouds in the peaks of the wave depending the humidity of the air and the amplitude of the wave.</p>

CC BY-SA 3.0
2016-08-09T02:36:18.223Z
2016-08-09T02:36:18.223Z
casey
<p>I recently visited Japan, where I observed a lenticular cloud above Mount Fuji:</p>

<p><a href="https://i.sstatic.net/es9cb.jpg" rel="noreferrer"><img src="https://i.sstatic.net/es9cb.jpg" alt="Mount Fuji and lenticular cloud"></a></p>

<p>Curious by the cloud, I looked it up and found it it's a <a href="https://en.wikipedia.org/wiki/Lenticular_cloud" rel="noreferrer">lenticular cloud</a>, that forms because of rising moist air coming into contact with cooler air. However, why is it circular? If I would look at it from above would it still be circular? Since air moves in a certain direction, I would expect the cloud to be laminar and elongate towards one direction rather than circular.</p>

<p>What makes it circular?</p>

CC BY-SA 3.0
2016-07-14T08:42:01.493Z
Gansub sounds to know more about it, but would think it's a typical biproduct of the necessary conditions: stable air and high moisture (otherwise the clouds would grow vertically into cumulus)... and such background conditions are heavily related to high pressure and fairly calm winds.  And while there's still some winds, if the vertical motion from the mountain (locally induced effect) is of a greater scale than the background winds, it'd tend to be quite circular.
JeopardyTempest
CC BY-SA 3.0
2016-07-14T08:42:27.167Z
That said, while most seem to be fairly circular, I was able to find pictures like http://4.bp.blogspot.com/--IBopprRXIo/VRAL3aFxa1I/AAAAAAAAAto/Cq1MtY2hf5s/s1600/lenticular2.jpg (albeit, not just one perfect peak) that suggest it is possible to have elongated clouds as you suggest more.
JeopardyTempest
CC BY-SA 4.0
2021-07-04T14:05:35.407Z
@gansub time stamp on the picture says 8 am, July 25 2016, but I don't know whether to believe it or not.
Gimelist
CC BY-SA 4.0
2021-07-05T01:30:20.370Z
@gansub the previous photo before it is us having dinner, so 8 am sounds reasonable
Gimelist
<atmosphere><clouds>
Why are lenticular clouds circular?
Gimelist
# Why are lenticular clouds circular?

(Asked by: username_0 on 2016-07-10)

I recently visited Japan, where I observed a lenticular cloud above Mount Fuji:

[![Mount Fuji and lenticular cloud](https://i.sstatic.net/es9cb.jpg)](https://i.sstatic.net/es9cb.jpg)

Curious by the cloud, I looked it up and found it it's a [lenticular cloud](https://en.wikipedia.org/wiki/Lenticular_cloud), that forms because of rising moist air coming into contact with cooler air. However, why is it circular? If I would look at it from above would it still be circular? Since air moves in a certain direction, I would expect the cloud to be laminar and elongate towards one direction rather than circular.

What makes it circular?

**Tags:** atmosphere, clouds

> **Comments:**
> **username_1**: Gansub sounds to know more about it, but would think it's a typical biproduct of the necessary conditions: stable air and high moisture (otherwise the clouds would grow vertically into cumulus)... and such background conditions are heavily related to high pressure and fairly calm winds.  And while there's still some winds, if the vertical motion from the mountain (locally induced effect) is of a greater scale than the background winds, it'd tend to be quite circular.  
> **username_1**: That said, while most seem to be fairly circular, I was able to find pictures like http://4.bp.blogspot.com/--IBopprRXIo/VRAL3aFxa1I/AAAAAAAAAto/Cq1MtY2hf5s/s1600/lenticular2.jpg (albeit, not just one perfect peak) that suggest it is possible to have elongated clouds as you suggest more.  
> **username_0**: @gansub time stamp on the picture says 8 am, July 25 2016, but I don't know whether to believe it or not.  
> **username_0**: @gansub the previous photo before it is us having dinner, so 8 am sounds reasonable  

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## 1 answers

### Answer by username_2 on 2016-08-09. Score: 4 | Accepted answer

Lenticular clouds don't form because "rising air comes in contact with cooler air" but rather from moist air cooling to its dew point as it is forced to rise over the terrain. Unlike most clouds that move with the wind, clouds like these are "anchored" to the terrain and the air moves through them. If you observe them closely you can see new cloud forming as air flows upward and into the cloud on the windward side of the slope and evaporating as it exits descending on the lee side.

As air rises, pressure decreases and the air expands (volume increases). This results in a temperature decrease in the parcel. When the rising air cools to its dew point the cloud will form. In this case air is only rising because it is forced over the mountain. Once past the peak, the air descends the lee slope and is compressed and warmed. Once it warms up and relative humidity is below 100% the cloud evaporates. If the streamlines parallel the contours of the peak, this results in a roughly circular region where the air is suitable for the cloud to form.

If the terrain was a long ridge instead of an isolated peak, the lenticular cloud will elongated as a line over the ridge tops.

You generally won't see the cloud elongated downwind, as the air is forced down the lee slope. What you might instead see are clouds at regular intervals downstream of the mountain if the mountain (and atmospheric stability) has caused a standing wave to form. In this case you might see clouds in the peaks of the wave depending the humidity of the air and the amplitude of the wave.


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