Holes in a cloud


A week ago, I showed iridescence in a lenticular cloud (colours in a wave cloud). Here is another uncommon feature of such clouds: lacunosus, that is, the cloud is potmarked with holes. (Lacunosus is Latin for: full of lacunae, that is holes.)

The holes are caused by convective bubbles of warm air from the clear air below the cloud that then rise through it and punch holes in it. Normally this wouldn’t happen as the temperature of the clear air below the cloud is about the same as that in the cloud. However, the cloud sits at the crest of a wave in the atmosphere that resulted from the air flowing over a mountain, and sometimes the wave begins to collapse. This causes the cloud and surrounding air to descend. 

Now, an odd thing happens. As air descends, it is compressed by the higher pressure it encounters at a lower elevations. The air temperature rises as a result of this compression. Interestingly, the temperature rise in the clear air above and below the cloud is greater than that in the cloud, with the result that the warmer lower air rises in little bubbles and punches holes in the cloud.

A collapsing mountain wave produces lacunosus in the lenticular cloud.


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3 Responses to Holes in a cloud

  1. Stu Heard says:


  2. As always, many thanks.

    So I gather from what you say that lacunose clouds must be observed more frequently in mountainous regions than elsewhere. (Not that I would know, having been fortunate enough to spend practically my entire life in the company of mountains).

    • Alistair says:

      Trevor, what is need for lacunosus is a fairly thin layer of cloud that is rapidly descending. A wave cloud on a collapsing mountain wave serves. Yet, it is not the only source. Often vigorous thunderstorms will produce the same thing. As the air in the storm ascends, some cloud is deposited in thin layers at stable layers in the mid-troposphere. However, the air in which these thin cloud layers rest begins to descend in compensation for the rapid updrafts in the thunderstorm, itself. Lacunosus forms.

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