Mottled clouds again

[jack.h...@gmail.com]

https://youtu.be/EwQAFwqvld0

They begin to show about 10 seconds into the time-lapse and then almost as quickly dissipate.

I published a video of the same type of cloud a week or so.  Seems to occur only in the evening.  Any explanation?

Jack

[Freddie]

I would say coincidence.  I've observed them at most times of the day, including at night.

[jack.h...@gmail.com]

Yes, time is likely pure coincidence.  But what causes them, and why so short-lived a phenomenon?

Jack

[Freddie]

They're formed by turbulence generated in layers of air of different densities - typically (but not always) just under a temperature inversion.  It is the turbulence that is the transient factor - the moisture is in the air all the while, but it doesn't become visible until cooling occurs due to uplift and then condensation takes place.  The dappled effect is a handy visualisation of where the turbulent air is ascending (cloud develops) or descending (no cloud, or cloud clears).  It involves small-scale processes, so models find it difficult to capture the detail.  Most stratocumulus and altocumulus is formed in this way.

[Len]

They look to me like altocumulus cells formed somewhere upwind from a sheet of altostratus which has had its top cooled radiatively {longwave).

This longwave cooling from the top destabilises the layer and it breaks into small convection cells.

Len

Wembury

On Thursday, 19 June 2025 at 01:03:03 UTC+1 jack.h...@gmail.com wrote:

[Len]

Freddie,

You may in part be right Freddie but turbulence suggests randomness and these cloud look rather organised in terms of spacing.

Convective cells spring to mind but convection we see of course nearly always is a mix of thermal and mechanical.

Len