I love looking at clouds. I love trying to describe them and compare what I see day to day.

I have dozens of questions about the things I’ve observed and I would really like to understand the physics of the world around me. Today I would like to focus on the surface dynamics of cumulus clouds. Why do some cumulus clouds have distinct textured surface while others are softer and wispy at the edge?

I imagine it probably comes down to temperature and pressure. It may also be an effect of direct sunlight, I’m not sure. I notice the soggy cotton clouds are more common in the evening and the billowy ice cream clouds are more common mid-day.

Do clouds have surface tension like liquid water?

Does the flow of water either into or out of a cloud affect its surface characteristics?

Textured cloud Soft cloud

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    $\begingroup$ Good question! I would also ask whether the type of cloud particles play a role - ice or water droplets? $\endgroup$ – Gordon Stanger Aug 29 '16 at 23:32
  • $\begingroup$ Don't forget the differences of scale. Your left picture is taken from a much larger distance, so you won't notice any 'fluffiness' around it. Can you replace one or the other picture with a different example so that we are not comparing oranges and apples? $\endgroup$ – Jan Doggen Aug 30 '16 at 11:12
  • $\begingroup$ Sure thing Jan Doggen, I'll see if I can find some more pictures. I'll make a note here if I change anything. $\endgroup$ – Patrick Staight Aug 30 '16 at 16:04

No, clouds don't really have a 'surface' that could have tension like a body of water. The different looks in these two examples (left Cumulonimbus Calvus and right Cumulus Humilis) are greatly dependent on how they have formed and how are they evolving now.

The large Cumulonimbus is still growing in a relatively rapid speed. The cloud is reaching higher and higher upwards carrying moist air. The moist air due to turbulent flows and expanding of the rising air gets mixed with the cold air and instantly forms/extends cloud as it reaches saturation (saturation is reached with less water in colder air and air is colder in higher altitudes). If there wasn't the expansion then flows of dry air to the cloud would desaturate the cloud and more small scale variation could be seen as in the Cumulus Humilis. Also the Cumulonimbus being much larger and further away looks different just due to distance. The steady state like situation of the Cumulus Humilis where it isn't growing (perhaps a little on the top) and the fact that the lower atmosphere has stronger turbulent motions equals to the appearance where more cotton candy pieces 'drift' from the cloud.

Neither of these are ice droplet clouds (they look different just because they are made of ice and not water droplets), but all clouds that are in freezing temperatures have some very small percentage of ice in them too. The Cumulus Humilis might be below 0 Celsius and the Cumulonimbus definitely is at some height. A crude estimate is that temperature drops 6 Celsius every kilometer. The base of both clouds is probably somewhere between 1 and 2.5 kilometers and the Cumulonimbus might reach over 10 kilometers.

  • $\begingroup$ Supplementary question to MofM: Can the difference be caused by whether the cloud-clear air interface is dominated by diffusion as opposed to advection? $\endgroup$ – Gordon Stanger Aug 30 '16 at 9:32
  • $\begingroup$ I would say advection plays only a secondary role by in providing the Cumulonimbus updraft. There definitely is advection that moves the Cumulus Humilis horizontally, but the air in front of the cloud moves of course at the same rate. In both cases there is turbulent diffusion, but only the Cumulonimbus expands. $\endgroup$ – Communisty Aug 30 '16 at 10:14

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